Doppler Shift

Saturday, July 25, 2009

Precise measurement of the velocity or change of position of stars tells us the extent of the star's movement induced by a planet's gravitational tug. From that information, scientists can deduce the planet's mass and orbit.

Why does a planet cause a star to sway? If a star has a single companion, both move in nearly circular orbits around their common center of mass. Even if one body is much smaller, the laws of physics dictate that both will orbit the center of the combined star and planet system. The center of mass is the point at which the two bodies balance each other.

The radial velocity method measures slight changes in a star's velocity as the star and the planet move about their common center of mass. In this case, however, the motion
detected is toward the observer and away from the observer. Astronomers can detect these variances by analyzing the spectrum of starlight. In an effect known as Doppler shift, light waves from a star moving toward us are shifted toward the blue end of the spectrum. If the star is moving away, the light waves shift toward the red end of the spectrum.

This happens because the waves become compressed when the star is approaching the observer and spread out when the star is receding. The effect is similar to the change in pitch we hear in a train's whistle as it approaches and passes.

The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. That's why many of the first planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their parent stars.

Astrometric Measurement

As with the radial velocity technique, this methods depends on the slight motion of the star caused by the orbiting planet. In this case, however, astronomers are searching for the tiny displacements of the stars on the sky.

The planets of our solar system have this effect on the Sun, producing a to-and-fro motion that could be detected by an observer positioned several light years away.

An important goal of the Space Interferometry Mission is to detect the presence of Earth-size planets orbiting nearby solar type stars via narrow angle astrometry. Similarly, the Keck Interferometry will conduct an astrometric survey of hundreds of stars to search for planets with masses as small as Uranu

Transit Method

If a planet passes directly between a star and an observer's line of sight, it blocks out a tiny portion of the star's light, thus reducing its apparent brightness.

Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice-versa. A terrestrial planet in an Earth-like orbit, for example, would produce a minute dip in stellar brightness that would last just a few hours

Gravitational Microlensing

This method derives from one of the insights of Einstein's theory of general relativity: gravity bends space. We normally think of light as traveling in a straight line, but light rays become bent when passing through space that is warped by the presence of a massive object such as a star. This effect has been proven by observations of the Sun's gravitational effect on starlight.

When a planet happens to pass in front of a star along our line of sight, the planet's gravity will behave like a lens. This focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star
Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.

Science - Finding Planets

Since planets do not give off their own light, observing them directly presents formidable challenges. Missions such as Terrestrial Planet Finder will rely on advanced technologies that can harness special properties of light to extend our vision. For a more detailed discussion of planet imaging

Science - Finding Planets

Since planets do not give off their own light, observing them directly presents formidable challenges. Missions such as Terrestrial Planet Finder will rely on advanced technologies that can harness special properties of light to extend our vi

How to Take Snapshots of Distant Worlds

One of the grand challenges of NASA's search for new worlds is to develop technologies that will allow us to obtain the first images of planets circling distant stars.

While the parent star is the source of light that will make any planet visible, its glare is between a million and 10 billion times brighter than the faint little speck we are looking for. Therefore, any detailed study of extrasolar planets will require methods to cover up or otherwise control the glare of the parent star so that we can study its immediate surroundings.

Another challenge stems from the fact that, compared to the separation between most things in the universe, planets are located extremely close to their parent stars. For this reason, we need very high resolution to separate the planet from its nearby host.

The following is an overview of several techniques in development that could overcome these obstacles and make extrasolar planet imaging a reality.


Originally invented to study the Sun, a coronagraph is a telescope designed to block light coming from the solar disk, in order to see the extremely faint emission from the region around the Sun, called the corona. It was invented in 1930 by B. Lyot to study the Sun's corona at times other than during a solar eclipse. The coronagraph, at its simplest, is an occulting disk in the focal plane of a telescope or out in front of the entrance aperture that blocks out the image of the solar disk, and various other features to reduce stray light so that the corona surrounding the occulting disk can be studied.

However, this technology is now being refined and adapted for the purpose of studying the region around distant stars in search of planets themselves or spectral evidence of planets. One challenge with this approach lies in the diffraction of light around the edges of the occulting shape, which detracts greatly from the potential angular resolution of the image.

The diffraction pattern of a simple round telescope, for example, is a series of concentric rings with a bright central spot. Blocking the light from a star in order to see an orbiting planet requires suppressing the first several bright rings without blocking out the planet. By using a different shape, the diffraction pattern can be controlled so that the starlight is much dimmer closer to the center in some areas, and brighter in others. The telescope can be rotated about its line-of-sight so that the planet image passes in an out of the regions where the starlight is dim.

Managing this diffraction pattern isn't too difficult -- there are a number of options available to accomplish this. So, the technologies under study include various tricks to block out as much of the starlight as possible, while managing the diffraction pattern such that the planet can be seen peeping out from beyond the diffraction bands.

Other proposed solutions for dealing with scattered light within the telescope include novel-shaped apertures, odd-shaped pupils, pupil masks to suppress some of the diffraction, and deformable mirrors.

To appreciate the difficulty the phenomenon of diffraction presents to the development of a coronagraph technology for studying other solar systems, see A closer look at diffraction .

Another possibility is to combine techniques of coronagraphy with interferometry. A coronagraph could also incorporate a spectrometer, so that chemical signs of life could be sought within the light reflected from a planet.

Interferometers and Nulling

An alternative way to get a picture of a distant planet is to replace one large mirror with a number of smaller mirrors and combining their light in a process called interferometry.

Using optical interferometers to study distant planets would allow for smaller mirrors, which can obtain a resolution equal to a single telescope as big as the largest separation between the individual telescopes.

To get enough of this information to build up a good picture, the interferometer must rotate around to different relative positions and repeat the "exposures." As well as taking a picture, an interferometer can obtain spectra of the targets it is looking at.

Interferometers provide extremely good angular resolution. That means they are very good at sorting out which light waves come from which part of the star system. Additionally, an interferometer can be "tuned" so that the light coming from the exact center in the field of view (where the star is) will be blanked out or nulled, while the light from any other area will be viewed normally.

The form of Universal Gravitation

Wednesday, July 22, 2009

The first thing to note is that the force of gravity must be independent of the type of material since all objects fall with the same acceleration, g. Therefore, the gravitational force must depend on mass alone. The dependence appears to be linear since Fgrav. = mg for an object of mass m. Consider two point masses, m1 and m2, as shown below.
The force acting on m1 should be F12 = m1k1, where k1 depends, at the very least, on the distance between the point masses. We have already established that k1 must be proportional to 1/r122, the distance between the point masses and that the force should be directed along a line connecting the two and pointing toward m2 (the latter comes from the fact that we assumed that the force of attraction for a circular orbit was a central one, i.e. the force connects the centers-of-mass). By the same reasoning, the force acting on m2 should be F21 = m2k2. But, by Newton's Third Law, we must have F21 = F12 and oppositely directed along the line between the point masses and pointing toward m1. The simplest consistent mathematical form is to have
k1 = Gm2/r 122
k2 = Gm1/r 122

where G is the universal constant of gravitation and must have units of Nt*m2/kg2. Therefore, Newton asserted that the gravitational force between two point masses is proportional to the product of the masses and inversely proportional to the square of the distance between the masses.

Introduction of The Solar System

Monday, July 20, 2009

Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system.

The Sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space. The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune. The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are Uranus and Pluto, which are tipped on their sides.

Does life exist on other planets beyond our Solarsystem

Does life exist on other planets beyond our Solarsystem? There is a high probability that life does exist on other planets than Earth.
But what do mean by 'life'? When we're talking about life we mean life as we know it: carbon based organic life forms that needs liquid water to exist.
So not all planets are capable of sustaining life. We know that already for quite some time, because in our Solarsytem Earth is the only planet of which we certain know that it sustains life. Mars could also have had life on it, but that isn't for sure. A planet has to meet certain conditions to be able to support life; the main condition is that the planet has to lie in the habitable zone. This is the region around a star in which life-supporting planets can exist; the boundaries are named the inner and outer edge of the habitable zone.
This means the habitable zone of a star requires certain conditions for a planet:
the star has to be a main sequence star (i.e. a star burning steadily light elements into heavy ones) the planet has to be solid to allow for a liquid-solid interface, this to enhance the exchange between molecules the planet has to be at the right distance from the star to allow for liquid water (temperature dependence) With the formula below (J. Schneider)¹ we can calculate the equilibrium temperature of a planet orbiting a certain star. A planet acquires, by heating, an equilibrium temperature Tp given by: ,Where A is the mean albedo (reflectance) of the planet surface at a distance a around a star with radius Rs and temperature Ts. On this page I will outline some things related to the question on top of this page.
The habitable zone (HZ) is the region around a star in which life-supporting planets can exist (Huang 1959,1960).

The habitable zone for Earth-like planets orbiting main sequence stars, is determined by water loss on the inner edge and by CO2 condensation, leading to runaway glaciation, on the outer edge. Planetary habitability is critically dependent on atmospheric CO2 and its control by the carbonate-silicate cycle. Conservative estimates for the boundaries of the Sun's (G type star) current HZ are 0.95 AU for the inner edge and 1.37 AU for the outer edge. The actual HZ width is probably greater, but is difficult to determine an exact value because of uncertainties regarding clouds which affect the planetary albedo. HZ widths around other stars in the spectral classification range of interest, F to M (~7200 to ~3000 Kelvin), are approximately the same if distances are expressed on a logarithmic scale (i.e. if you plot the distances from the inner and outer edges of the CHZs for different stars on a logarithmic axis, you will find that the widths of the CHZs for the different stars is about the same on this scale). If planets exist around other stars (they do) and if planetary spacing is logarithmic, as in our Solar System, the chances that one or more planets will be found within a star's HZ are fairly good.
The continuously habitable zone (CHZ) is the HZ that stays the HZ during the lifetime of the star. Because the star evolves the boundaries of the HZ will change slightly too, the CHZ will not change so the width of the CHZ will be smaller than the width of the HZ.
The width of the continuously habitable zone (CHZ) around a star depends on the time that a planet is required to remain habitable and on whether a planet that is initially frozen can be cold-started by a modest increase in stellar luminosity. CHZs are generally narrower than HZs because the boundaries of the CHZ migrate outward as a star ages. Despite this, the 4.6 Gyr CHZ around our own Sun extends from at least 0.95 to 1.15 AU and is probably considerably wider. CHZs around early K stars should be somewhat wider (in log distance) than around G stars because the K stars evolve more slowly. Equivalently, one could say that their CHZs are longer-lived. Since there are approximately three times as many K stars as G stars, this suggests that the majority of habitable planets may reside around K stars. Late K stars and M stars would have even wider CHZs, but the planets within them are susceptible to tidal damping and will probably rotate synchronously after a few billion years. F stars should have narrower CHZs than do G stars (on a log distance scale) because they evolve more rapidly. High ultraviolet flux are another potential problem for life around F stars. Stars earlier than ~F0 have main sequence lifetimes of less than 2 Gyr, so their planets are probably not suitable for evolving intelligent life. But 'simple life' could evolve here.

Discovered planets

Most extra-solar planets that have been discovered have been found by using Doppler technique. I've listed a table and a schematic diagram of the recently discovered planets around main sequence stars. I've also made a table with some data on Jupiter and Earth. These are planets who are the most likely to support life. As you can see most planets have small orbital values, and the mass is also quite big. Solid planets most have masses of ~15 Earthmasses, planets with higher mass are mostly of the gaseous type. This means the surface temperature would be way to high to support life, and the planets would all be of the gaseous type. None of them is likely to be solid and none of them is likely to be a candidate for a life sustaining planet.
There have also been planets found orbiting pulsars. A pulsars is a radio source that emits signals in very short, regular bursts; it's a highly magnetic, rotating star of extremely high density and small size that is composed mainly of very tightly neutrons (neutron star, mass no bigger than ~3 solar masses). We expect here more extreme conditions, and a habitable zone is not very likely. The object orbiting these pulsars are most Earth like masses and solid, but there have also Jupiter like masses been found; data can be found at Darwin Project and Extra-solar Planets Catalog.
Objects with mass > 13 Jupiter masses are commonly named Brown Dwarfs. This is a very low mass objects (~0.01-0.08 solar mass) of low temperature and luminosity that never becomes hot enough in its core to ignite thermonuclear reactions. So you can't really call them planets, they are some kind of stars that have failed to become a star. Several of these kind of object have also been found orbiting stars; data can be found at Darwin Project and Extra-solar Planets Catalog.
But why have only these kind of planets been found orbiting main sequence stars? The answer lies in the Doppler technique used to find these planets. These kind of planets are easiest to discern using this observation technique. To discover less massive planets in more high orbit you would need more high-precision Doppler observations, but that isn't conceivable yet. You could also use more precise observation techniques like micro-lensing, but micro-lensing events are more rare and there's only one chance to collect the data.
Let's make some assumptions for the quantities in the formula above to estimate the planets surface temperature.
The stars listed in the table are all of the F and G type. This means the temperatures of the stars ranges from ~5100 to ~7200 degrees Kelvin. Use this for Ts. The mean albedo A for earth is 0.39, that of Jupiter is 0.51. Use a value of the same size here also. The radius Rs of typeV G and F stars is about the same as the radius of the sun, 6.96 .10^5 km (range is about 1.3 Rsun (F0V) to 0.85 Rsun (G9V)). The value for a is given in the table below, the radius of the orbit. 1 AU = 1.496 .10^8km The estimated temperatures of the stars and the calculated temperatures of the planets are listed in table 1. One can see as the orbit becomes bigger the temperature drops. Some of the planets have high eccentricity's, this means that the temperature will vary a lot, because of the smaller and greater distance from the star. Since all planets are probably gaseous, you wouldn't expect life to evolve there.

table 1: Some data of planets around main sequence stars (data from Darwin Project and Extra-solar Planets Catalog) :
As you might notice, the mass is given in Jupiter mass·sin i, where i is the inclination of the planet's orbit. Because the orbital inclination cannot be retreived from the observations, the mass range could be quite big. For the temperature estimate is used: Albedo = 0.5, Rs = Rsun (~ 4.74 .10^-3 AU) then Tp = (0.041/a^1/2).Ts, a in AU

Which primary conditions are necessaryfor life as we know it?

Going to extremes

Life can exist in the strangest inhospitable places on earth. It are mostly bacteria or algae that apperently choose extreme lifeconditions.Especially in the deep-seas live so-called "super-thermophilic" (=extremely-heatloving) micro-organisms, these not only exist, but thrive at temperatures even beyond 150 ° C (Baross & Deming, 1993). They inhabit pressurized enviroments beneath deep-sea hydrothermal vents. At this temperatures you might expect the water to boil, but it doesn't because of the immense pressure. Recently bacteria were discovered which live at an astonishing 169 ° C! It's not only the temperature, lots of organisms at places where no sunlight ever comes use chemicals like hydrogen sulfite as their energy source. The bacteria inturn sustain larger organisms in the ventcommunity. Also well into the Earths crust, the toplayer of the earth, microbes are found. The mounting pressure has little direct effect on them even at several kilometres below groundlevel. It is the increasing temperature that limits the depth of life beneath the surface. In the oceanic crust the temperature rises about 15 ° C per kilometre. So microbial life extends on average about 7 kilometres below the sea floor. For continental crust the microscopic life should reach almost 4 kilometres into the earth, for the surface temperature is approximately 20 ° C and it rises with 25 ° C per kilometre. However the amount of micro-organisms will vary from place to place.
BacteriaFrom biology-courses we know that bacteria have an optimum curve, which means that at certain temperatures they have a peak where they thrive most. At too cold temperatures they go into somesort of stasis, you could compare it with hibernation, and they do not show any characteristics of life. If the temperatures increase they become active again. At too hot temperatures, however, they are damaged too severly that the damage is irretrivable.

Conditions necessary for life

If we want to investigate the possibility of life on other planets we have to figure out what exactly we are looking for. You cannot go marching off looking only for planets that resemble Earth as it is nowadays. We shall have to go back to the formation of the Earth and life's origin, things were very different then...

Origin of life

Earth formed about 4.6 billion years ago (= 4.6 Giga years) as third planet in a series of nine circling around a star, the Sun. The Sun and the planets are made from stellardebris of stars that came to the end of their existance. Everything we see around us is actually recycled stardust.After the formation Earth was a pockmarked planet of roughly uniform composition and had an early atmosphere of mainly hydrogen (H2). Then radioactive heating began to melt the interior and the core was formed. Now this heating had as a consequense that degassing from the planets interior created a second atmospere rich in water (H2O), carbon dioxide (CO2), methane (CH4) and Ammonia (NH3). When the surface had cooled enough intense rains began to fall and created the oceans.
It is generaly believed in science that this "prebiotic soup" as it is called is where life originated. The famous Miller-Urey experiment duplicated the conditions of early Earth in a laboratory (Orgel, 1994). In a self-contained apparatus an "atmosphere" consisting of hydrogen, water, methane and ammonia was created above an "ocean" of water. These gasses were subjected to "lightning" in the formof an electrical discharge. They found that 10% of the carbon (C) in the system had converted into organic compounds and 2% of this carbon went on to make amino-acids, the buildingblocks of our carbon-based life.Although doubt has arisen because recent investigations indicate that Earth's early atmosphere may have contained more gasses than in the experiment, like CO2 (Orgel, 1994)(De Jager, 1995). But it is still the best theory we have.These amino-acids that formed on Earth are very important, they are the buildingblocks of the nucleic-acids RNA and DNA which in their turn carry the genetic information of organisms. Whether RNA arose spontanous or replaced some earlier geneticsystem is not quite clear. But its development was probably the key in the development of life. It very likely led to the synthesis of proteins, the formation of DNA and the emergence of a cell that could be the ancestor of all

current lifeforms as theory implies. (Orgel, 1994).
Nowadays one of the vital needs for survival of complex organisms is the presence of oxigen (O2). Yet if one looks at Miller-Urey's experiment one sees that no free oxigen has been included in the initial mixture of gasses. Free oxigen was little or not present at the creation of life on the Earth. For it is an agressive element, it oxidizes other chemicals; it subtracts hydrogen from existing molecules. Therefore under oxidizing conditions amino-acids do not or very little form.The atmosphere plays a major part in the creation of life and sustaining it

Development of life

What has to be said, is that simple models of the evolution of life and its development do not apply. Webs and chains in the past are so intricate and full of random and chaotic events. All scientists have to make assumptions especially on this matter.
As stated before, Earth formed approximately 4.6 billion years ago. It had a to endure heavy bombardments of cosmicdebris. These heavy bombarments ended about 3.8 billion years ago (Kasting, 1993). We know this because the oldest rocks date from that period. The large impacts before this date melted the Earth's surface so no solid rock could exist. Still the oldest rocks to hold cellular fossils date from approximately 3.5 billion years ago(Gould, 1994). Which means that life on Earth evolved quickly and is really old. Those first fossils were of bacteria. Bacteria represent more or less the simplest forms of life, so the only way to expand was in width and height. Or to put it differently, to expand in diversity and in complexity.
After the bacterial cells the cells belonging to the plant- and animal kingdom began to to evolve around 2 billion years ago. Yet, life remained unicellular for the first five sixths of its history. Some of the multicellular algae evolved a bilion yars ago. But no record can be found of multicellular animal organization for the span of 3 bilion years. Even more surprisingly, all major stages organizing animal life's multicellular design occured in a very small timespan. It began less than 600 milion years ago and lasted until 530 million years ago, but the steps are not gradually, they're discontinous. Though it actually took only five million years of intense creativity to develop, called the Cambran explosion,followed by 500 million years of variation.It is not known how nature came up with these anatomical designs so quickly. But this first period of both internal and external flexability gave a range of invertabrate anatomies that may have outnumbered the full range of animal form in all Earth's enviroments today.
The question is why did most of these early experiments die out, while others survived. It's more by luck than by a predictable struggle for existance that those organisms we know survived. For mass extinction mark the boundaries of divisions of geologic-timescale. It is thought that these extinctions were mainly caused by impacts of large extraterrestrial objects which smashed into the Earth (the last of these, about 65 million years ago, is thought to have wiped out the dinosaures). Mass extinctions are not randomly distributed in their inpact on life. Some descendants die and others survive as a practical outcome on presence or absence of evolved characteristics. But if the triggering cause of the extinction is sudden catastrophic, reasons for death or life may lie very close to eachother, they may be random. (Gould, 1994).

What do we call life?

A reasonable biological definition of life: Living systems are capable of: metabolism, growth, reaction to stimuli, reproduction, mutation and reproduction of its mutations.

Necessity of water

It seems that for life on Earth water was and is of the utmost important to us. Earth is clearly distinct from other (terrestrial) planets by its wetness.We will now look at the right distance from a star to allow a planet to have liquid water.We have a planet orbiting at a star. This star has a radius Rs and a temperature Ts. The planet has an albedo (=reflectance) A of its surface. Then its equilibrium temperature will be Tp. The distance between the star and the planet we call a.
Tp = { (1-A) / sqrt2}power 1/4 • (Rs / a)power 1/2 • Ts (1) (J. Schneider, 1995).
If we fill in the values known for Earth we find that A=0.39, Tsun =5770 K, so Tp=280 K (which is very close to the actual 287 K)
So from equation (1) we have a planet having a temperature of approximately 300 ± 20 K to allow for liquid water must be located at a distance from the star given by
a = Rs ( Ts / 300)² (2)
where the albedo is A=1. This distance depends on the type of centralstar. So it ranges from approximately 0.1 Au (1Au is the distance between the Sun and the Earth) for cool stars with Ts = 3000 K to about 2 Au for hot stars with Ts = 6500 K. in the next section we will see that there is more to be dealth with in calculating the habitable distance .
However some assumptions were made in forwarding the two equations we just saw. The one of main interest to us, is that it has we assume to have a solid planet, which would exclude giant gaseous planets like Jupiter. But is this assumption correct? What do we know about Jupiter anyway?
Moreover in 1995 an anouncement was made that large ammount a gassious alcohol had been discovered around a star in its initial formationfase. The temperature of the gas was 125 K, very warm for conditions in interstellarspace. The alcohol and other complicated molecules had probably been formed on dustparticles, when the star got larger and heated the dustparticles, the precipitated gasses evaporated.In meteorites, some even older than the solarsystem itself, complicated organic molecules were found aswell. (De Jager, 1995). It seems that no prebioticsoup was needed to create those. But wether life would be able to originate in anyotherway, without the aid of water remains a mystery.

What's the difference between a star and a planet?

IntroductionIf you want to examine what conditions are necessary for a planet to support live, an important question will be: When do you call an object a planet? Or what is the difference between a star and a planet? In this article I will try to explain the exact difference between the two and I will look at the atmosphere, the mass and the temperature, to examine if live is possible on these objects. Star formationA star forms when a very big cloud of gas contracts under the influence of its own gravitational force. As this contraction takes place the object emits energy. This energy is called fall energy. As a result of this contraction the core gets denser and hotter. When the core reaches a temperature of about 3 million Kelvin, it starts to emit light, because of nuclear fusion ( Krane, K.S) reactions in the core. At this stage the gas cloud will stop its contraction because now the gravitational force is in equilibrium with the pressure build up by the hot gas. When this starts to happen you can say that a new star is born. Planet formationA planet on the other hand is build up out of the dust that surrounds a star. When a star is formed there is still a disk of gas surrounding it. As this gas cools, it condenses and forms solid grains. These grain particles accrete into large bodies called planetesimals, which then collide and accrete to make protoplanets. These protoplanets evolve into planets like the planets in our own solar system. So the formation of a star is totally different from that of a planet. This is the main difference between a star and a planet. If an object has a mass of 0.084 times the mass of our own sun (85 times the mass of Jupiter) the core reaches a point where it can start the process of nuclear fusion in its core (see fig.1). If the mass is smaller than this, the lowest temperature to support nuclear fusion will never be reached and the object will never shine like a star. But can we call all of these objects planets? No, objects with a mass between 85*Mj (85 times the mass of Jupiter) and 13*Mj can't sustain nuclear fusion of elements like hydrogen (H) and helium (He) but can support the fusion of two protons into deuterium (D), early in their lifetime. These objects are called brown dwarfs. They form the transition between stars and planets.

Brown dwarfs

Brown dwarfs form like stars so you can't call them planets, but they have a mass that is to small to sustain the nuclear fusion process that takes place inside a star, so they aren't really stars either. As such an object contracts under the influence of it's own gravity, it doesn't reach the temperature that is needed to start the nuclear fusion from H-nuclei into He-nuclei. But it does reach a high enough temperature for the fusion of protons into deuterium. Because of this fusion it emits light during the first period of it's lifetime. It also emits light because of the fall energy that is produced as a result of the contraction of the gas, but this forms only a minor contribution to the total emission of light. Because very little or no fusion takes place, the core of the star can't build up enough pressure to prevent the star from further contraction under the influence of the gravitational forces that are working on the gas. The gas in the center of the star gets so dense that it degenerates. Now the star won't contract any further because of the pressure that is build up by the degenerated matter. This pressure is called electron degeneracy pressure (EDP) (Kulkarni, S.R.). The origin of this pressure is explained by quantum mechanics as arising from oscillations of confined electrons. Because the fusion and contraction have stopped, the only light emission that is left is due to the cooling of the atoms of the star. During the rest of it's lifetime the star will get dimmer and dimmer and eventually and up as a cool object emitting light in the infrared. The chemical composition of the atmosphere of a brown dwarf strongly depends on it's temperature. But for an atmosphere similar to that of the first detected brown dwarf (gl229B), chemical equilibrium calculations indicate that the upper layers of it's atmosphere mainly exist out of methane (CH4), ammonia (NH3), water (H2O) hydrogen sulfide (H2S) and phosphine (PH3). However deep in the atmosphere methane is "replaced" by carbon monoxide (CO) and ammonia is "replaced" by nitrogen (N2) (Marley, M.S.).


Planets can be divided into two different groups. The smaller, solid terrestrial planets (like the earth) and the large, liquid Jovian planets (like Jupiter and Saturn). I will concentrate on the jovian planets because the border between brown dwarfs and planets lies in the mass range of these planets. I already explained how planets are formed, but Jupiter and Saturn may have formed in another way. They may have formed like stars. That is they may have formed out of a gas cloud that contracted under the influence of the gravitational force working on it. In this case the only difference between brown dwarfs and jovian planets is the fusion of protons into deuterium in the core of the brown dwarfs. The jovian planets are large gas bulbs with a small massive core, or in the case of bigger planets, the core may exist out of degenerated material. The very thick atmosphere is build up out of several different layers. The principal constituents of the atmospheres of the jovian planets are molecular hydrogen (H2) and helium (He). The outer most layer of the atmosphere (the photosphere), is build up out of a mixture of these gasses. Underneath this layer is a thick layer of liquid hydrogen. Then you get a layer liquid metallic hydrogen and in the center there possibly exists a rocky core. Although most of the atmosphere consists out of hydrogen and helium there are a lot of other molecules in the atmospheres of the jovian planets. As already mentioned, which elements there are, strongly depends on the temperature of the planets. The temperatures of the extra solar planets that have been discovered until now, differ very much from each other, with values ranging from 100-1500 Kelvin. So the chemical composition of the atmospheres are also very different. Figure 3 gives a rough plot of the chemical species that are likely to condense near the photosphere for a given effective temperature. It is most likely that these kinds of planets can't support life because they aren't solid like terrestrial planets, but for more information on this subject you have to visit Saskia's page.


Mercury is the planet closest to the Sun in our Solar System. This small, rocky planet has almost no atmosphere. Mercury has a very elliptical orbit and a huge range in temperature. During the long daytime (which lasts 58.65 Earth days or almost an entire Mercurian year, which is 88 days long), the temperature is hotter than an oven; during the long night (the same length), the temperature is colder than a freezer. Mercury is so close to the Sun that you can only see it near sunrise or sunset.

Craters on the surface of Mercury.
Mercury is a heavily cratered planet; its surface is similar to the surface of our Moon. Cratering on Mercury triggered volcanic eruptions that filled much of the surrounding area. Mercury does have a magnetic field (probably generated by a partly-liquid iron core).


Mercury's thin atmosphere consist of trace amounts of hydrogen and helium. The atmospheric pressure is only about 1 x 10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.
Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no "greenhouse effect" on Mercury. When the sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.

Mercury is about 3,031 miles (4,878 km) in diameter. It is the smallest planet in our solar system (it used to be considered the second-smallest planet, when Pluto was still considered to be a planet). Mercury is a bit over one third of the diameter of the Earth. Mercury is only slightly larger than the Earth's moon

Mercury's mass is about 3.3 x 1023 kg. This is about 1/20th of the mass of the Earth.
The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person would weigh only 38 pounds on Mercury. To calculate your weight on Mercury, just multiply your weight by 0.38 (or go the planetary
Mercury is closest planet to our Sun and the fastest moving planet in our Solar System. Mercury is just over a third as far from the sun as the Earth is; it is 0.387 A.U. from the sun (on average). Mercury's orbit is very eccentric; at aphelion (the point in the orbit farthest from the sun) Mercury is 70 million km from the sun, at perihelion Mercury is 46 million km from the sun.
There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet's orbit. Since Mercury's axis is directly perpendicular to its motion (not tilted), it has no seasons.
If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth!


Mercury has a huge range in temperatures. Its surface ranges in temperature from -270°F to 800°F (-168°C to 427°C). During the very long daytime (88 Earth-days long), the temperatures are very high (the second-highest in the Solar System - only Venus is hotter); during the long night, the thin atmosphere lets the heat dissipate, and the temperature drops quickly.
Mercury has no moons.


Venus is the second planet from the sun in our solar system. It is the hottest planet in our Solar System. This planet is covered with fast-moving sulphuric acid clouds which trap heat from the Sun. Its thick atmosphere is mostly carbon dioxide. Venus has an iron core but only a very weak magnetic field. This is a planet on which a person would asphyxiate in the poisonous atmosphere, be cooked in the extremely high heat, and be crushed by the enormous atmospheric pressure.Venus is also known as the "morning star" or the "evening star" since it is visible and quite bright at either dawn or dusk. It is only visible at dawn or dusk since it is closer to the sun than we are.
Like the moon, Venus' appearance from Earth changes as it orbits around the Sun. It goes from full to gibbous to crescent to new and back.
Venus is about 7,521 miles (12,104 km) in diameter. This is about 95% of the diameter of the Earth. Venus is the closest to Earth in size and mass of any of the other planets

Venus' mass is about 4.87 x 1024 kg. The gravity on Venus is 91% of the gravity on Earth. A 100-pound person would weigh 91 pounds on Venus. The density of Venus is 5,240 kg/m3, slightly less dense than the Earth and the third densest planet in our Solar System (after the Earth and Mercury).

Venus rotates VERY slowly. Each day on Venus takes 243 Earth days. A year on Venus takes 224.7 Earth days. It takes 224.7 Earth days for Venus to orbit the sun once. The same side of Venus always faces Earth when the Earth and Venus are closest together.


Venus is 67,230,000 miles (108,200,000 km) from the sun. Venus has an almost circular orbit. On average, Venus is 0.72 AU, 67,230,000 miles = 108,200,000 km from the sun.
Venus rotates in the opposite direction of the Earth (and the other planets, except possibly Uranus). Looking from the north, Venus rotates clockwise, while the other planets rotate counterclockwise. From Venus, the Sun would seem to rise in the west and set in the east (the opposite of Earth). No one knows why Venus has this unusual rotation
Venus is the hottest planet in our Solar System. Its cloud cover traps the heat of the sun (the greenhouse effect), giving Venus temperatures up to 480°C. The mean temperature on Venus is 726 K (452°C = 870°F).
Venus has no moons
Venera 3 (from the U.S.S.R.) was the first manmade object to reach Venus. This Soviet spacecraft was launched on November 16, 1965. On March 1, 1966 , the spacecraft arrived at Venus and the capsule parachuted down to the planet, but contact was lost just before entry into the atmosphere.
This is the symbol of the planet Venus. Venus was named after the Roman goddess of love.


The Earth is the third planet from the Sun in our Solar System. It is the planet we evolved on and the only planet in our Solar System that is known to support life


The Earth is about 7,926 miles (12,756 km) in diameter. The Earth is the fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and Neptune). Eratosthenes (276-194 BC) was a Greek scholar who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and Alexandria. He properly assumed that the Sun's rays are virtually parallel (since the Sun is so far away). Knowing the distance between the two locations, he calculated the circumference of the Earth to be 250,000 stadia. Exactly how long a stadia is is unknown, so his accuracy is uncertain, but he was very close. He also accurately measured the tilt of the Earth's axis and the distance to the sun and moon.


The Earth has one moon. The diameter of the moon is about one quarter of the diameter of the Earth.
The moon may have once been a part of the Earth; it may have been broken off the Earth during a catastrophic collision of a huge body with the Earth billions of years ago


The Earth's mass is about 5.98 x 1024 kg.

The Earth has an average density of 5520 kg/m3 (water has a density of 1027 kg/m3). Earth is the densest planet in our Solar System.
To escape the Earth's gravitational pull, an object must reach a velocity of 24,840 miles per hour (11,180 m/sec).

Each day on Earth takes 23.93 hours (that is, it takes the Earth 23.93 hours to rotate around its axis once - this is a sidereal day). Each year on Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once).
The Earth's rotation is slowing down very slightly over time, about one second every 10 years.

Orbital Eccentricity

The Earth has an orbit that is close to being circular; its orbital eccentricity is 0.017. (Eccentricity is a measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; an eccentricity between 0 and 1 represents an elliptical orbit.)


The Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four seasons of the year: Summer, Spring, Winter and Autumn. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year. This affects the amount of sunlight each receives. For more information on the seasons, click here.

At the equator, the Earth's surface moves 40,000 kilometers in 24 hours. That is a speed of about 1040 miles/hr (1670 km/hr). This is calculated by dividing the circumference of the Earth at the equator (about 24,900 miles or 40,070 km) by the number of hours in a day (24). As you move toward either pole, this speed decreases to almost zero (since the circumference at the extreme latitudes approaches zero).

The temperature on Earth ranges from between -127°F to 136°F (-88°C to 58°C; 185 K to 311 K). The coldest recorded temperature was on the continent of Antarctica (Vostok in July, 1983). The hottest recorded temperature was on the continent of Africa (Libya in September, 1922).
The greenhouse effect traps heat in our atmosphere. The atmosphere lets some infrared radiation escape into space; some is reflected back to the planet.


The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases.
The atmosphere was formed by planetary degassing, a process in which gases like carbon dioxide, water vapor, sulphur dioxide and nitrogen were released from the interior of the Earth from volcanoes and other processes. Life forms on Earth have modified the composition of the atmosphere since their evolution.

MARS.The Red Planet

Mars, the red planet, is the fourth planet from the sun and the most Earth-like planet in our solar system. It is about half the size of Earth and has a dry, rocky surface and a very thin atmosphereMARS' SURFACEThe surface of Mars is dry, rocky, and mostly covered with iron-rich dust. There are low-lying plains in the northern hemisphere, but the southern hemisphere is dotted with impact craters. The ground is frozen; this permafrost extends for several kilometers. The north and south poles of Mars are covered by ice caps composed of frozen carbon dioxide and water. Scientists have long thought that there is no liquid water on the surface of Mars now, but recent photos from Mars indicate that there might be some liquid water near the surface. The surface of Mars shows much evidence of the effects of ancient waterways upon the landscape; there are ancient, dry rivers and lakes complete with huge inflow and outflow channels. These channels were probably caused by catastrophic flooding that quickly eroded the landscape.
Scientists think that most of the water on Mars is frozen in the land (as permafrost) and frozen in the polar ice caps.
G. Schiaparelli was an Italian astronomer who first mapped Mars (in 1877) and brought attention to the network of "canali" (Italian for canals or channels) on Mars. These "canals" were later found to be dry and not to be canals at all. A Martian impact crater (Crater Schiaparelli, 461 km = 277 mi in diameter) and a hemisphere of Mars have been named after Schiaparelli.

Mars is about 4,222 miles (6790 km) in diameter. This is 53% (a little over half) of the diameter of the Earth.


Crust and Surface: Mars' surface is composed mostly of iron-rich basaltic rock (an igneous rock). Mars has a thin crust, similar to Earth's.
Mantle: Silicate rock, probably hotter than the Earth's mantle at corresponding depths.
Core: The core is probably iron and sulphides and may have a radius of 800-1,500 miles (1,300-2,400 km). More will be known when data from future Mars missions arrives and is analyzed


Mars' mass is about 6.42 x 10^23 kg. This is 1/9th of the mass of the Earth. A 100-pound person on Mars would weigh 38 pounds.


Each day on Mars takes 1.03 Earth days (24.6 hours). A year on Mars takes 687 Earth days; it takes this long for Mars to orbit the sun once.

Mars is 1.524 times farther from than the sun than the Earth is. It averages 141.6 million miles (227.9 million km) from the sun. Its orbit is very elliptical; Mars has the highest orbital eccentricity of any planet in our Solar System except Pluto.


Mars has a very thin atmosphere. It consists of 95% carbon dioxide (CO2), 3% nitrogen, and 1.6% argon (there is no oxygen). The atmospheric pressure is only a fraction of that on Earth (about 1% of Earth's atmospheric pressure at sea level), and it varies greatly throughout the year. There are large stores of frozen carbon dioxide at the north and south poles. During the warm season in each hemisphere, the polar cap partly melts, releasing carbon dioxide. During the cold season in each hemisphere, the polar cap partly freezes, capturing atmospheric carbon dioxide.
The atmospheric pressure varies widely from season to season; the global atmospheric pressure on Mars is 25% different (there is less air, mostly carbon dioxide) during the (northern hemisphere) winter than during the summer. This is mostly due to Mars' highly eccentric orbit; Mars is about 20% closer to the Sun during the winter than during the summer. Because of this, the northern polar cap absorbs more carbon dioxide than the southern polar cap absorbs half a Martian year later.
Occasionally, there are clouds in Mars' atmosphere. Most of these clouds are composed of carbon dioxide ice crystals or, less frequently, of frozen water crystals.
There are a lot of fine dust particles suspended in Mars' atmosphere. These particles (which contain a lot of iron oxide) absorb blue light, so the sky appears to have little blue in it and is pink/yellow to butterscotch in color.

Mars' surface temperature averages -81 °F (-63 °C). The temperature ranges from a high of 68° F(20° C) to a low of -220° F(-140° C). Mars is much colder than the Earth.

Mars has 2 tiny moons, Phobos and Deimos. They were probably asteroids that were pulled into orbit around Mars.


Mariner 4 was the first spacecraft to visit Mars (in 1965). Two Viking spacecraft landed in 1976. Mars Pathfinder landed on Mars on July 4, 1997, broadcasting photos.


Mars has been known since ancient times.


Jupiter is the fifth planet from the Sun and the largest planet within the Solar System.[11] It is a gas giant with mass slightly less than one-thousandth that of the Sun but is two and a half times more massive than all of the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian planets.
The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after the Roman god Jupiter.[12] When viewed from Earth, Jupiter can reach an apparent magnitude of −2.8, making it on average the third-brightest object in the night sky after the Moon and Venus. (Mars can briefly exceed Jupiter's brightness at certain points in its orbit.)
Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. Because of its rapid rotation, Jupiter's shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.
Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed and adjust its trajectory toward Pluto, thereby saving years of travel. Future targets for exploration in the Jovian system include the possible ice-covered liquid ocean on the moon Europa.


Jupiter is one of the four gas giants; that is, it is not primarily composed of solid matter. It is the largest planet in the Solar System, having a diameter of 142,984 km at its equator. Jupiter's density, 1.326 g/cm³, is the second highest of the gas giant planets, but lower than any of the four terrestrial planets.


Jupiter's upper atmosphere is composed of about 88-92% hydrogen and 8-12% helium by percent volume or fraction of gas molecules (see table to the right). Since a helium atom has about four times as much mass as a hydrogen atom, the composition changes when described in terms of the proportion of mass contributed by different atoms. Thus the atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and five percent other elements by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.[13][14] Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.[15]
The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. However, neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun.[16] Helium is also depleted, although only to about 80% of the Sun's helium composition. This depletion may be a result of precipitation of these elements into the interior of the planet.[17] Abundances of heavier inert gases in Jupiter's atmosphere are about two to three times that of the sun.
Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other gas giants Uranus and Neptune have relatively much less hydrogen and helium.[18] However, because of the lack of atmospheric entry probes, high quality abundance numbers of the heavier elements are lacking for the outer planets beyond Jupiter


Jupiter is 2.5 times more massive than all the other planets in our Solar System combined — this is so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). Although this planet dwarfs the Earth (with a diameter 11 times as great) it is considerably less dense. Jupiter's volume is equal to 1,317 Earths, yet is only 318 times as massive.[19][20] A Jupiter mass (MJ) is used to describe masses of other gas giant planets, particularly extrasolar planets.
Theoretical models indicate that if Jupiter had much more mass than it does at present, the planet would shrink. For small changes in mass, the radius would not change appreciably, and above about four Jupiter masses the interior would become so much more compressed under the increased gravitation force that the planet's volume would actually decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition is achieved as in high-mass brown dwarfs around 50 Jupiter masses.[21] This has led some astronomers to term it a "failed star", although it is unclear whether or not the processes involved in the formation of planets like Jupiter are similar to the processes involved in the formation of multiple star systems.
Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter.[22][23] In spite of this, Jupiter still radiates more heat than it receives from the Sun. The amount of heat produced inside the planet is nearly equal to the total solar radiation it receives.[24] This additional heat radiation is generated by the Kelvin-Helmholtz mechanism through adiabatic contraction. This process results in the planet shrinking by about 2 cm each year.[25] When it was first formed, Jupiter was much hotter and was about twice its current diameter.[26]

Internal structure

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen.Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen.[25] Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths (see below). In 1997, the existence of the core was suggested by gravitational measurements.[25] indicating a mass of from 12 to 45 times the Earth's mass or roughly 3%-15% of the total mass of Jupiter.[24][27] The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation involving initial formation of a rocky or icy core that is massive enough to collect its bulk of hydrogen and helium from the protosolar nebula. Assuming it did exist, it may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely.[25][28]
The uncertainty of the models is tied to the error margin in hitherto measured parameters: one of the rotational coefficients (J6) used to describe the planet's gravitational moment, Jupiter's equatorial radius, and its temperature at 1 bar pressure. The JUNO mission, scheduled for launch in 2011, is expected to narrow down the value of these parameters, and thereby make progress on the problem of the core.[29]
The core region is surrounded by dense metallic hydrogen, which extends outward to about 78 percent of the radius of the planet.[24] Rain-like droplets of helium and neon precipitate downward through this layer, depleting the abundance of these elements in the upper atmosphere.[17][30]
Above the layer of metallic hydrogen lies a transparent interior atmosphere of liquid hydrogen and gaseous hydrogen, with the gaseous portion extending downward from the cloud layer to a depth of about 1,000 km.[24] Instead of a clear boundary or surface between these different phases of hydrogen, there is probably a smooth gradation from gas to liquid as one descends.[31][32] This smooth transition happens whenever the temperature is above the critical temperature, which for hydrogen is only 33 K (see hydrogen).
The temperature and pressure inside Jupiter increase steadily toward the core. At the phase transition region where liquid hydrogen (heated beyond its critical point) becomes metallic, it is believed the temperature is 10,000 K and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K and the interior pressure is roughly 3,000–4,500 GPa.[24]

Planetary rings

Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring.[46] These rings appear to be made of dust, rather than ice as is the case for Saturn's rings.[24] The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter because of its strong gravitational pull. The orbit of the material veers towards Jupiter and new material is added by additional impacts.[47] In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the gossamer ring.[47Orbit and rotationJupiter is the only planet that has a center of mass with the Sun that lies outside the volume of the Sun, though by only 7% of the Sun's radius.[50] The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun, or 5.2 AU) and it completes an orbit every 11.86 years. This is two-fifths the orbital period of Saturn, forming a 5:2 orbital resonance between the two largest planets in the Solar System.[51] The elliptical orbit of Jupiter is inclined 1.31° compared to the Earth. Because of an eccentricity of 0.048, the distance from Jupiter and the Sun varies by 75 million km between perihelion and aphelion, or the nearest and most distant points of the planet along the orbital path respectively.
The axial tilt of Jupiter is relatively small: only 3.13°. As a result this planet does not experience significant seasonal changes, in contrast to Earth and Mars for example.[52]
Jupiter's rotation is the fastest of all the Solar System's planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope. This rotation requires a centripetal acceleration at the equator of about 1.67 m/s², compared to the equatorial surface gravity of 24.79 m/s²; thus the net acceleration felt at the equatorial surface is only about 23.12 m/s². The planet is shaped as an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is 9275 km longer than the diameter measured through the poles.[32]
Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation. The rotation of Jupiter's polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere; three systems are used as frames of reference, particularly when graphing the motion of atmospheric features. System I applies from the latitudes 10° N to 10° S; its period is the planet's shortest, at 9h 50m 30.0s. System II applies at all latitudes north and south of these; its period is 9h 55m 40.6s. System III was first defined by radio astronomers, and corresponds to the rotation of the planet's magnetosphere; its period is Jupiter's official rotation.[53]


Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian, meaning "Jupiter-like", planets.
Saturn is named after the Roman god Saturn (that became the namesake of Saturday), equated to the Greek Kronos (the Titan father of Zeus) the Babylonian Ninurta and to the Hindu Shani. Saturn's symbol represents the god's sickle (Unicode: ♄).
The planet Saturn is composed of hydrogen, with small proportions of helium and trace elements.[11] The interior consists of a small core of rock and ice, surrounded by a thick layer of metallic hydrogen and a gaseous outer layer. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter. Saturn has a planetary magnetic field intermediate in strength between that of Earth and the more powerful field around Jupiter.
Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-one known moons orbit the planet, not counting hundreds of "moonlets" within the rings. Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.

Physical characteristics
Due to a combination of its lower density, rapid rotation, and fluid state, Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10%—60,268 km vs. 54,364 km.[4] The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm³ due to the gaseous atmosphere. Saturn is only 95 Earth masses,compared to Jupiter, which is 318 times the mass of the Earth[13] but only about 20% larger than Saturn

Internal structure
Though there is no direct information about Saturn's internal structure, it is thought that its interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer of liquid hydrogen and helium, and in the outermost 1000 km a gaseous atmosphere.[15] Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth.[16] Saturn has a very hot interior, reaching 11,700 °C at the core, and it radiates 2.5 times more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.[17]

The outer atmosphere of Saturn consists of about 96.3% molecular hydrogen and 3.25% helium.[18] Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected.[19] The upper clouds on Saturn are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH4SH) or water.[20] The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.
The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.[21]

Orbit and rotation

The average distance between Saturn and the Sun is over 1 400 000 000 km (9 AU). With an average orbital speed of 9.69 km/s,[4] it takes Saturn 10 759 Earth days (or about 29½ years), to finish one revolution around the Sun.[4] The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth.[4] Because of an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155 000 000 km between perihelion and aphelion,[4] which are the nearest and most distant points of the planet along its orbital path, respectively.
The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as in Jupiter's case): System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet in the period of the Voyager flybys, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.
However, a precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased appreciably, to approximately 10 h 45 m 45 s (± 36 s).The cause of the change is unknown—it was thought to be due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than because of a change in Saturn's rotation.
Later, in March 2007, it was found that the rotation of the radio emissions did not trace the rotation of the planet, but rather is produced by convection of the plasma disc, which is dependent also on other factors besides the planet's rotation. It was reported that the variance in measured rotation periods may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and "weighs down" Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet itself. At the time it was stated that there is no currently known method of determining the rotation rate of Saturn's core.
The latest estimate of Saturn's rotation based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes was reported in September 2007 is 10 hours, 32 minutes, 35 seconds.


Uranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός) the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit.[14] Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. This was also the first discovery of a planet made using a telescope.
Uranus is similar in composition to Neptune, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus's atmosphere, while similar to Jupiter's and Saturn's in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with traces of hydrocarbons.[10] It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.[10] In contrast the interior of Uranus is mainly composed of ices and rock.[9]
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, its north and south poles lie where most other planets have their equators.[15] Seen from Earth, Uranus's rings can sometimes appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appeared edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants.[15] However, terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).[16]


Uranus had been observed on many occasions before its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed the planet at least six times, cataloging it as 34 Tauri. The French astronomer, Pierre Lemonnier, observed Uranus at least twelve times between 1750 and 1769,[17] including on four consecutive nights.
Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy),[18] but initially reported it (on 26 April 1781) as a "comet".[19] Herschel "engaged in a series of observations on the parallax of the fixed stars",[20] using a telescope of his own design.
He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".[21] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".[22] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:[23]
“ The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed. ”
Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".[24]
While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell estimated its distance as 18 times the distance of the Sun from the Earth, and no comet had yet been observed with a perihelion of even four times the Earth–Sun distance.[25] Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".[26] Bode concluded that its near-circular orbit was more like a planet than a comet.[27]
The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System."[28] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so that the Royal Family could have a chance to look through his telescopes.[29]

Maskelyne asked Herschel to "do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, & which we are so much obliged to you for the discovery of."[30] In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.[31] He explained this decision in a letter to Joseph Banks:[28]


The pronunciation of the name Uranus preferred among astronomers is with stress on the first syllable as in Latin Ūranus;[37] in contrast to the colloquial / with stress on the second syllable and a long a, though both are considered acceptable. Because, in the English-speaking world, ū·rā′·nəs sounds like "your anus", the former pronunciation also saves embarrassment: as Dr. Pamela Gay, an astronomer at Southern Illinois University, noted on her podcast, so as avoid "being made fun of by any small schoolchildren ... when in doubt, don't emphasise anything and just say ūr′·ə·nəs. And then run, quickly.
Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. The adjective of Uranus is "Uranian". Its astronomical symbol is . It is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars.[40] Its astrological symbol is , suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star

Orbit and rotation
Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400 that on Earth.[43] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.[25] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus's orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.
The rotational period of the interior of Uranus is 17 hours, 14 minutes
However, as on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. In effect, at some latitudes, such as about two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.

Internal structure
Uranus's mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets, while its density of 1.27 g/cm³ makes it the second least dense planet, after Saturn. Though having a diameter slightly larger than Neptune's (roughly four times Earth's), it is less massive. These values indicate that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus's interior is not precisely known, as different figures emerge depending on the model chosen; however, it must be between 9.3 and 13.5 Earth masses. Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses. The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by rocky material
The standard model of Uranus's structure is that it consists of three layers: a rocky core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20 percent Uranus's; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20 percent of Uranus's radius. Uranus's core density is around 9 g/cm³, with a pressure in the center of 8 million bars (800 GPa) and a temperature of about 5000 K.The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants.
While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow science to determine which model is correct. The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.[9] However, for the sake of convenience, a revolving oblate spheroid set at the point at which atmospheric pressure equals 1 bar (100 kPa) is conditionally designated as a "surface". It has equatorial and polar radii of 25 559 ± 4 and 24 973 ± 20 km, respectively. This surface will be used throughout this article as a zero point for altitudes.

Internal heat

Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux. Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun.[16] Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere.In fact, Uranus's heat flux is only 0.042 ± 0.047 W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m².[58] The lowest temperature recorded in Uranus's tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.
Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature.[59] Another hypothesis is that some form of barrier exists in Uranus's upper layers which prevents the core's heat from reaching the surface. For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.
Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope that is accessible to remote sensing is called its atmosphere.Remote sensing capability extends down to roughly 300 km below the 1 bar (100 kPa) level, with a corresponding pressure around 100 bar (10 MPa) and temperature of 320 K.[60] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.[61] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; (10 MPa to 10 kPa), the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10–10 bar (10 kPa to 10 µPa), and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface. There is no mesosphere.
The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of molecular hydrogen and helium.[10] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03[12] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.[10][58] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,[62] indicating that helium has not settled in the center of the planet as it has in the gas giants.[10] The third most abundant constituent of the Uranian atmosphere is methane (CH4).[10] Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.[10] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.The mixing ratio[e] is much lower in the upper atmosphere owing to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.[64] The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. However they are probably also higher than solar values.In addition to methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[66] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), diacetylene (C2HC2H).[64][67][68] Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets


Neptune is the eighth planet from the sun in our solar system. This giant, frigid planet has a hazy atmosphere and strong winds. This gas giant is orbited by eight moons and narrow, faint rings arranged in clumps. Neptune's blue color is caused by the methane (CH4) in its atmosphere; this molecule absorbs red light. Neptune cannot be seen using the eyes alone. Neptune was the first planet whose existence was predicted mathematically (the planet Uranus's orbit was perturbed by an unknown object which turned our to be another gas giant, Neptune).


Neptune is about 30,775 miles (49,528 km) in diameter. This is 3.88 times the diameter of the Earth. If Neptune were hollow, it could hold almost 60 Earths.
Neptune is the fourth largest planet in our Solar System (after Jupiter, Saturn, and Uranus).

Neptune's mass is about 1.02 x 1026 kg. This is over 17 times the mass of the Earth, but the gravity on Neptune is only 1.19 times of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared).
A 100-pound person would weigh 119 pounds on Neptune.


Each day on Neptune takes 19.1 Earth hours. A year on Neptune takes 164.8 Earth years; it takes almost 165 Earth years for Neptune to orbit the sun once.
Since Neptune was discovered in 1846, it has not yet completed a single revolution around the sun.


Neptune is about 30 times farther from the sun than the Earth is; it averages 30.06 A.U. from the sun. Occasionally, Neptune's orbit is actually outside that of Pluto; this is because of Pluto's highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.At aphelion (the point in Neptune's orbit farthest from the sun) Neptune is 4,546,000,000 km from the sun, at perihelion (the point in Neptune's orbit closest from the sun) Neptune is 4,456,000,000 km from the sun.
Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun (this is few degrees more than the Earth). This gives Neptune seasons. Each season lasts 40 years; the poles are in constant darkness or sunlight for 40 years at a time.

The mean temperature is 48 K

Neptune's existence was predicted in 1846, after calculations showed perturbations in the orbit of Uranus. The calculations were done independently by both J.C. Adams and Le Verrier. Neptune was then observed by J.G. Galle and d'Arrest on September 23, 1846