Planets

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.