All stars visible to the naked eye owe their momentary brightness to nuclear reactions occurring in their interior. While this certainly makes them jewels of the night skies, it will eventually lead them to a tragic end, in which they will explode to become either degenerate white dwarfs, neutron starsor black holes. Another, more numerous but barely visible population has chosen to lead a dull but quiet and almost eternal life: these are careful not to ever become dependent on hydrogen to shine. Some, in their youth, do burn less energetic substances as deuterium and lithium, but they rapidly get short of supply. As a consequence, they steadily cool and contract, retaining intact most of theelements that made them. These brown dwarfs and giant planets form an entirely new class of astronomical objects. They ﬁll a gap between stars and the planets of our Solar System. Their study informs us on our origins, the formation of stars and planets. It can also help us to understand or test theories from high pressure physics, to atmospheric dynamics, tides, condensation and cloudformation...etc. The course focuses on some physical aspects related to the theoretical study of these substellar objects: I detail their hydrostatic evolution and how it is modeled, what we can learn from Jupiter, Saturn, Uranus and Neptune, how the atmospheres of brown dwarfs and giant planets are key to their appearance and cooling, what we can learn from the recent observations of brown dwarfs andextrasolar planets, and how this aﬀects our view of planet formation.
2 “Our” Giant Planets as a Basis for the Study of Substellar Objects
2.1 Origins: Role of the Giant Planets for Planet Formation The Solar System contains our Sun, which possesses more than 98% of the mass of the system, and eight planets orbiting around it in the same plane and
Guillot T (2006), Physics of substellar objectsInteriors, atmospheres, evolution. In: Mayor M, Queloz D, Udry S and Benz W (eds) Extrasolar planets. Saas-Fee Adv Courses vol 31, pp 243–368 c Springer-Verlag Berlin Heidelberg 2006 DOI 10.1007/3-540-29216-0 2
same direction with quasi-circular orbits. The planets contain 99.5% of the angular momentum of the system. The four inner planets, Mercury, Venus, Earth and Mars havethe highest densities, but more than 99.5% of the mass of the planetary system is in its four outer planets, Jupiter, Saturn, Uranus and Neptune. Most of the planets have moons, or natural satellites. Orbiting around the Sun, one also ﬁnds asteroids, Kuiper belt objects (including Pluto) and comets. A picture emerges naturally from these observations: the formation of the planets in acircumstellar disk: the protosolar nebula. Planets formed close to the Sun naturally contain less volatiles and ices, while the outer planets were favored by the abundant presence of ices and could therefore grow fast enough to get hold of the surrounding hydrogen and helium of the nebula before its dissipation. In this picture, asteroids, Kuiper belt objects and comets all represent leftovers from anineﬃcient planet formation mechanism. By their masses, the giant planets Jupiter, Saturn, Uranus and Neptune played a key role in this story. While the inner, terrestrial planets took tens of millions of years to reach their present masses, the giant planets had to form rapidly, before the gas of the protosolar nebula disappeared onto the star or was swept away from the system. They led to the ejection ofnumerous material, preventing the formation of a planet between Mars and Jupiter, and sending planetesimals into the Oort cloud, from where these remains of planetary formation come back once in a while as comets. Their study therefore informs us on our origins. It also allows us to extend our knowledge beyond the frontiers of the Solar System and to model with conﬁdence the other giant...