Abstract
The outer solar system extends beyond a heliocentric distance of 5 AU. It contains the giant planets and their systems (rings and satellites), the Kuiper belt, the comets (except those which approach episodically the inner solar system) and, at its outer edge, the Oort cloud. The outer solar system physically corresponds to the region located outside the « snow line » which corresponded to the distance of ice condensation in the protodolar disk, and thus made the frontier between the terrestrial and the giant planets at the time of the planets’ formation. The outer solar system is charaterized by a very large variety of ob jects, even within a given class of ob jects. Each of the giant planet has its own properties, as well as each of the outer satellites and the ring systems ; all are the products of specific conditions which determined their formation and evolution processes. The existence of the Kuiper belt, suspected on theoretical bases since the 1940s, has been confirmed since 1992 with the observation of over 1200 trans-neptunian ob jects. Thanks to the the developments of more and more performing groundbased instrumentation and the use of large telescopes, these ob jects are now studies in a statistical way, both dynamically and physically, and these studies are precious for constraining the early formation models of the solar system.
Highlights
A simple look at the orbital and physical characteristics of the eight solar-system planets shows that all these objects fall in two main categories (Table 1)
Since the 1990s, we know that Pluto is one of the biggest representatives of a new population of outer solar-system objects, called the trans-neptunian objects and located in the Kuiper belt
All giant planets are characterized by a tropospheric region, where the convection dominates with a thermal gradient close to the adiabatic value, which extends from deep levels up to the tropopause, at about 0.1 bar (Fig. 4)
Summary
A simple look at the orbital and physical characteristics of the eight solar-system planets shows that all these objects fall in two main categories (Table 1). At low heliocentric distance (Rh < 2 AU), the terrestrial planets (Mercury, Venus, the Earth and Mars) have relatively small sizes, low densities, and a small number of satellites ; their atmosphere (apart from Mercury which has none) is only a negligible fraction of their total mass. At Rh > 5 AU, the giant planets (Jupiter, Saturn, Uranus and Neptune) have larges masses, low densities, a ring system and a large number of satellites ; their atmosphere is, in all cases, a significant fraction of their total mass. Since the 1990s, we know that Pluto is one of the biggest representatives of a new population of outer solar-system objects, called the trans-neptunian objects and located in the Kuiper belt. We will review the different classes of outer solar-system objects : giant planets, rings and satellites, comets and Kuiper-Belt objects, with special emphasis on the atmospheres of planets, outer satellites and comets
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