PLANETS form out of a `protoplanetary' discs of dust and gas that exist around a new-born star. Such embryonic discs have been seen around young stars, both in infrared and in visible light. The system of orbits of the planets in the solar system is actually the trace of the skeleton of such a discs that encircled the new-born sun. Collision of dust particles in the disc causes material to agglomerate and form solid bodies or `planetisimals' that continue to accrete debris.
In the case of our solar system, this led to eight major bodies. The ninth planet, Pluto, is believed to be a survivor of an early subclass of bodies in the solar system called icy dwarfs. The more massive planets are found farther from the sun, though not so far where material did not have time to aggregate. The size of the solar system or the distance of the farthest planet is probably determined by the distance at which the orbital period of the disc matter was sufficient for collisions to occur and agglomeration to take place and not too slow that collisions were rare.
Unlike asteroids, which are cold chunks of debris from the solar system, a planet is massive enough to have had at least once a molten core that differentiated the planet's interior. This is a process in which heavier elements sank to the centre and lighter elements floated to the surface. Accordingly, planets would have dense rocky/metallic cores and, depending on how far they formed from their parent star, they may retain a dense mantle of primordial hydrogen and helium. In the solar system this mechanism establishes two families of planets: the inner-rocky or terrestrial planets such as the earth and Mars, which have solid surfaces, and the outer gas-giant planets Jupiter and Saturn.
Massive planets are still contracting gravitationally and shine in infrared light.
On the basis of the above simple theory of planet formation, one expects giant planets to form outside the so-called `snow line' where there was a larger density of rock/ice to allow rapid planet formation. Since planets are believed to have formed out of discs with circular orbits, their orbits should also be close to circular orbits. But the discovery of many `hot Jupiters' (circling close to the parent star) as well as frequently non-circular orbits is at variance with the expected picture.
The most popular explanation for the existence of `hot Jupiters' is `orbital migration', according to which giant planets form at large radii within the protoplanetary disc (several A.Us.), lose energy and angular momentum and migrate to present orbits closer to the star. Mechanisms that could lead to orbital migration include `gravitational scattering' by other planets or planetesimals in an initially unstable planetary system, and gravitational interactions of a single planet with the protoplanetary disc soon after planet formation.
