The Earth, Mercury, Venus, and Mars are known as the rocky planets because they are composed predominantly of rocks which contain silicon, iron, and other heavy elements. Jupiter and Saturn, the gas giants, are mostly hydrogen, the lightest and most abundant element. Finally the ice giant planets, Uranus and Neptune, make up the third category of planetary types. They are made of compounds like water, methane, ammonia that form ices when cold, and which are less abundant than hydrogen but more common than rocky materials. All planets in the universe, even ones around other stars, ought to fall roughly into these three solar-system categories that are determined for the most part by the relative
abundances and properties of the chemical elements themselves. Astronomers, however, have only a partial understanding of how planets form and develop in the first place, and why they follow the orbits we see them in today. Might an earth-like rocky planet be someday found in the outer regions of a stellar system where in our solar system only the ice giants reign?
SAO astronomer and Hubble Fellow Eric Ford and his colleague have been studying how and where the ice giants formed in the gas and debris disk that surrounded the young Sun. The simple, gradual coagulation of small grains in the outer solar disk takes way too long - about one hundred times longer than the age of the solar system. Furthermore, since these giants also contain significant quantities of hydrogen, they must have formed before the early disk lost most of its hydrogen, estimated by astronomers to take only a few tens of millions of years. Two solutions to the problem of forming ice giants have been suggested in the past: either they form much closer to the Sun where densities are higher, and then migrate out to the distant realms they now occupy, or they acquire most of their mass by accreting from a massive disk of much smaller planetesimals that facilitates the rapid growth of such ice planets. Each idea has problems,
though: in particular, if ice giants form quickly and efficiently, there is a high probability of forming many more than just the two of them we see - Uranus and Neptune.
Ford and his colleague write in the latest issue of the Astrophysical Journal that their simulations provide new insight into these puzzles. Their results show that Uranus and Neptune probably formed about twice as close to the Sun as they are now, and did so in the likely presence of planetesimals that facilitated the formation process. They are then pushed outward into their current orbits by the gravitational effects of Jupiter, Saturn, and the disk of planetesimals, a process that simultaneously ejects many other of the ice giants from the solar system altogether. For a certain range of disk parameters, the scientists find that the simulations often end up with only two ice giants in about the right location. The new results not only show possible paths of formation for Uranus and Neptune, they also help us better understand the environment of the young Earth.