Migrating planet

Phil Armitage studies the formation and migration of planets around stars outside our solar system. During star formation, about 10% of the available mass doesn't end up in the star; this material becomes the building blocks of new planets. In searches for planets circling more than a thousand nearby stars, astronomers have discovered more than a hundred massive gas planets like Jupiter and Saturn. (Small, rocky planets like Earth are too small to be detected, even with the Hubble Space Telescope.) What's intriguing is that most of these gas planets are much closer to their star than planets are in our solar system. In addition, few of them travel in the nearly circular orbits characteristic of Earth and its neighbors. Armitage and his group recently investigated the possibility that large planets form at a similar distance from their star as Jupiter, and then migrate either toward or away from it. They have studied two mechanisms that could cause such a migration: (1) the interaction of a large planet with the protoplanetary disk in which it was formed and (2) the interaction between two or more massive planets.

The researchers have discovered that planets around stars with high metal concentrations typically form early in the lifetime of the protoplanetary disk, coalesce quickly, and frequently migrate. When planets form early, gravitational interactions between the new planet and the still-dense accretion disk can pull a planet toward, or even into, its star. In contrast, when planets form late, there isn't enough mass left in the disk to carry the planet toward the star. Thus, planets like Jupiter must have formed near the end of the lifetime of the Sun's protoplanetary disk. As a result of this research, Armitage believes that the circular planetary orbits that characterize our solar system are uncommon, perhaps found in only about 10% of stellar planetary systems. He also wonders whether astronomers might someday discover a massive gas planet in orbit around our Sun out beyond Pluto.