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 several hundred planets, many like Jupiter and Saturn but some much smaller, down to masses just a few times that of the Earth. The smaller planets may well have a rocky composition like Earth’s.

Few extrasolar planets travel in the nearly circular orbits characteristic of Earth and its neighbors. Armitage and his group are investigating mechanisms that produce extra solar planetary systems different from our own. For instance, they are looking into 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.

Some gas planets are much closer to their star than are similar planets in our solar system. They can wind up in blistering proximity to their Sun-like parents, orbiting them in 1.2 to 8 days. Such orbits are well inside the cavities created by magnetic fields that typically separate such stars from their planet-forming accretion disks. Because of incredibly high temperatures and the complete lack of planet-building materials in these cavities, there is no way these planets could have formed so close to their stars. In studies of these "roaster" planets, Armitage discovered that many planets migrate in towards their star. However, many of these, especially the largest ones, fly all the way into the stars. In contrast, lower-mass planets should pile up in close-in stable orbits. Of course, smaller extrasolar planets are much harder to detect, so it may be a while before this prediction can be verified with observations.

Another goal of Armitage’s research is to use both simulations and observations made with the Spitzer Space Telescope and the ALMA radio telescope to develop an empirical understanding of protoplanetary disks, including their structure, composition, and evolution over time. For instance, the Armitage group 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, the central 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. In related work, the Armitage group studies the physics of planet formation. The goal is to understand not only how terrestrial and gas planets form, but also how different environments affect planet formation.

As astrophysicists learn more about extrasolar planetary systems, Armitage has begun to explore how the solar system fits in with its counterparts around other stars. It is not yet known whether many features of the solar system are common throughout the galaxy or, in fact, relatively rare. Whether rare or commonplace, did such features as our large moon play a key role in the evolution of life on Earth? Also, was the near-circular nature of our solar system’s planets a prerequisite to Earth’s being habitable? Or, was the location of Jupiter relative to the Earth critical for Earth’s surface consisting of 70% water? Armitage is investigating the complex ways in which different parts of the solar system have influenced each other over the eons.

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