A radical new theory that planets are born within a massive veil of gas may help explain how recently discovered extrasolar planets developed their stunning diversity of sizes and locations.
In the theory, planets are born under wraps, hidden at the centers of giant gas clouds far from their parent stars. A star’s gravity then reels in the planetary cloud, stripping away some or all of the gas to reveal the planet inside.
Depending on how much of the gas is removed in the process, the unveiled planet would resemble a gas giant like Jupiter, a solid core with a layer of gas like Neptune or a solid body like Earth. Sergei Nayakshin of the University of Leicester in England describes the process in an upcoming Monthly Notices of the Royal Astronomical Society as well as in several papers posted online at arXiv.org.
Such a beginning might explain the abundance of small-to-middling extrasolar planets — including many Neptune-sized planets — recently spotted by NASA’s Kepler spacecraft orbiting within roasting distance of their stars (SN: 2/26/11, p. 18). Standard planet-formation models are facing unprecedented challenges because they can’t easily account for the many types of extrasolar planets described since 1995, notes theorist Aaron Boley of the University of Florida in Gainesville. “At the end of the day, we need to explain this diversity of planetary systems,” he says.
Nayakshin’s theory, along with a similar one by Boley and his collaborators, borrows ideas from two more traditional models. In a scenario known as core accretion, bits of solid particles coalesce within the disk of gas and dust surrounding a young star and form a solid core that resembles Mercury or Earth. The core may then snare enough gas to form a Jupiter. In the other model, known as gravitational instability, gas within the planet-forming disk suddenly fragments to form a giant blob, forming a Jupiter in one fell swoop.
In contrast, Nayakshin begins with the giant blob of gas generated by gravitational instability. Then, he suggests, dust settles to the core of the blob, ultimately forming a solid body that snares some of the gas around it, as in the core accretion model. The gassy envelope around the core is initially fluffy and easily removed. As the clump migrates inward toward its star and reaches a distance similar to that separating Mars from the sun, some of the gas may be stripped away by the star’s gravitational tidal forces. Removing the outer layers of gas produces rocky planets similar to those in the inner solar system.
In other cases, when migration is slower, the fluffy gas envelope has enough time to contract and become denser, resisting stripping. In that case, only if the blob moves very close to the star, within half the average separation of Mercury from the sun, can the star’s tidal forces remove part or all of the dense gas envelope. Such a downsized planet sits within roasting distance of its star and could be a hot version of Jupiter, Neptune or Earth, as recently observed by Kepler and other telescopes searching for extrasolar planets.
Kepler has shown that Neptune-sized exoplanets are very common at distances closer to their parent stars than Earth is (and even at half the Earth-sun distance), rather than being confined to the chilly outer parts of a planetary system as previously thought, says Boley. “Determining whether tidal downsizing can explain a sizable fraction of these planets … is a direction worth exploring,” he says.
Jack Lissauer of the NASA Ames Research Center in Mountain View, Calif., however, says that he considers “the hypothesis to be quite weak” and “the idea of forming planetary cores in this manner is far from demonstrated.” In addition, he says, the theory would have trouble explaining some types of extrasolar planets that have been observed, such as a gas-rich one with a relatively small rocky core that closely orbits its parent star.
Alan Boss, who developed the gravitational instability model and is based at the Carnegie Institution for Science in Washington, D.C., takes a longer view. “I am all for having a thousand flowers bloom in the world of exoplanet formation theory — at this time it is hard to pick the eventual winners and losers.”
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In a new theory of planet formation, planets develop inside the cores of giant gas clumps. Showing the birth of a super-Earth (a planet several times the mass of Earth), the beginning of this simulation shows gas clumps (yellow) developing at different distances from the parent star. As the clumps move inward, dust settles at their cores and forms solid objects (black). As clumps are pulled toward the parent star, the gassy envelope is stripped off and a solid planet is unveiled. Credit: Seung-Hoon Cha, S. Nayakshin/http://arxiv.org/abs/1010.1489