Centauri Dreams' Paul Gilster considers, in light of the recent discovery of rocky and close-orbiting exoplanet Alpha Centauri Bb, new models of planetary formation. Our solar system, with its well-spaced rocky planets orbiting close to the sun and gas and ice giants orbiting more distantly, really is an anomaly. One pair of astronomers' explanation for this anomaly has interesting implications for potential Earth-like worlds, at Alpha Centauri and elsewhere.
Maybe the reason exoplanets so often surprise us is that we base our thinking on our own Solar System, and the minimum-mass Solar nebula from which it grew, considering this a template. The rest of the galaxy may have other ideas. Consider that close-in super-Earths are common. Planets like these, showing up in abundance in Kepler data and Doppler velocity surveys, are a challenge to explain. Laughlin and Chiang say that more than half, if not nearly all Sun-like stars have planets with radii between 2 and 5 times that of Earth and orbital periods of less than 100 days.
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The problem, then, is that our Solar System has no planets inside Mercury’s 88-day orbit. Is it possible our Solar System did not undergo the same kind of formation history that may be the dominant mode in the galaxy? To explore this, the researchers look at migration issues, for it is commonly thought that short-period planets formed several AU out from their stars and then migrated to their present location. But disk migration is poorly understood, and while it may be necessary to explain hot Jupiters, Laughlin and Chiang say it may not be the mechanism to explain the majority of planetary systems with super-Earths in inner orbits.
The alternative: Forget orbital migration and consider the possibility that super-Earths form right where they are, in circumstellar disks that extend inward from 0.5 AU.
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Hot young stars should lack close-in super-Earths because they would be too hot for planetesimal-building dust to survive. Brown dwarfs and M dwarfs should have close-in super-Earths and Earths orbiting them. Close-in planets grown from close circumstellar disks should also have orbital planes aligned with the equatorial planes of their host stars. I’ll send you to the paper to go through the entire list of predictions, all of which should allow these ideas to be probed, but I do want to mention one last prediction having a bearing on Centauri B b. For if Laughlin and Chiang are right, then binary systems offer a good test.
After all, close binary systems should make planetary migration extremely difficult. The close companion would disrupt planet formation at large distances from the star. Planets orbiting Centauri B inside the 0.5 AU boundary would be incompatible with migration, and of course, we now have such a planet, along with the likelihood of finding more. Here I drop back to the Science News article, which quotes Laughlin on Centauri B: “I think that the odds that there’s an interesting planet, a truly interesting planet in the system, are very high, given that this one is there.” And if he’s right, that interesting, potentially habitable world may serve as further evidence for the theory that such planets formed right where they are found.
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