So-called ‘super-Earths,’ planets larger than the Earth but smaller than Neptune, pose problems to our theories of planet formation. The most recent illustration of this came in the announcement that the candidate planets found by Kepler had now reached 2,326. Remember, many of these will not be confirmed — they’re candidates — but taken in the aggregate, what is interesting here is that one-third to one-half of these candidates fit the super-Earth category. And just as we had a problem with ‘hot Jupiters’ in trying to figure out their orbital position, many of these new planets are likewise in orbits close to their parent star, where the models say they shouldn’t be.
Things seemed so much simpler when we just had a single solar system to worry about, our own. Then, the idea of core accretion could readily account for everything we saw. The dust in the protoplanetary disk was thought to have aggregated into small planetesimals which, in the course of time and numerous collisions, bulked up into planets. It made sense that planets in the inner system would be smaller because the inner part of the disk was thought to have less material for growth, whereas an outer planet would become larger, growing into a gas giant massive enough to pull in a thick atmosphere from the surrounding disk.
As this article by Eric Hand in Nature News points out, the basic model was challenged by finding gas giants in tight orbits, forcing the development of a migration model in which these huge worlds formed out beyond the ‘snow line’ and later made their way into the inner system. Now we have to account for the latest super-Earth findings, as the article points out:
…these models predicted that anything reaching super-Earth size should either become a gas giant or be swallowed by its star, creating a ‘planetary desert’ in this size range. Kepler’s discoveries wreck those predictions. “It’s a tropical rainforest, not a desert,” says Andrew Howard, an astronomer at the University of California, Berkeley. “We hope the theory is going to catch up.”
Hand goes on to talk to Jack Lissauer (NASA Ames), who suggests that some super-Earths could have begun as smaller cores in the outer system that simply never reached the kind of runaway growth that would lead to a Jupiter-class world. Lissauer thinks a planet like this could grow to super-Earth size, and his ideas could explain some of the super-Earths we’ve found with low densities, implying a small rocky core surrounded by a large gas envelope. Even so, we’re still not able to explain denser super-Earths of the kind that are now beginning to be detected.
Kepler’s gradual revelation of smaller and smaller worlds keeps pointing us in the direction of new planet formation models like Norm Murray’s. The astrophysicist (University of Toronto) has tweaked the migration model to have the process of planet formation occur after planetesimals have migrated inward on their own, with the subsequent accretion occurring near the host star. Who knows whether this theory will stand up to the next wave of Kepler results, but if there is one thing we’ve learned from the exoplanet hunt so far, it’s that surprises are abundant, and too doctrinaire a view at this stage is simply asking for revision down the road.