NASA's Kepler telescope has now identified an insane 2,326 possible exoplanets. What's particularly shocking is how many of these planets are giant rocky planets known as Super-Earths, which defies everything we thought we knew about planet formation.
Of all the recent discovered candidate planets, anywhere from a third to half of them are super-Earths, which are rocky planets larger than Earth but smaller than Neptune. What's more, many of them are found in extremely close proximity to their stars. Now, it may be that super-Earths aren't actually quite as common as these observations suggest - after all, it's easier to spot planets close to a Sun, and it's easier to see larger rocky planets than smaller ones.
The problem, however, is that the existence of any super-Earths so near their stars doesn't fit with our understanding of planet formation, and to find so many seemingly impossible planets represents a glaring hole in our astrophysical knowledge. Both these planets' existence and their position don't fit with our standard planet formation models, and this isn't a matter of simply tweaking the existing models to accommodate the new observations - this requires a much deeper reevaluation of the physics involved.
Part of the problem is that astrophysicists developed models of planet formation when the only example we had was that of our own solar system, in which there's a very clear delineation between large gas planets and small rocky ones, and even Mercury is located at a fairly safe distance from the Sun. This led to theories of "core accretion", which held that the dust and rock in embryonic solar systems were distributed such that only small planets could form near the sun, with larger rocky planets possible further out and gas giants only possible at much greater distances from their star.
But the discovery, starting in 1995, of gas giants even bigger than Jupiter in close proximity to their stars threw a massive wrench in the core accretion model. Astronomers patched this gap by saying that these so-called Hot Jupiters could form far from their star and then migrate much closer in. But they still assumed that any planet larger than Earth would either become a gas giant or be swallowed by its star. And that's where Kepler is creating massive trouble for astronomers.
Writing for Nature, Eric Hand details the latest findings from Kepler, including a new hint on just what makes up these super-Earths:
For a handful of the super-Earths detected by Kepler, ground-based observations have also determined mass, by tracking the wobble of the host star induced by the planet's gravity. And some of these super-Earths seem to have very low densities - indicating that they may have small rocky cores surrounded by large gas envelopes.
Kepler astronomer Jack Lissauer, of Ames, thinks that they may have begun as small cores in the outer parts of their solar system, accreting a large amount of gas without reaching the point of runaway growth that leads to a true gas giant. Without the gravitational heft of a giant to hold in gas, such a planet would have a large, low-density atmosphere, but it could still grow to super-Earth size by a cooling process that shrinks the atmosphere and allows more gas to be drawn in, he says.
But there's still a couple major problems with our current planet formation models, as Hand explains. First, these super-Earths aren't just low density, semi-gassy worlds. Some are actually very massive and still larger than Earth, and we've got no real way to explain their formation. What's more, the super-Earths found orbiting very close around their suns - at distances far smaller than the orbit of Mercury - have no business existing in our current models, which say they should either be further away or swallowed up by their star.
One possibility being explored by Norm Murray of the University of Toronto is that it's actually the raw materials that migrate inwards from the outer solar system, and then the super-Earth actually forms in a very close but stable orbit around its star. It might resolve the present conundrum, but it seems likely that Kepler has plenty more surprises up its sleeves - and it's anyone's guess whether our current grasp of astrophysics will be enough to explain them.
Via Nature. Artist's conception by Lynette Cook.