Super-Earths give theorists a super headache

Nature : Out of the 2326 exoplanets identified so far by NASA’s Kepler space telescope, between one-third and one-half of them are in the emerging and perhaps most numerous “super-Earth” category. Bigger than Earth but smaller than Neptune, they contradict conventional models of planet formation. Early planet-formation models used our own solar system as an example and were based on the idea of core accretion. Dust in a star’s protoplanetary disk aggregates into small cores of rock and ice. Whereas the inner part of the disk doesn’t contain enough material for the cores to grow much larger than Earth, cores farther out can form planets 10 times as massive as Earth. Those outer planets attract large volumes of gas to become Jupiter-like gas giants. After the detection of Jupiter-sized exoplanets with orbits as short as a few Earth days, the model was revised to allow those planets to move closer to their star after forming farther out. But that modification still doesn’t account for super-Earth-sized planets that have not become gas giants or gotten swallowed up by their star. No current theory can explain how super-Earths can orbit so close to their stars.