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.
