Super-dense alien planet could be entirely new type

Among the most mysterious finds of NASA’s Kepler space mission to find exoplanets are bodies too heavy for their size. In some cases, planets the size of Earth are denser than pure iron, according to a report in the journal Nature.

No standard theories about planet formation could explain such dense bodies. “There is no way to explain that in the Solar System,” says Olivier Grasset, a geophysicist at the University of Nantes in France.

But scientists believe these planets could be the "fossil cores" of ice giants similar to Neptune that veered too close to their suns, according to research presented this week at a meeting on exoplanets at the Royal Society in London. These cores would have formed under the intense pressure of their outer layers -- 5 million times the atmospheric pressure on Earth - and temperatures up to 6,000 kelvin.

Grasset and colleagues Antoine Mocquet, a planetary scientist also at Nantes, and Christophe Sotin, a planetary geologist at NASA’s Jet Propulsion Laboratory, created a computer simulation to see what happens when these outer layers are stripped away by increased heat closer to their suns.

The team found that if the outer layers of an ice giant are removed over billions of years, the core materials would "relax," expanding to ordinary densities. But if the stripping occurred over a geologically short time, the sudden cooling would keep the core super dense. “If the process is short, you end up with a very compressed super-Earth,” says Grasset.

William Borucki, a scientist at NASA's Ames Research Center and leader of the Kepler mission, says the idea is plausible, though there could be other ways for the outer layers of an ice giant to be ripped away. The process could be the result of a collision with another large object, or the high-density cores could even point to processes similar to star formation. Whatever the implications, he says, Kepler's findings are exciting. “This is why we do science.”