Nature calls them, poetically enough, the skeletons of “wandering ice giants”:
Among the most puzzling finds of NASA’s Kepler space mission to find exoplanets, which launched in 2009, are bodies too heavy for their size. In some of the rare cases in which astronomers can estimate both the mass and the size of distant planets discovered by the probe, the objects have radiuses similar to that of Earth but are denser than pure iron.
[Olivier Grasset, a geophysicist at the University of Nantes in France] and his collaborators now say that the strange bodies could be the “fossil cores” of planets that were once much larger, an idea that was first proposed by researchers in 2011. These planets would have been ice giants that formed in the outer parts of a star system and then migrated inwards — as their orbits were affected by interactions with surrounding gas and dust — perhaps getting as close to their suns as Mercury is to ours.
The team found that if the outer layers of an ice giant are removed over billions of years, the materials would ‘relax’, expanding back to more ordinary densities. But if the stripping occurred over a geologically short time, the sudden cooling would keep the core locked into its dense state essentially forever. “If the process is short, you end up with a very compressed super-Earth,” says Grasset.
Like memories. Heavy ghosts.