Super-Earths, meet superpuffs, a lighter weight class of planet

Superpuff
SOO-per-puf n.

A gassy planet, close to its star, with a puzzlingly small mass.

Not to be confused with a generic breakfast cereal, superpuffs are fluffy planets snuggled up to their suns. A superpuff develops its inflated persona by forming far from its star and then wandering inward, astronomers Eve Lee and Eugene Chiang of the University of California, Berkeley suggest online November 1 at arXiv.org. Starting out in the outskirts of a fledgling planetary system, the researchers say, would mean more cold gas is available to stick to a rocky core.

Superpuffs are quite large for their mass. They typically weigh a few times as much as Earth but stretch up to 10 times as wide. That’s about as large as Jupiter but roughly one one-hundredth of its mass. These puffy worlds are the opposite of the rocky heavyweights known as super-Earths (SN: 4/18/15, p. 17), which have surprisingly little gas and live a bit farther out than where superpuffs end up. Unlike superpuffs, super-Earths probably stay where they form, Lee and Chiang suggest, but begin to form later, after much of the gas swirling around their young sun has dissipated.