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

Underweight, oversized planets formed in outskirts of star systems

BIG PERSONALITY  Superpuffs (one illustrated) are extra fluffy planets that formed far from their star and subsequently wandered in closer, a new study suggests.

NASA, ESA, G. BACON/STSCI

Magazine issue: Vol. 188, No. 12, December 12, 2015,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.