Oversized planet or undersized star?

“These new spin measurements suggest that if these bodies are massive planets located far away from their stars, they have properties that are very similar to those of the smallest brown dwarfs,” said Heather Knutson, professor of planetary science at Caltech and a co-author of the study, in a press release.

The researchers put forth two possible explanations for why the planetary-mass companions were spinning at the same rate as their brown-dwarf counterparts.

The first possibility is that the companions are, in fact, brown dwarfs. Brown dwarfs form just like regular stars do, out of collapsing clouds of gas and dust. However, unlike stars, brown dwarfs do not have enough mass to kick-start nuclear fusion within their cores, so they never shine with the same blinding intensity as regular stars.

The second possibility is that the companions were formed in the same way as regular planets, out of the swirling disks of material that surround young stars. But if this is the case, then why do the planets have spin rates nearly identical to those of the brown dwarfs?

“It’s a question of nature versus nurture,” said Knutson. “Were the planetary companions born like brown dwarfs, or did they just end up behaving like them with similar spins?”

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