“It is very likely that the planet lost everything at the very beginning,” says JPL’s Raissa Estrela, a study co-author. “But the transit observations show spectral features which means there definitely is an atmosphere.” And those features, she adds, suggest that the gases are rich in hydrogen and low in oxygen content — which suggests they might be explained by volcanic outgassing.
Volcanic origins
Detecting a secondary atmosphere derived from volcanic activity would be a first for exoplanet studies.
Before GJ 1132 b, all the exoplanet atmospheres we’ve ever seen are thought to have formed the same way: during the initial formation of the local system, protoplanets grow by accreting material from the disk of gas around their host stars, and their atmospheres are derived from a leftover envelope of gas.
Since their modelling ruled out the possibility of this primordial atmosphere surviving on GJ 1132 b, Swain and his colleagues turned to a 2019 paper that proposes a stage in the process where a nascent planet accreting a hydrogen atmosphere can absorb that hydrogen into its molten mantle. This reservoir of hydrogen, the team proposes, could be released later through volcanic activity.
There is some evidence for this cycle happening on the primordial Earth, the team says, which had a very different air composition than it does today. “There are some rocks that have been dug up from the Earth’s mantle that show a very low oxygen content,” says Swain. Many geologists think that these rocks formed when Earth had a primordial, hydrogen-rich atmosphere — which eventually went deep into the ground.
The team modelled this possibility for GJ 1132 b and found that if this hydrogen-rich magma released its gas above ground, it could produce what they observed in the atmosphere. This includes features like the planet’s unusually high levels of hydrogen cyanide, which makes up about 0.5 percent of the planet’s total atmosphere.
The study is the first to connect atmospheric observations to formation theories of a planet’s mantle, which also has wider implications for studying exoplanet formation. One line of thought holds that many super-Earths are actually the leftover cores of sub-Neptunes — a class of gaseous planet whose growth stalls out before it can reach Neptune size — that have lost their primordial envelope of gas. This study raises the possibility that these worlds may still host atmospheres that astronomers can study, even when those planets are very close to their host stars.
“What our results provide is observational evidence that, at least in some cases, this class of planet can reestablish an atmosphere,” says Swain.
A promising follow-up target
Leslie Rogers, an astrophysicist at the University of Chicago who was not involved in this study, doesn’t think the evidence for a secondary atmosphere is definitive. “I think they could have better quantified the statistical significance of their results,” she says.