Exoplanets come in various sizes, from gas giants larger than Jupiter to small rocky planets the size of Earth or Mars. Recently, astronomers using NASA’s Hubble Space Telescope have identified two different cases of “mini-Neptune” planets.

These two mini-Neptune planets – TOI 560.01 (also known as HD 73583b) and HD 63433c – meet their bloated atmospheres and are likely to become super-Earths. Both planets are located in the star system called TOI 560, located 103 light-years away.

They used the W. M. Keck Observatory in Hawaii to study one of the star system’s two mini-Neptune planets. They found that planets are losing their atmosphere due to radiation from stars. Radiation from its star causes a hot gas to rise like steam from a pot of boiling water. This suggests that they could be becoming super-Earths.

Michael Zhang, lead author of both studies and a graduate student at Caltech, said: “Most astronomers suspected that young mini-Neptunes must have evaporating atmospheres. But no one had detected one in the process of doing so until now.”

Caltech professor of planetary science Heather Knutson, an adviser to Zhang and co-author of the study, said: “This was unexpected, as most models predict that gas should flow away from the star. We still have a lot to learn about how these output streams work in practice”.

Astronomers were able to detect the leaking atmospheres by watching mini-Neptunes cross in front of, or transit through, their host stars. Planets can’t be seen directly, but when they pass in front of their stars, as seen from our vantage point on Earth, telescopes can look for absorption of starlight by atoms in the planets’ atmospheres.

In the case of mini-Neptune TOI 560.01, scientists found helium signatures. In the case of the star system HD 63433, they found hydrogen signatures on the outermost planet they studied, called HD 63433c, but not on the inner planet, HD 63433b.

According to Zhang, “the inner planet may have already lost its atmosphere.”

“The velocity of the gases provides evidence that the atmospheres are escaping. The helium observed around TOI 560.01 is moving up to 20 kilometers per second, while the hydrogen around HD 63433c is moving up to 50 kilometers per second. The gravity of these mini-Neptunes is not strong enough to retain such fast-moving gas The extent of outflows around the planets also indicates the existence of escape atmospheres, the gas cocoon around TOI 560.01 is al least 3.5 times the radius of the planet, and the cocoon around HD 63433c is at least 12 times the radius of the planet.”

Sources, credits and references:

Michael Zhang et al, Escaping Helium from TOI 560.01, a Young Mini-Neptune, The Astronomical Journal (2022). DOI: 10.3847/1538-3881/ac3fa7

Michael Zhang et al, Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune, The Astronomical Journal (2022). DOI: 10.3847/1538-3881/ac3f3b