It’s great to narrow down what we need to be looking for as we search for Earth 2.0, but there’s a teeny, tiny problem. We have this gap in our planetary catalog between exoplanets about the size of Earth and planets 2-4 times the size, what we call mini-Neptunes. So if we’re looking for something less than six times the size of Earth, we might be scrounging for targets.

But two new papers published in The Astronomical Journal may have solved that planetary gap problem. Well, not solved so much as found out why it exists in the first place, which may not help in the search for Earth 2.0, and I really need to stop calling it that, but is still useful in helping to understand this whole planetary formation process that keeps confounding us.

The new studies involve mini-Neptune planets in two systems – TOI 560 and HD 63433. These systems are between 70 and 100 light-years away or so. An analysis of the atmospheric gas around the plants shows that the gas is, well, escaping from them. That could mean that these mini-Neptunes are becoming super-Earths.

Do you know what’s awesome? We’ve suspected this was the case for a few years now, and it’s great to get confirmation. As lead author and graduate student Michael Zhang notes: Most astronomers suspected that young, small mini-Neptunes must have evaporating atmospheres. But nobody had ever caught one in the process of doing so until now.

Except one of these planets is losing its gas toward its star, which was not expected.

There is a great possible explanation for this whole puffy planet to rocky world process in the press release: Mini-Neptunes are theorized to be cocooned by primordial atmospheres made of hydrogen and helium. The hydrogen and helium are left over from the formation of the central star, which is born out of clouds of gas. If a mini-Neptune is small enough and close enough to its star, stellar X-rays and ultraviolet radiation can strip away its primordial atmosphere over a period of hundreds of millions of years, scientists theorize. This would then leave behind a rocky super-Earth with a substantially smaller radius, which could, in theory, still retain a relatively thin atmosphere similar to that surrounding our own planet.

That’s not to say that there aren’t other possible explanations for the planetary gap, but these new discoveries strengthen the evidence for the transition hypothesis.

More Information

Caltech press release

Hubble press release

Keck Observatory press release

NASA Goddard press release

“Escaping Helium from TOI 560.01, a Young Mini-Neptune,” Michael Zhang et al., 2022 January 17, The Astronomical Journal

“Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune,” Michael Zhang et al., 2022 January 17, The Astronomical Journal