Why is there nothing in between mini-Neptunes and super-Earths?

At the end of January it was thirty years ago that the first exoplanets (planets outside our solar system) were discovered. Since that discovery, thousands of (candidate) exoplanets have been found, in various shapes and sizes. Most of them are close to their stars (which is where we can most easily spot them with current detection methods). And if we zoom in on the smallest, rocky specimens, we quickly see something striking. They can generally be divided into two groups: the mini-Neptunes (smaller versions of ‘our’ Neptune: a large rocky core surrounded by a thick blanket of gases) and super-Earths. The super-Earths are up to 1.75 times the size of our planet, while the mini-Neptunes are between 2 and 4 times larger. And what is striking is that to date hardly any planets have been discovered that are in between in terms of size. One possible explanation for that mysterious hole in our exoplanet catalog was that mini-Neptunes gradually transform into super-Earths. But evidence for this was lacking. Until now. Because scientists have discovered two mini-Neptunes that are losing their thick atmospheres and thus appear to be turning into super-Earths.

Observations

“Most astronomers suspected that young, tiny mini-Neptunes had evaporating atmospheres,” said study researcher Michael Zhang. But observational evidence was lacking. Until astronomers – including Zhang – recently used the Near Infrared Spectrograph from the Keck Observatory and the Hubble Space Telescope took a closer look at the mini-Neptunes TOI 560.01 and HD 63433 c, respectively. The astronomers noticed that both atmospheres were ‘leaking’: atmospheric gas flowed away from the planets. In the case of TOI 560.01, it was helium rushing away at up to 20 kilometers per second. In HD 63433 c, it was hydrogen that disappeared at a speed of up to 50 kilometers per second.

super earth

The observations suggest that both mini-Neptunes are in danger of losing their entire atmosphere. And what’s left is a rocky core that, based on its size, would be classed as one of the super-Earths.

Radiation

The fact that the mini-Neptunes are losing their atmosphere is due to the intense radiation from the nearby parent star. Astronomers suspect that radiation could cause a mini-Neptune to lose most of its atmosphere within a few hundred million years. The remaining rocky core should theoretically still be able to hold a small portion of that atmosphere, but the remaining atmosphere would not be much thicker than ours.

Hole

The research may help explain why very few planets have been discovered to date that are somewhere between super-Earths and mini-Neptunes in size. “A planet in that hole would have an atmosphere thick enough to intercept a lot of stellar radiation and thus be doomed to lose mass quickly,” Zhang said. At the same time, however, the atmosphere is also too thin to be able to cope with this mass loss for a long time. “The atmosphere is so thin that it disappears relatively quickly. And that’s why a planet can’t stay in that ‘hole’ for long.”

With the observations, the researchers have found for the first time evidence for the theory that mini-Neptunes transform relatively quickly into super-Earths and that we therefore find few planets in between in size in terms of size. However, more observations are needed to definitively confirm this theory. Moreover, the new study also raises new questions. For example, the escaping atmosphere of mini-Neptunes was believed to move away from the parent star. But observations from TOI 560.01 indicate that the gases leaking from the atmosphere actually flow towards the parent star. Future observations of other mini-Neptunes should reveal whether TOI 560.01 is an exception or not. “As exoplanet researchers, we’ve learned to expect the unexpected,” said researcher Heather Knutson. “These exotic worlds are constantly surprising us with new physical processes that go beyond what we see here in our own solar system.”