Gaseous mini-Neptunes may become rocky super-Earths when they lose their atmospheres. Now, astronomers have caught that process in action.

a grey orb floats away from a bright orb with apurple dust surrounding them against a black background
An artist's animation shows the mini-Neptune TOI 560.01, located 103 light-years away in Hydra. Astronomers have discovered that the mostly rocky planet is losing its puffed-up atmosphere — and that the gas is unexpectedly flowing toward the star.
W. M. Keck Observatory / Adam Makarenko

Astronomers appear to have solved a long-standing mystery surrounding the universe’s most common planets. Using both the Keck Observatory and the Hubble Space Telescope, a team led by Caltech researchers caught two planets called mini-Neptunes seemingly in the process of transforming into super-Earths.

In our own solar system, the planets can be neatly divided into small, rocky worlds or gigantic, gaseous ones. A real surprise from our haul of exoplanet discoveries so far is that as much as 80% of planets have sizes that fall somewhere in between.

In turn, these middling worlds can be split into two types: rocky super-Earths between 1 and 1.75 Earth radii and mini-Neptunes between 2 and 4 Earth radii. Our best theories of planet formation have so far struggled to account for their existence. Even more puzzlingly, astronomers have also been vexed as to why there are very few planets between 1.75 and 2 Earth radii — something called the Fulton gap.

Now, at last, we could have the answers: Super-Earths and mini-Neptunes may be the same planet, just in different stages of their lives. Initially, the planet gathers up a lot of gas from the disk of material in which it’s formed. Then, over time, the intense ultraviolet and X-ray radiation from their host star strips that atmosphere away to expose the rocky world below.

“Most astronomers suspected that young, small mini-Neptunes must have evaporating atmospheres,” says lead researcher Michael Zhang (Caltech). “Nobody had ever caught one in the process of doing so until now.”

The team took spectra of the planets in question, TOI 560.01 (103 light-years away) and HD 63433c (73 light-years away), as they transited in front of their host stars. Zhang and his team spotted gas escaping from both worlds, measuring helium rushing away from TOI 560.01 at 20 kilometres per second (45,000 mph) and hydrogen from HD 63443c moving at 50 kilometres per second. The results are published in two papers in the Astronomical Journal.

It’s not an open and shut case, because it might not apply to all mini-Neptunes and super-Earths. Perhaps some super-Earths form straight away and never gather up enough gas to puff up in the first place. Nevertheless, Björn Benneke (University of Montreal), who wasn’t involved in this study, says that these atmosphere-shedding mini-Neptunes are “a significant discovery”.

Such a rapid flow of gas might explain why we see very few planets in the Fulton gap. “A planet in the gap would have enough atmosphere to puff up its radius, making it intercept more stellar radiation and thereby enabling fast mass loss,” Zhang says. “But the atmosphere is thin enough that it gets lost quickly . . . a planet wouldn’t stay in the gap for long.”

a grey orb emerges from a purple dust cloud against a black background
In this artistic animation, the mini-Neptune TOI 560.01 is shown transforming into a super-Earth. The planet is about 2.8 times the size of Earth and has a puffy atmosphere, made up of mostly hydrogen and helium.
W. M. Keck Observatory / Adam Makarenko

Given that these medium-sized worlds make up such a huge fraction of the exoplanet census, Benneke says that “understanding their formation and evolution history is absolutely critical to developing a fundamental understanding of planets overall.” He goes on to add that these results “represents a critical first step towards observationally probing the mass loss process that likely drives the transition from mini-Neptune to super-Earth.”

These tentative initial observations are already throwing up surprises. The helium from TOI 560.01 seems to be escaping toward its star. “Most models predict that the gas should flow away from the star,” says team member Heather Knutson. “We still have a lot to learn about how these outflows work.”

As they get to know our exoplanetary neighbours better, astronomers continue to be astounded. “These exotic worlds are constantly surprising us with new physics that goes beyond what we observe in our solar system,” says Knutson. Given how common these exoplanets are, perhaps it is us who are the real oddballs after all.



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