Mini-Neptunes And Super-Earths: How Puffy Planets Could Be The Missing Link

When astronomers look at planets outside our solar system, called exoplanets, often they have only minimal information they can use to classify them. Two key factors they often do know are the planets' size and their mass, from which they can determine their density. This helps to categorize them into groups similar to the planets we know, such as rocky planets like Earth or Mars, or gas giants like Jupiter or Saturn.

Adam Makarenko (Keck Observatory)

But there are also other types of exoplanets which are unlike those found in our solar system. Two examples are super-Earths, which are planets that are rocky but are much larger than Earth, and mini-Neptunes or gas dwarfs, which are smaller than Neptune but have gaseous atmospheres over a rocky core.

Recently, new research using the Hubble Space Telescope and the W. M. Keck Observatory has shown how these different types of planet may evolve. Astronomers have spotted two mini-Neptune planets which seem to be transforming into super-Earths as they lose their atmospheres (published in The Astronomical Journal).

The two planets are located in different systems, one orbiting star HD 63433, located 73 light-years away, and the other orbiting star TOI 560, located 103 light-years away. Astronomers observed gas escaping from both of the mini-Neptunes, which they believe is evidence they are losing their atmospheres. This could be putting them on the road to become super-Earths.

"Most astronomers suspected that young, mini-Neptunes must have evaporating atmospheres," said Caltech graduate student Michael Zhang, lead author of the research (via NASA). "But nobody had ever caught one in the process of doing so until now."

This could help to explain an astronomical oddity. Most exoplanets discovered have fallen into distinct size categories, with super-Earths being typically up to 1.6 times the size of Earth and mini-Neptunes being between 2 and 4 times the size of Earth. But there are very few planets discovered in between these two sizes. This could be because mini-Neptunes are turning into super-Earths, and the differences in atmosphere explains the differences in sizes.

"A planet in the size-gap would have enough atmosphere to puff up its radius, making it intercept more stellar radiation and thereby enabling fast mass loss," said Zhang. "But the atmosphere is thin enough that it gets lost quickly. This is why a planet wouldn't stay in the gap for long."

There are more oddities to note in one of these systems. In the TOI 560 system, the gasses being given off by the planet were not flowing away from the star, as would be expected, but were actually flowing toward the star, and the researchers don't know why that is.

"As exoplanet scientists, we've learned to expect the unexpected," co-author Heather Knutson of Caltech said. "These exotic worlds are constantly surprising us with new physics that goes beyond what we observe in our solar system."