Astronomers have identified two different cases of “mini-Neptune” planets that are losing their puffy atmospheres and likely transforming into super-Earths.
Radiation from the planets’ stars is stripping away their atmospheres, driving hot gas to escape like steam from a pot of boiling water.
“Most astronomers suspected that young, small mini-Neptunes must have evaporating atmospheres,” said Michael Zhang, lead author of both studies and a graduate student at Caltech. “But nobody had ever caught one in the process of doing so until now.”
The findings were published in two separate papers in The Astronomical Journal: one is based on data from W. M. Keck Observatory on Maunakea, and the other paper involves observations from NASA’s Hubble Space Telescope. Together, the studies help paint a picture of how exotic worlds like these form and evolve.
Mini-Neptunes are a class of exoplanets, which are planets that orbit stars outside our solar system. These worlds, which are smaller, denser versions of the planet Neptune, consist of large rocky cores surrounded by thick blankets of gas.
In the new studies, a team of astronomers led by Caltech used Keck Observatory’s Near-Infrared Spectrograph to study one of two mini-Neptune planets in the star system called TOI 560, located 103 light-years away; and they used Hubble to look at two mini-Neptunes orbiting HD 63433, located 73 light-years away.
Their results show that atmospheric gas is escaping from the innermost mini-Neptune in TOI 560, called TOI 560.01, and from the outermost mini-Neptune in HD 63433, called HD 63433 c.
Furthermore, Keck Observatory data showed the gas around TOI 560.01 was escaping predominantly toward the star.
“This was unexpected, as most models predict that the gas should flow away from the star,” said Professor of Planetary Science Heather Knutson, Zhang’s advisor and a co-author of the study. “We still have a lot to learn about how these outflows work in practice.”
Since the first exoplanets orbiting Sun-like stars were discovered in the mid-1990s, thousands of others have been found. Many of these orbit close to their stars, and the smaller, rocky ones generally fall into two groups: mini-Neptunes and super-Earths. The super-Earths are as large as 1.6 times the size of Earth, while the mini-Neptunes are between two and four times the size of Earth. Few planets with sizes between these two planet types have been detected.
One possible explanation for this gap is that mini-Neptunes are transforming into super-Earths. The 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.
The astronomers were able to detect the escaping atmospheres by watching the mini-Neptunes cross in front of, or transit, their host stars. The planets cannot be seen directly but when they pass in front of their stars as seen from our point of view on Earth, telescopes can look for absorption of starlight by atoms in the planets’ atmospheres. In the case of the mini-Neptune TOI 560.01, the researchers found signatures of helium. For the star system HD 63433, the team found signatures of hydrogen in the outermost planet they studied, called HD 63433 c, but not the inner planet, HD 63433 b.
