These mini-Neptune planets are transforming into super-Earths

Observations revealed that atmospheric gas is escaping from TOI 560.01, the innermost mini-Neptune in TOI 560 as well as HD 63433c, the outermost mini-Neptune in HD 63433, suggesting that they could be turning into super-Earths.

Astronomers have identified two different cases of Mini-Neptunes, smaller and denser versions of Neptune, that are losing their puffy atmospheres of mostly hydrogen and helium, likely transforming into super-Earths - rocky planets several times Earth's mass. The discovery represents the first direct evidence to support the theory that mini-Neptunes are indeed turning into super-Earths.

Using NASA's Hubble Space Telescope, the astronomers studied two mini-Neptunes orbiting HD 63433, a star located 73 light-years away. The team used the W. M. Keck Observatory in Hawaii to study one of two mini-Neptune planets in the star system called TOI 560, located 103 light-years away.

Observations revealed that atmospheric gas is escaping from TOI 560.01, the innermost mini-Neptune in TOI 560 as well as HD 63433c, the outermost mini-Neptune in HD 63433, suggesting that they could be turning into super-Earths.

The study also revealed that the gas lost from TOI 560.01, which is about 2.8 times the size of Earth, was flowing toward the star. Astronomers believe that the planet could lose the vast majority of its atmosphere after several hundred million years, leaving behind a super-Earth.

By observing these planets cross in front of, or transit, their host stars, the researchers were able to detect the escaping atmospheres. The team found signatures of helium in TOI 560.01 and hydrogen in HD 63433c. However, there were no signatures of hydrogen in HD 63433's inner planet, HD 63433b.

"The speed of the gases provides the evidence that the atmospheres are escaping. The observed helium around TOI 560.01 is moving as fast as 20 kilometres per second, while the hydrogen around HD 63433c is moving as fast as 50 kilometres per second. The gravity of these mini-Neptunes is not strong enough to hold on to such fast-moving gas," explained Michael Zhang, lead author of both studies and a graduate student at Caltech.

The findings of the studies are being published in two separate papers in The Astronomical Journal.

“Mini-Neptune” ➡️ “Super-Earth”Astronomers have identified cases of exoplanets (planets orbiting stars beyond our Sun) losing their puffy atmospheres of mostly hydrogen & helium, which might transform them into large, rocky planets called super-Earths: https://t.co/hrxVx9CSFq pic.twitter.com/env4A549dW

— Hubble (@NASAHubble) February 3, 2022