Astronomers have identified two mini-Neptune exoplanets — TOI-560b and HD 63433c — that are losing their puffy atmospheres and likely transforming into super-Earths. The results appear in two papers in the February 1, 2022 edition of the Astronomical Journal.
Mini-Neptunes are smaller, denser versions of Solar System’s Neptune.
These worlds are thought to consist of large rocky cores surrounded by thick atmospheres.
“Most astronomers suspected that young, mini-Neptunes must have evaporating atmospheres. But nobody had ever caught one in the process of doing so until now,” said Dr. Michael Zhang, graduate student at Caltech.
Using the NASA/ESA Hubble Space Telescope, Zhang and colleagues looked at two mini-Neptunes orbiting HD 63433, a G5-type star located 73 light-years away in the constellation of Gemini.
They also used the Near-Infrared Echelle Spectrograph (NIRSPEC) on the Keck II Telescope at the W.M. Keck Observatory in Hawaii to study one of two mini-Neptunes around TOI-560, a K4-type star some 103 light-years away in the constellation of Hydra.
The results show that atmospheric gas is escaping from the innermost mini-Neptune in the TOI-560 system, called TOI-560b (also known as TOI-560.01 and HD 73583b), and from the outermost mini-Neptune in the HD 63433 system, called HD 63433c.
This suggests that TOI-560b and HD 63433c could be turning into super-Earths.
The astronomers also found that the gas around TOI-560b was escaping predominantly toward the parent star.
“This was unexpected, as most models predict that the gas should flow away from the star,” said Caltech Professor Heather Knutson.
“We still have a lot to learn about how these outflows work in practice.”
The astronomers were able to detect the escaping atmospheres by watching the mini-Neptunes cross in front of, or transit, their stars.
In the case of TOI-560b, they found signatures of helium. For HD 63433, they found signatures of hydrogen in HD 63433c, but not the inner planet, HD 63433b.
“The inner planet may have already lost its atmosphere,” Zhang said.
“The speed of the gases provides the evidence that the atmospheres are escaping.”
“The observed helium around TOI-560b is moving as fast as 20 km/sec, while the hydrogen around HD 63433c is moving as fast as 50 km/sec.”
“The gravity of these mini-Neptunes is not strong enough to hold on to such fast-moving gas.”
“The extent of the outflows around the planets also indicates escaping atmospheres; the cocoon of gas around TOI-560b is at least 3.5 times as large as the radius of the planet, and the cocoon around HD 63433c is at least 12 times the radius of the planet.”
The observations also revealed that the gas lost from TOI-560b was flowing toward the star.
Future observations of other mini-Neptunes should reveal if this exoplanet is an anomaly or whether an inward-moving atmospheric outflow is more common.
“As exoplanet scientists, we’ve learned to expect the unexpected,” Professor Knutson said.
“These exotic worlds are constantly surprising us with new physics that goes beyond what we observe in our Solar System.”
Michael Zhang et al. 2022. Escaping Helium from TOI 560.01, a Young Mini-Neptune. AJ 163, 67; doi: 10.3847/1538-3881/ac3fa7
Michael Zhang et al. 2022. Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune. AJ 163, 68; doi: 10.3847/1538-3881/ac3f3b
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