‘Mini-Neptunes’ evolving into ‘super-Earths’ spotted – studies – Reuters

Studies of ‘evaporating atmospheres’ on two distant ‘mini-Neptune’ exoplanets seem to confirm the theory that they will eventually become ‘super-Earths’

Astronomers have discovered two separate cases of “mini Neptune” planets whose atmospheres are stripped away by solar radiation “like steam from a pot of boiling water,” suggesting that they transform into bodies known as “super-Earths”.

The findings, published in two recent papers in The Astronomical Journal, help shed light on the formation and evolution of mini-Neptune exoplanets ā€“ which orbit stars beyond our solar system. These worlds are smaller and denser than the planet Neptune, with large rocky cores surrounded by a thick gaseous atmosphere thought to be composed of hydrogen and helium.

According to scientific theory, a mini-Neptune close enough to its star would be bombarded with X-rays and ultraviolet radiation. Over several hundred million years, it would slowly lose its gaseous atmosphere as it transformed into a rocky ‘super-Earth’ – which can be about 1.75 times the size of Earth. Mini-Neptunes are between two and four times larger than our planet.

Only a few planets that fall between these two types of exoplanets in size have been found, suggesting that mini-Neptunes eventually become super-Earths.

While researchers “suspected that young mini-Neptunes must have evaporating atmospheres”, the studies’ lead author, Michael Zhang, a graduate student at the California Institute of Technology (Caltech), noted that “No one had ever caught one doing it until now.”

In the new studies, Caltech astronomers examined one of two mini-Neptune planets in the “TOI 560” star system, located about 103 light-years from Earth. They also used NASA’s Hubble Space Telescope to study two other mini-Neptunes orbiting the star “HD 63433”, about 73 light-years away.

The team found atmospheric gas escaping from both the innermost mini-Neptune in TOI 560 (named TOI 560.01) and the outermost mini-Neptune in HD 63433 (named HD 63433 c) – hinting at their evolution into super-Earths. Helium signatures were detected in the case of TOI 560.01, while hydrogen signatures were observed in HD 63433 c.

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ā€œGas velocity provides evidence that atmospheres are escaping. The helium observed around TOI 560.01 moves at a speed of 20 kilometers per second, while the hydrogen around HD 63433 c moves at a speed of 50 kilometers per second. Zhang said, adding that the gravity of the planets was “not strong enough to hold gas that fast.”

Interestingly, the waste gas from TOI 560.01 was found to be flowing towards its star – the opposite of what “most models predict” said co-author Professor Heather Knutson. Further studies will show if this phenomenon is an anomaly in the mini-Neptunes.


RT

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