Where did the hot neptunes go? Astronomers explain

Where did the hot neptunes go? Astronomers explain
This artist's illustration shows a giant cloud of hydrogen streaming off a warm, Neptune-sized planet just 97 light-years from Earth. © NASA, ESA, and D. Player (STScI)

Where did the hot neptunes go? Astronomers from the University of Geneva (UNIGE), Switzerland asked this question and have made observations which suggest that hot neptunes have lost their atmosphere and turned into smaller planets called super-Earths.

The research has been published in Astronomy & Astrophysics, and supports the theory that hot neptunes lose much of their energy and become super-Earths.

The ‘desert’ of hot neptunes

While searching for explanets, astronomers have observed a large number of hot planets the size of Jupiter and numerous super-Earths (which are a little larger than the Earth), but no planets close to their star the size of Neptune. The ‘desert’ of hot Neptunes suggests two explanations: either these alien worlds are rare, or they used to be plentiful but have since disappeared.

Vincent Bourrier, researcher in the Astronomy Department of the Faculty of Science of the UNIGE, member of the European project FOUR ACES** and first author of the study, said: “Until now we were not sure of the role played by the evaporation of atmospheres in the formation of the desert.”

Explaining the abundance of super-Earths

The discovery of several warm Neptunes at the edge of the desert losing their atmosphere supports the idea that the hot Neptunes are short-lived. “This could explain the abundance of hot super-Earths that have been discovered,” added David Ehrenreich, associate professor in the astronomy department of the science faculty at UNIGE and co-author of the study.

Observing the evaporation of two warm Neptunes is a positive step towards confirming the theory, however it is still necessary to study more of them to confirm the predictions about what happens to hot neptunes.

Researchers plan to use the Hubble space telescope to look for other traces of atmospheric escape, because hydrogen could drag upward heavier elements such as carbon.

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