Observations have shown that mini-neptunian planets can lose their atmosphere to the radiation of their stars, changing to a group of rocky super-Earth planets.
There are no such worlds in the Solar System, but mini-Neptunes make up a large proportion of the total number of planets throughout the Galaxy. These are gas dwarfs, similar to our Uranus and Neptune, but much smaller. Mini Neptunes are surrounded by a dense primordial atmosphere of hydrogen and helium and are usually a fair distance from their parent stars. Closer to them, more compact planets, super-Earths, which are as rocky as ours but more massive, are more common.
The boundary between super-Earths and mini-neptunes is in the region of one and a half to two Earth radii. However, mass observations of exoplanets by the Kepler Space Telescope have shown that there are very few planets of such borderline size. The phenomenon is called the 'Fulton Failure' - after astronomer Benjamin Fulton, who discovered it about 10 years ago.
After more research, scientists have linked the lack of planets with a 1.5-2.0 Earth radius to the loss of their atmospheres. Most likely, such worlds are too small to hold their original, primordial gas envelope and it quickly escapes into space, leaving a more compact, rocky super-Earth. But what serves as the main driver of this loss? It could be heat emanating from the planet's interior, for example, or radiation from the star, which is particularly intense in the early stages of their existence.
New work by Michael Zhang and colleagues at the California Institute of Technology (Caltech) has helped make sense of the mystery. The scientists talk about it in a paper due for publication in The Astronomical Journal and is so far available from the arXiv Preprint Library.
The astronomers have made spectroscopic observations of four young and not too distant mini-neptunes orbiting near young stars, orange dwarfs. They are TOI 560b measuring 2.8 Earth radius, TOI 1430.01 2.1 Earth radius, TOI 1683.01 and TOI 2076b 2.3 and 2.5 radius respectively. The work has shown that all four planets are rapidly losing helium from their shells, a rate too great to be explained by their own bowels. Consequently, radiation from the parent star is the main culprit.
At this rate, the atmospheres of mini-Neptunians only fully erode in a few hundred million years. "We conclude that most, if not all, of these planets will lose their hydrogen-rich envelopes and become super-Earths," write Zhang and his co-authors. - These results show that most mini-neptunes orbiting solar-type stars initially have primordial atmospheres, but under the influence of radiation they lose them and become super-Earths."