The theories
What could be causing mini-Neptunes to lose their atmospheres?
Exoplanets are not losing their atmospheres
According to David, this theory suggests that super-Earths are not the product of planets that have lost their atmospheres. Instead the theory explains that they avoided accreting atmospheres to begin with: “In this theory, whether or not a planet becomes a rocky superEarth or a gaseous miniNeptune depends on whether or not the core formed early enough to accrete and hold onto a substantial amount of gas.”
It’s caused by high-energy radiation
This assumes that the exoplanets have accreted atmospheres. As David says, the accretion will have happened when the gaseous protoplanetary disc was still around – that is, in the first 10 million years. In this theory, X-ray and ultraviolet radiation, due to close proximity with the star, heat away molecules from the planet’s upper atmosphere in a process known as photoevaporation. This will take the planet down towards its core. Astronomers think it’s a more likely explanation.
It’s due to leftover heat from an exoplanet’s formation
Remnant heat from a planet’s formation caused by collisions with rocky bodies is said to escape the planetary core and transfer to its atmosphere. In a similar process as predicted by the photoevaporation theory, the outflow of energy departs into space. Again this is a leading theory, but there is a fourth, less advanced suggestion that collisions directly cause net atmospheric losses if they are energetic and frequent enough.