After formation, a planet does not always stay in the same orbit around its host star. Various interactions can cause a planet to migrate nearer to or further from its host star. As a result, a planet formed outside the snow line (i.e. the region of the protoplanetary disk beyond which it is cold enough for water and other volatiles to condense into ice) can potentially be brought nearer to its host star, to within the habitable zone (HZ). Due to the large abundance of ices beyond the snow line, a planet coming into the HZ from there should contain a lot of water - an important criterion for habitability. Nonetheless, such a planet may also have acquired a thick gaseous envelope of hydrogen and helium, making it uninhabitable. A planet like this can be termed a “mini-Neptune”.
Mini-Neptunes are basically icy/rocky cores surrounded by massive gaseous envelopes. In a way, they are miniature versions of Neptune. A study by Luger et al. (2015) show mini-Neptunes that migrate into the HZ of M-dwarf stars can naturally shed their thick gaseous envelopes of hydrogen and helium, effectively transforming into potentially habitable, volatile-rich Earth-mass and super-Earth-mass planets. The pre-main sequence (PMS) phase of a star is the period during which the star has yet to fully contract. Sun-like stars spend less than 50 million years in the PMS phase, while M-dwarf stars can spend hundreds of millions of years in the PMS phase where they remain super-luminous. This can lead to greater atmospheric mass loss for planets around M-dwarf stars.
M-dwarf stars remain active for many hundreds of millions of years after formation. During this period they emit high levels of X-ray/extreme ultraviolet (XUV) radiation. Exposure to high levels of XUV radiation for such a long period of time can cause a mini-Neptune that has migrated into the HZ to lose its thick gaseous envelope of hydrogen and helium to space. After losing their massive gaseous envelopes, these objects are termed “habitable evaporated cores” (HECs). Such a process is most likely to occur for mini-Neptunes with solid cores ~1 Earth-mass or so and up to 50 percent hydrogen/helium by mass.
Mini-Neptunes with core masses grater than roughly twice the mass of Earth are not expected to form HECs because they are sufficiently massive to retain their thick gaseous envelopes. HECs are also unlikely to form around K-dwarf and G-dwarf stars because they have shorter super-luminous PMS phases and shorter XUV timescales. With abundant water content, HECs are expected to be water worlds with very deep oceans and are therefore not true terrestrial planets. On Earth, geochemical cycles are crucial for life. However, the oceans on HECs can be deep enough for high-pressure ice to form at the bottom, separating the ocean from the underlying mantle, possibly inhibiting geochemical cycling. Nevertheless, geochemical cycling can still go on via solid state ice convection.
Luger et al., “Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs”, Astrobiology, Volume: 15, Issue 1, January 15, 2015