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.
Reference:
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