Earth is perfectly balanced for organic life.
It makes sense that similar planets orbiting far-away stars may also sustain life.
However, proving the truth is challenging.
One of the most reliable ways of discovering extraterrestrial life will be analyzing the atmospheres of inhabited exoplanets, but Earth is really small for a planet, and it has a thin atmosphere compared to more giant planets.
It will be considerably easier to analyze the atmospheres of gas planets, but could such worlds host life? A recent paper in Universe suggests that it could.
Exobiologists have long speculated that we shouldn’t just assume that all life in the Universe will occur on planets that are similar to ours. The subsurface oceans of Ganymede and Enceladus may possibly host terrestrial life, and Titan presents an intense methane chemistry that may host exotic life.
However, most recipes for life start with three elements – energy, water, and a surface.
Life must use some sort of energy source to survive, regardless of its origin (geothermal or solar), and water may be an adequate solution to grant complex molecules a chance to interact.
The requirement of a surface is somewhat more subtle.
It isn’t required for life to flourish, as numerous organisms can survive in water or air for their whole life.
Instead, it appears that it may be mandatory for life to commence.
Surface chemistry is particularly essential at creating large organic molecules, even in space.
It possibly takes a rocky surface for the building blocks of life to form.
The new paper analyzed warm sub-Neptune worlds, which are between 8 and 10 times more massive than our planet, and are probably minor gas planets with thick atmospheres but no terrestrial surfaces.
Some of the planets have been discovered in the possibly habitable zone of the star, like K2-18b, which orbits a red dwarf star nearby.
The team proved that warm sub-Neptunes like K2-18b likely contain lots of water and organic molecules required to develop a habitable area within their atmosphere.
Being smaller gas planets, the habitable layer may be somewhat stable, granting any life to stay alive long enough to reproduce before sinking into hostile depths.
Comparable arguments led to the possibly habitable layer of Venus’s atmosphere.
K2-18b is different from Venus because it doesn’t appear to have a surface.
Though life may thrive on a sub-Neptune world, the question of how it would get there got scientists thinking.
The team speculates that asteroids may be some of the main suspects.
If an exoplanetary system were accompanied by an unstable asteroid belt, meteorite impacts with the sub-Neptune world and smaller terrestrial worlds would both be mundane.