Habitability and biosignature claims on exoplanets

5 problems

• Is there a sharp boundary between airless and atmosphere-bearing planets?

  Open

  In the Solar System, bodies that are small or strongly heated have no air, and the dividing line follows a simple rule linking gravity and sunlight. Whether exoplanets follow the same line is untested.

• How common are Earth-size planets in habitable zones of Sun-like stars?

  Open

  The fraction of Sun-like stars with an Earth-size planet at a distance where water could be liquid sets how many targets future telescopes can study. Current estimates differ by about a factor of ten.

• Which non-biological processes can mimic oxygen or methane biosignatures?

  Open

  Oxygen and methane can sometimes be made by sunlight splitting water or by volcanic and rock-water chemistry, without life. A sign of life must be distinguishable from these.

• Does the atmosphere of K2-18 b contain dimethyl sulfide?

  Contested

  A team using JWST reported hints of dimethyl sulfide, a gas made on Earth almost only by marine life, on the planet K2-18 b. Other teams analyzing the same data find no reliable signal.

• Can rocky planets around red dwarf stars keep atmospheres?

  Open

  Red dwarfs are the most common stars and the easiest around which to study small planets, but they emit intense ultraviolet light and flares that may strip planetary atmospheres. Whether their rocky planets keep any air is unknown.