Kepler's survey found a number of super-Earths, planets several times Earth's mass, and therefore too small to be gassy so-called ice giants like Uranus and Neptune. But what might the spectral fingerprint of a nearby super-Earth look like? And could we unequivocally deduce the planet is inhabited to everyone's satisfaction?
GALLERY: Exquisite Exoplanetary Art
Astrobiologists are now modeling super-Earth atmospheres for planets orbiting in the habitable zone of red dwarf stars. It's time to begin to try and understand and predict the expected signature of an alien biosphere.
A set of models developed by J. L. Grenfell of the Zentrum fur Astronomie und Astrophysik, Technische Universitat Berlin, and colleagues, start out with the assumption the planet is blanketed with life (unlike Mars where apparently nothing survives on the surface). This would likely require that the target world be several billion years old to allow for the evolution of life to expand, diversify, and significantly modify the planet's atmosphere.
Life on any planet would use chemical reactions to extract energy, store it, and release certain gases as a byproduct of their metabolism. On Earth the biggest chemical signature of metabolism is oxygen produced by photosynthesis. Other strong signals would come from the presence of ozone and nitrous oxide. Other biotracers, carbon dioxide and methane (already detected on and exoplanets by the Hubble Space Telescope) can also be produced by non-biological processes.