Photosynthesis on a lush exoplanet. Source: Doug Cummings, Caltech
Astrobiologists have been using Earth as a benchmark for habitability, for the obvious reason that it is so far the only place where life has been observed. But what if our planet is in fact not as habitable as it could be? There may be worlds out there that are in fact more habitable than Earth. Heller and Armstrong in their paper Superhabitable Worlds argue that other planets or moons may have ecosystems more benign to life than our home planet.
Published in the Astrobiology journal, the paper by René Heller and John Armstrong, of Canada's McMaster University and Weber State University respectively, conclude that K dwarfs are the most likely parent stars for superhabitable worlds, after considering a range of physical effects. Such worlds will be older and more massive than Earth, for one thing. Alpha Centauri B is a favorite parent star of the paper's authors. A K dwarf believed to host an Earth-sized planet, Alpha Centauri B belongs to the Sun's closest stellar system.
Age, then, is one of the main considerations for a world's superhabitable status. Why would a planet or moon older than Earth be more friendly towards life? Because, Heller and Armstrong argue, older worlds will have had longer to evolve more biodiverse environments, creating ecosystems that harbor conditions more conducive to life.
For an extrasolar planet to be habiltable, it would have to be free from excessive tidal heating; too much ice would also render a world inhospitable to extraterrestrial life. Terms such as 'habitable zone' (HZ) may be too vague, the authors state, since worlds with the HZ may not be habitable, and worlds that are habitable need not be within the HZ.
Astronomers and astrobiologists alike will be keen to see the images sent back from the James Webb Space Telescope, a joint project by NASA, ESA, and CSA due to launch 2018. Among other objectives, the JWST will gather images and data relating to planetary systems and the origins of life.
