A team of astronomers from the University of Washington and NASA Astrobiology Institute’s Virtual Planetary Laboratory has created the ‘Habitability Index for Transiting Exoplanets’ to help guide the ongoing search for extraterrestrial life.
This artistic concept shows Kepler-186f. Image credit: NASA Ames / SETI Institute / JPL-CalTech.
“Basically, we’ve devised a way to take all the observational data that are available and develop a prioritization scheme, so that as we move into a time when there are hundreds of targets available, we might be able to say: OK, that’s the one we want to start with,” explained team leader Prof. Rory Barnes of the University of Washington and the Virtual Planetary Laboratory.
The habitability of an exoplanet is traditionally assessed by comparing a planet’s semi-major axis to the location of its parent star’s ‘habitable zone’ – more informally called the ‘Goldilocks zone’ – the shell around a star for which terrestrial planets can possess liquid surface water.
NASA’s Kepler Space Telescope has discovered numerous exoplanet candidates near the habitable zone, and many more are expected from missions such as ESA’s PLATO (PLAnetary Transits and Oscillations of stars) and NASA’s TESS (Transiting Exoplanet Survey Satellite). These exoplanets require significant follow-up observations for validation, so prioritizing candidates for habitability from transit data has become an important aspect of the search for alien life in the Universe.
Prof. Barnes and co-authors developed a novel method that allows astronomers to compute a number the team calls the Habitability Index for Transiting Exoplanets (HITE), which represents the relative probability that an exoplanet could support liquid surface water.
In creating it, the scientists factored in estimates of a planet’s rockiness, rocky planets being the more Earth-like.
They also accounted for a phenomenon called ‘eccentricity-albedo degeneracy,’ which comments on a sort of balancing act between the a planet’s albedo – the energy reflected back to space from its surface – and the circularity of its orbit, which affects how much energy it receives from its host star.
“A life-friendly energy equilibrium for a planet near the inner edge of the habitable zone – in danger of being too hot for life,” Prof. Barnes explained, “would be a higher albedo, to cool the world by reflecting some of that heat into space.”
“Conversely, a planet near the cool outer edge of the habitable zone would perhaps need a higher level of orbital eccentricity to provide the energy needed for life,” he said.
The astronomers ranked in this way exoplanets so far found by Kepler, in its original mission as well as its K2 follow-up mission.
They found that the best candidates for habitability and life are those exoplanets that get about 60 percent to 90 percent of the solar radiation that the Earth receives from the Sun, which is in keeping with current thinking about a star’s habitable zone.
The HITE index is introduced in a paper accepted for publication in the Astrophysical Journal (arXiv.org preprint).
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Rory Barnes et al. 2015. Comparative Habitability of Transiting Exoplanets. ApJ, accepted for publication, arXiv: 1509.08922
