Our notions of habitability are built around environments like our own, which is why the search for planets with temperatures that support liquid water at the surface is such a lively enterprise. But as we saw yesterday, it is not beyond possibility that many places in our Solar System could have sub-surface oceans, even remote objects in the Kuiper Belt. And that raises the question of how we assess astrobiological environments, an issue studied by Dirk Schulze-Makuch (Washington State University) and Abel Mendez (University of Puerto Rico at Arecibo), working with an international team of researchers in a paper suggesting a new approach.
Schulze-Makuch makes the situation clear:
“Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star. For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life.”
To avoid overlooking potentially habitable worlds as we discover more and more exoplanets, the authors propose two indices that help to provide a quantitative look at a given exoplanet’s chances for habitability. The first is an Earth Similarity Index that screens exoplanets in relation to all the factors that make our planet hospitable to life. The second is a Planetary Habitability Index, which describes chemical and physical parameters that may allow life to exist under conditions that vary markedly from Earth. Think Enceladus, or Europa. Think the exomoon of a gas giant. Think, in other words, as speculatively as possible.
The Planetary Habitability Index is based on ‘the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent,’ as the paper’s abstract notes. But it’s also based upon hypotheses about life’s viability in extreme environments that we’re as yet unable to test. Acknowledging this, the authors see their index as an ongoing work that can be updated as technology and knowledge about astrobiology advances. Interestingly enough, they apply their metrics to the provocative Gliese 581 system, finding that both GJ 581c and GJ 581d show an Earth Similarity Index comparable to that of Mars, and a Planetary Habitability Index somewhere between that of Europa and Enceladus.
Future space instrumentation should be able to tell us whether an Earth-class planet shows the signature of life, but how do we use those instruments to size up an icy exomoon when we can’t make the call on far closer worlds like Europa? What will change the game is finding proof in our own Solar System that life can occur in just this kind of extreme environment. Such a demonstration would make the Planetary Habitability Index far more interesting — and accurate — telling us that life can adapt to places utterly unlike our own planet. Until that occurs, constructing the PHI seems like an intriguing but premature exercise.
The paper is Davila et al., “A Two-Tiered Approach to Assessing the Habitability of Exoplanets,” accepted by Astrobiology (abstract).