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Q&A: How to Rate the Habitability of Other Planets


These days, every exoplanet discovery is still rich with excitement, as astronomers scrutinize each distant world and consider its possible characteristics. But this could get tedious pretty soon, as the number of confirmed exoplanets climbs into the thousands. When that happens, astronomers and especially astrobiologists will have to start sifting planets according to their interestingness. A new paper to be published next month describes a new two-step ranking system to make this process easier. We spoke to astrobiologist Dirk Schulze-Makuch to get some details.

Unlike other astrobiology criteria, this new system doesn’t assume that habitability only applies to a rocky place with liquid water. Dirk Schulze-Makuch, first author on the new paper and an astrobiologist at Washington State University, said the team wanted to be as open-minded as possible.

Their proposed Earth Similarity Index looks at the size, density and orbital distance of a planet or moon, as well as its star’s size and temperature, and compares these with Earth. The companion Planetary Habitability Index is based on the presence of some type of stable substrate, an energy source, appropriate chemistry, and the potential for holding a liquid solvent, not necessarily water. Both use a scale from 0 to 1.

So what’s the most Earthlike habitable place (other than here) in the cosmos? In the burgeoning exoplanet pantheon, it’s Gliese 581d, with an index of 0.74. The PHI has some different results, with the best option apparently Titan, with an index of 0.64. Mars rings in at 0.59, incidentally.

These numbers are only as meaningful as the data used to inform them, however, so the researchers stress there’s room for updates and improvements.

PopSci talked with Schulze-Makuch to get some more details about the twin indices and how they could help astronomers make intergalactic classifications.

PopSci: Why do you think there are several habitable exoplanets?

Dirk Schulze-Makuch: Right now, the exoplanets we have discovered are still heavily biased toward gas giants, because they are easier to detect. But the longer we research, we are getting closer and closer to Earth-like planets. Most of them are still super-Earths.


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