New data helps narrow the search for habitable exoplanets

An evaluation of exoplanets shows that many have an eccentric orbit around their star which probably precludes life.

The research is in a new paper in the Proceedings of the National Academy of Sciences.

M dwarf stars, which make up 70% of all known stars, are at most about half the size of our Sun. They’re also much cooler, so in order to have liquid water and therefore life, exoplanets around those stars need to get very close.

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The problem is that, because they’re so close, many potentially life-holding exoplanets are subject to “tidal heating”: if their orbit is too oval-shaped, they’re subject to different gravitational pull at different points in their orbit, which causes friction to heat the planet too much and boil its oceans. So while the planet is technically in the habitable zone, its orbit stops it from being actually habitable.

The less circular an orbit, the more “eccentric” it is. Most planetary orbits aren’t perfectly circular, including Earth’s, which gets closest to the Sun in January and furthest in July.

But Earth is far enough away from the Sun that its eccentricity doesn’t have a big effect on its weather. This isn’t true for planets whizzing around M-dwarf stars.

“It’s only for these small stars that the zone of habitability is close enough for these tidal forces to be relevant,” says paper co-author Professor Sarah Ballard, an astronomer at the University of Florida, US.

Ballard and her co-author, doctoral student Sheila Sagear, used data from NASA’s Kepler Telescope and the ESA’s Gaia Telescope to examine the eccentricity of 163 M-dwarf exoplanets.

They found that two-thirds of the planets had orbits that were too eccentric to harbour life – but one-third did not, and could still be hospitable.

Star systems with multiple planets were much more likely to have habitable, circular orbits, while lone exoplanets were the most likely to be eccentric.

“I think this result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars,” says Sagear.

“These stars are excellent targets to look for small planets in an orbit where it’s conceivable that water might be liquid and therefore the planet might be habitable.”