A team of U.S. astronomers has discovered that two-thirds of the planets around M dwarfs (red dwarf stars) could be roasted by extreme tidal forces, sterilizing them. But that leaves one-third of the planets that could be in an orbit close enough, and gentle enough, to hold onto liquid water and possibly harbor life.
An artist’s impression of the Gliese 414 system. Image credit: Sci.News.
“I think this result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars,” said University of Florida doctoral student Sheila 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.”
In their study, Sagear and her colleague, University of Florida’s Professor Sarah Ballard, measured the eccentricity of a sample of more than 150 planets around M dwarfs.
“If a planet orbits close enough to its star, at about the distance that Mercury orbits the Sun, an eccentric orbit can subject it to a process known as tidal heating,” they explained.
“As the planet is stretched and deformed by changing gravitational forces on its irregular orbit, friction heats it up.”
At the extreme end, this could bake the planet, removing all chance for liquid water.
“It’s only for these small stars that the zone of habitability is close enough for these tidal forces to be relevant,” Professor Ballard said.
For the study, the astronomers analyzed data from NASA’s Kepler space telescope and ESA’s Gaia telescope.
To measure the planets’ orbits, they focused especially on how long the planets took to move across the face of the stars.
“The distance is really the key piece of information we were missing before that allows us to do this analysis now,” Sagear said.
The authors found that the stars with multiple planets were the most likely to have the kind of circular orbits that allow them to retain liquid water.
Stars with only one planet were the most likely to see tidal extremes that would sterilize the surface.
“Since one-third of the planets in this small sample had gentle enough orbits to potentially host liquid water, that likely means that the Milky Way has hundreds of millions of promising targets to probe for signs of life outside our Solar System,” the researchers said.
Their paper will be published in the Proceedings of the National Academy of Sciences.
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Sheila Sagear & Sarah Ballard. 2023. The orbital eccentricity distribution of planets orbiting M dwarfs. PNAS, in press; doi: 10.1073/pnas.2217398120
