A third of the most common planets in the galaxy could be in the habitable zone

Our familiar warm yellow sun is relatively rare in the Milky Way. The most common stars are much smaller and cooler, containing only about half the mass of our sun at most. Billions of planets orbit these common dwarf stars in our galaxy. To capture enough heat to be habitable, these planets would have to clump close to their young stars, making them vulnerable to intense tidal forces.

To capture enough heat to be habitable, these planets would have to clump close to their young stars, making them vulnerable to intense tidal forces. In a new analysis based on the latest data from the telescope, astronomers at the University of Florida have found that two-thirds of the planets around these ubiquitous small stars could be burned up by these tidal extremes, rendering them sterilizing. But that leaves a third of the planets, hundreds of millions across the galaxy, that could be in a moderate orbit soon enough, smooth enough to contain liquid water and possibly support life.

UF astronomy professor Sarah Ballard and doctoral student Sheila Sager published their findings the week of May 29 in a journal. Proceedings of the National Academy of Sciences. Ballard and Sager have long studied exoplanets, those worlds that orbit stars other than our Sun. “I think this result is really important for the next decade of exoplanet research, because eyes are on this group of stars,” Sager said. “These stars are excellent targets for the search for small orbiting planets where it is conceivable that water is liquid and therefore the planet is habitable.”

Sager and Ballard measured the eccentricity of a sample of more than 150 planets around these M dwarf stars, which are about the size of Jupiter. The more elliptical the orbit, the weirder it is. If a planet orbits close to its star, about as far as Mercury goes around the sun, its eccentric orbit can subject it to a process known as tidal heating. When a planet is stretched and deformed by changing gravitational forces in its irregular orbit, friction causes its temperature to rise. Eventually, the planet could ferment, eliminating all possibility of liquid water. “Only for these young stars is the habitable zone close enough that tidal forces are relevant,” Ballard said.

The data comes from NASA’s Kepler telescope, which captures information about exoplanets as they move in front of their host stars. To measure the orbits of the planets, Ballard and Sagear focused specifically on how long it took for the planets to move across the surface of the stars. Their study was also based on new data from the Gaia telescope, which measures the distance to billions of stars in the galaxy. “Distance is really the key piece of information that we were missing before that allows us to do this analysis now,” Sagir said.

Sagir and Ballard found that stars with multiple planets are more likely to have some sort of circular orbit that allows them to retain liquid water. Stars with only one planet were the most likely to see extreme tidal waves that would sterilize the surface.

source: https://www.pnas.org/doi/10.1073/pnas.2217398120