Are common plants in a habitable zone?

Researchers found, in the Milky Way, the yellow sun is comparatively uncommon. The vast majority of stars are far smaller, colder, and only have up to half the sun's mass. In our galaxy, countless planets orbit these familiar dwarf stars.

These planets would have to be extremely close to their little stars in order to accumulate enough heat to be habitable, making them vulnerable to strong tidal forces.

The study was published in the journal, 'Proceedings of the National Academy of Sciences.

The University of Florida astronomers have found that two-thirds of the planets near these commonplace tiny stars might be sterilized by these tidal extremes by roasting them in a new analysis based on the most recent telescope data. But that still leaves one-third of the planets--hundreds of millions around the galaxy--that might be in an orbit just right for holding onto liquid water and perhaps supporting life.

The week of May 29, research by UF astronomy professor Sarah Ballard and doctoral candidate Sheila Sagear was published in the Proceedings of the National Academy of Sciences. Exoplanets, or planets that orbit stars other than the sun, have long been the subject of research by Ballard and Sagear.

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

Sagear 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 oval-shaped an orbit, the more eccentric it is. 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. 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," Ballard said.

Data came from NASA's Kepler telescope, which captures information about exoplanets as they move in front of their host stars. To measure the planets' orbits, Ballard and Sagear focused especially on how long the planets took to move across the face of the stars. Their study also relied on new data from the Gaia telescope, which measured the distance to billions of stars in the galaxy.

"The distance is really the key piece of information we were missing before that allows us to do this analysis now," Sagear said.

Sagear and Ballard found that 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.