One-third of galaxy's most common planets could be in habitable zone

In the Milky Way, our beloved, cozy, golden sun is rather uncommon. The vast majority of stars are far smaller, colder, and only have up to half the mass of the sun. In our galaxy, many planets orbit these common dwarf stars.

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

University of Florida astronomers have shown that two-thirds of the planets near these commonplace tiny stars might be sterilized by these tidal extremes by roasting them in a new study based on the most recent telescope data. But that still leaves one-third of the planets—hundreds of millions around the galaxy—that may be in an orbit that's 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.

Because attention is turning to this group of stars, Sagear said, "I think this result is really important for the next decade of exoplanet research." These stars are great places to seek for tiny planets in orbits where it's possible that there may be liquid water, which would make the planet potentially livable.

A sample of more than 150 planets orbiting these Jupiter-sized M dwarf stars were analyzed for eccentricity by Sagear and Ballard. An orbit's eccentricity is directly correlated with how oval it is. An eccentric orbit can cause a planet to experience tidal heating if it circles its star sufficiently closely, at a distance similar to that of Mercury's orbit around the sun. Friction warms the planet as it is stretched and twisted by shifting gravitational forces on its erratic orbit. At its most extreme, this might roast the Earth and completely eliminate any prospect of liquid water.

Ballard claimed that only for tiny stars is the zone of habitability close enough for these tidal pressures to be significant.

The information was gathered by NASA's Kepler telescope, which records data on exoplanets as they pass in front of their home stars. Ballard and Sagear concentrated particularly on how long it took the planets to cross the faces of the stars in order to calculate the planets' orbits. New information from the Gaia telescope, which calculated the distance to billions of stars in the galaxy, was also used in their analysis.

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

According to research by Sagear and Ballard, stars with many planets are more likely to have the type of circular orbits necessary to support liquid water. The likelihood of tidal extremes that might sterilize the surface was highest for stars with only one planet.

Since one-third of the planets in this sample had orbits that might potentially support liquid water, the Milky Way most certainly contains hundreds of millions of intriguing prospects for the search for extraterrestrial life.

Provided by University of Florida