Researchers have discovered that a yellow sun is relatively uncommon in the Milky Way. The vast majority of stars are much smaller, cooler, and have only half the mass of the Sun. Countless planets orbit these familiar dwarf stars in our galaxy.
These planets would have to be extremely close to their small stars 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.”
Astronomers at the University of Florida have found that two-thirds of the planets near these ordinary small stars could be sterilized by these tidal extremes by roasting them in a new analysis based on the latest telescope data. But that still leaves one-third of the planets—hundreds of millions around the galaxy—that could be orbiting just to hold liquid water and possibly support life.
The week of May 29, research by UF astronomy professor Sarah Ballard and PhD student Sheila Sagear was published in the journal 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 as eyes shift to this population of stars,” Sagear said. “These stars are excellent targets for searching for small planets in orbit where it is possible that water could be liquid and therefore the planet could 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 the orbit, the more eccentric it is. If a planet orbits close enough to its star, about the distance at which Mercury orbits the Sun, the eccentric orbit can subject it to a process known as tidal heating.
As the planet is stretched and deformed by changing gravitational forces in its irregular orbit, friction heats it up. In an extreme case, it could bake the planet and eliminate any chance of liquid water.
“Only for these small stars is the habitable zone close enough for these tidal forces to be relevant,” Ballard said.
The data comes from NASA’s Kepler space telescope, which captures information about exoplanets as they move in front of their host stars. In measuring the orbits of the planets, Ballard and Sagear focused in particular on how long it took the planets to travel across the faces 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 that we were missing before that allows us to do this analysis now,” Sagear said.
Sagear and Ballard found that stars with multiple planets are most likely to have circular orbits that allow them to hold liquid water. Stars with only one planet were the most likely to see tidal extremes that would sterilize the surface.