Hundreds of Millions of Habitable Planets May Exist in the Milky Way, Suggests New Study

Stars much smaller and redder than our Sun, known as M dwarfs or red dwarfs, make up the majority of stars in our galaxy. New research suggests that a significant proportion of these stars could host life-supporting planets.

A recent reevaluation of data from the Kepler space telescope, a planet-hunting mission, reveals that about one-third of planets orbiting M dwarfs could potentially support life. This implies that hundreds of millions of potentially habitable planets could exist within the Milky Way galaxy alone.

In their study, astronomers from the University of Florida combined data from the Kepler mission with new information from the European Space Agency's Gaia satellite, known for accurately measuring distances and movements of stars. This allowed them to refine the measurements of exoplanets' orbits, focusing on the 'eccentricity' parameter, which determines how elliptical a planet's orbit around its star is.

"The key piece of information that we lacked before, which now enables us to carry out this analysis, is the precise distance," said Sheila Sagear, a graduate student in astronomy at the University of Florida and lead author of the study.

Planets orbiting M dwarfs with high eccentricities— orbits which are greatly elongated or oval-shaped — can end up being scalded by their star, if close enough. This phenomenon, known as tidal heating, is caused by the gravitational forces of the star acting on the planet's irregular orbit, generating heat from friction much like rubbing one's hands together. Excessive heating could cause a planet to lose its water, significantly reducing the chances for life to evolve there.

For a planet orbiting an M dwarf to avoid extreme tidal heating, it would need to be located at a greater distance from its star. However, this would make the planet too cold for life. Consequently, exoplanets around M dwarfs must maintain a fine balance: they need to be close enough to their stars to sustain the necessary warmth for life, but not so close that they risk severe tidal heating due to an irregular orbit.

"It's only for these smaller stars that the habitable zone is near enough for tidal forces to be relevant," Sarah Ballard, an astronomer at the University of Florida and co-author of the study, explained.

Applying their improved measurement techniques to a collection of exoplanets discovered by the Kepler space telescope, Sagear and Ballard concluded that approximately two-thirds of planets around M dwarfs would be rendered inhospitable by their host stars' heat. However, this still leaves one-third of planets within the so-called Goldilocks zone, where liquid water, and thus potential life, could theoretically exist. The chances of a planet maintaining a stable, circular orbit in this Goldilocks zone increased if another exoplanet was orbiting the same star.

"I believe this finding is critical for the next decade of exoplanet research, as the focus is shifting towards this population of stars," Sagear commented. "These stars are excellent targets for finding small planets with orbits that could conceivably have liquid water, thereby making the planet potentially habitable."