The sun is an ordinary star, but it is not the only type of star that exists. A new study indicates that most of the stars in our galaxy are M dwarfs (sometimes called red dwarfs), which are smaller and redder than the Sun — and many of them could have the potential to host life.
A new analysis of Kepler’s Planet Search mission data shows that a third of the planets around M dwarfs may be suitable for life, which means that there are potentially hundreds of millions of habitable planets in Milky Way alone.
For the analysis, astronomers at the University of Florida incorporated new information from the European Space Agency’s Gaia satellite, which accurately measures the distances and motions of stars, to improve measurements of the orbits of exoplanets. The researchers wanted to determine a parameter for each orbit known as eccentricity, which is a measure of how far a planet’s trajectory extends around its star.
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“Distance is really the key piece of information that we lacked before that allows us to do this analysis now.” Sheila SagarUniversity of Florida astronomy graduate student and lead author of the study, V.A statement.
Planets around M dwarfs with large eccentricities — very long elliptical orbits — end up being fried by the star if they are close enough, in a process called tidal warming. Tidal heating is caused by the planet’s wobbly orbit, causing the star’s gravitational stretching and compression. Just like rubbing your hands together, all this motion generates heat through friction. If there is too much heat, the planet loses its water, as well as the chances of life developing on its surface. (Because water is essential to life as we know it, water is often central to the search for habitable worlds beyond Earth.)
If a planet orbited an M dwarf far away, that distance might prevent the warming doom — but then the planet would be too cold, lacking the heat needed for life. Therefore, exoplanets around M dwarfs must live close to their stars in order to have a chance of being warm enough for life, which puts them at risk of tidal surges if their orbit is not a proper circle.
“Only for these young stars is the habitable zone close enough for these tidal forces to be relevant,” Sarah BallardUniversity of Florida astronomer and co-author of the study, said in the release.
With their new and improved measurements of the large number of exoplanets discovered by the Kepler space telescope, Sagear and Ballard found that two-thirds of the planets around M dwarfs would be flaked with heat from their host stars, burning their chances of life. But that leaves a third of the planets in the so-called Goldilocks Zone where liquid water could theoretically exist – along with the possibility of life. The odds of a planet with a stable circular orbit in the Goldilocks zone also increase if it has another extrasolar friend around the same star.
“I think this result is really important for the next decade of exoplanet research, as eyes are turned to this group of stars,” Sagir said. “These stars are excellent targets for the search for small planets in orbit where it is conceivable that the water could be liquid, and thus the planet could be habitable.”
The results are published May 30 in the journal PNAS.
