Scientists from the University of California announced the number of planets in the habitable zone of small and cold stars in the Milky Way. It turned out that about a third of the planets revolve so close to their stars to have enough heat needed for life. The results of the work were published in the journal PNAS.

Stars like the Sun are relatively rare in our galaxy. Most of the luminaries have a much smaller mass, usually no more than half the mass of the sun. Obviously, in such conditions, the planets orbiting them need to be much closer in order to receive as much light and heat as necessary for the origin of life, as well as to have liquid water.

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To come up with a more specific number, the authors analyzed data from NASA's Kepler Space Telescope, which captures information about exoplanets as they move in front of host stars, and the Gaia telescope, which measures the distance to billions of stars in the galaxy.

Particular attention has been focused on the eccentricity of the planets around M dwarf stars, which are roughly the size of Jupiter. The more oval the orbit, the more eccentric it is. If a planet orbits close enough to its star, then it can undergo tidal heating - stretch and deform due to changes in gravitational forces in orbit.

The researchers found that stars with multiple planets are more likely to have circular orbits that allow them to store liquid water. Stars that had only one planet were the most likely to encounter tidal heat that destroyed everything on the planet. About two-thirds of the planets in the sample had sufficiently elongated orbits, but the remaining one-third had orbits that were shallow enough to potentially contain liquid water. This suggests that the Milky Way has hundreds of millions of promising objects to look for signs of life outside the solar system.