An analysis conducted by astronomers at the University of Florida (USA) showed that a third of all planets in our Milky Way are in orbits safe enough to support life (PNASMay 29).
Our familiar warm yellow sun is relatively rare in the Milky Way. The vast majority of stars are smaller and cooler, with at most half the mass of the Sun. Billions of planets orbit red dwarf stars. To absorb enough heat to be habitable, these planets would have to be very close to their stars. But this in turn makes them sensitive to very strong tidal forces.
Based on the latest data from the Kepler and Gaia space telescopes, Ballard and Sager determined that two out of three planets around young stars everywhere are so hot that they should be considered uninhabitable. But there are still hundreds of millions of people outer planets around where temperatures remain moderate enough to contain liquid water.
In their research, Sager and Ballard measured the aberrations of a sample of more than 150 planets, roughly the size of Jupiter, orbiting red dwarf stars. Eccentricity refers to how strongly the orbit of a celestial body deviates from circular shape: the greater the eccentricity, the more elliptical the orbit.
About the distance Mercury orbits our sun, such an eccentric orbit can cause what is called tidal heating. This means that the planet is being stretched and distorted by the changing gravitational forces it is exposed to in its long orbit. In the extreme case, the atmosphere can become so hot that all the liquid water on its surface evaporates.
Sagear and Ballard found that stars with multiple planets are more likely to have planets in circular orbits, or habitable exoplanets. In stars that only have one planet, tidal forces are often so intense that the chance of habitable conditions is slim. (EE)
A third of the most common planets in the galaxy could be in the habitable zone
