It seems that about 30% of the planets orbiting the most common stars in the Milky Way are in their habitable zone, that is, at a distance that allows the occurrence of liquid water. The discovery is by astronomers at the University of Florida, who have studied the orbits of more than 150 exoplanets.
It may not seem like it, but the Sun is a relatively rare star: the Milky Way has many more smaller and cooler stars than our star, most of which have only half the mass of the Sun. These dwarf stars are orbited by billions of exoplanets, the name given to worlds that orbit stars other than the Sun.
As the stars are cooler, the exoplanets in question would have to be very close to them to be warm enough and have liquid water, a requirement considered essential for life as we know it. On the other hand, these close orbits would leave the planets exposed to extreme gravitational forces from the stars.
In the study, astronomer Sarah Ballard and PhD student Sheila Sagear worked with data from the Kepler and Gaia telescopes, and analyzed the eccentricity of exoplanets’ orbits around the stars, to see how oval they were. If the planet is at a distance from it similar to that between Mercury and the Sun, an eccentric orbit could cause tidal heating on it.
This heating comes from the gravitational interactions between the planet and the star, which cause it to be deformed and suffer friction, generating heat; however, the heat can be so much that it may even end up ending the chances of liquid water. “It’s only in these small stars that the habitable zone is close enough for tidal forces to be relevant,” Ballard noted.
Stars with multiple planets were more likely to have circular orbits that would allow these worlds to have liquid water — stars with just one exoplanet were more likely to cause extreme tidal forces capable of sterilizing them. As 30% of the planets in the sample had orbits smooth enough to allow liquid water, this suggests that our galaxy likely has millions of promising worlds for the search for life.
The article with the results of the study was published in the journal Proceedings of the National Academy of Sciences.
Source: Proceedings of the National Academy of Sciences; Via: University of Florida
