Planets Like Earth, Millions in Our Galaxy

Rocky and not much older than our world, turn to the right distance from its star for liquid water on its surface. Astronomers have estimated their numbers for the first time.

Artist’s impression of a sunset on the Earth Gliese 667 super Cc

What if the Earth was not something so unique? An international team of astronomers has estimated, for the first time, the number of rocky planets not much larger than ours in our galaxy orbiting around red dwarfs, the most common stars. And the results can not be more striking. Astronomers believe that there must be tens of billions of planets of this type, called super-Earths, only in the Milky Way, and probably about one hundred in the vicinity of the solar system at distances less than 30 light years. And one more thing, perhaps the most important are in the habitable zone of its star, ie the right distance so that the existence of liquid water possible and who knows, perhaps life.

The astronomers used for research conducted observations with the HARPS spectrograph, the European Southern Observatory (ESO), installed in La Silla, Chile. The team looked exoplanets orbiting the most common stars in the Milky Way, red dwarfs, also known as M-type dwarfs These stars are faint and cold compared to our Sun, but very common and long-lived. In fact, account for 80% of all stars of the Milky Way.

“About 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone, an area that allows the existence of liquid water on the planet’s surface,” said Xavier Bonfils of Sciences of the Universe Observatory of Grenoble, in France, who led the team. “Because red dwarfs are so common, there are about 160,000 million in the Milky Way, this leads to the conclusion that there are tens of billions of such planets in our galaxy alone.”

Nine new super-Earths

The HARPS team analyzed over a period of six years a carefully selected sample of the southern sky, which appeared 102 red dwarf stars. We found a total of nine super-Earths (planets with masses between one and ten times the mass of Earth), including two in the habitable zone of Gliese 581 and Gliese 667 C respectively. Astronomers were able to estimate its weight and the distance to the host star around which orbited.

Combining all data, the team found that the frequency of the presence of super-Earths in the habitable zone is 41% ranging from 28% to 95%. However, more massive planets similar to Jupiter and Saturn in our solar system seem to be very common around red dwarfs. It is believed that less than 12% of red dwarfs have giant planets.

Stellar flares

“The habitable zone around a red dwarf, where the temperature is suitable for liquid water on the surface is closer to the star than in the case of Earth from the Sun,” explains Stéphane Udry ( Geneva Observatory researcher and team member). But red dwarfs are known to be subject to eruptions or stellar flares, “which would flood the planet X-ray or ultraviolet radiation and make it more difficult the existence of life.”

One of the planets discovered in the survey of red dwarf Gliese 667 HARPS is Cc. It is the second planet from the triple star system and appears to be located near the center of the habitable zone. Although this planet is more than four times heavier than Earth, is the closest to our planet from those found so far, and almost certainly has the right conditions for the existence of liquid water on its surface. This is the second super-Earth in the habitable zone of a red dwarf discovered during the survey of HARPS, following the announcement of the discovery of Gliese 581 d in 2007 and its subsequent confirmation in 2009.

“We hope that some of these planets pass in front of its parent star during its orbit around it, thus opening the exciting possibility of studying the atmosphere of these planets and signs of life,” concludes Xavier Delfosse.

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