Tens of Billions of Planets in Habitable Zones Around Red Dwarfs in the Milky Way

Posted on March 28, 2012

Astronomers estimate that there are tens of billions of rocky planets located in habitable zones around red dwarfs in our galaxy alone. Red dwarf stars are faint and cool compared to the Sun, but they are very common and long-lived. Red dwarfs account for 80% of all the stars in the Milky Way. The researchers estimate that about 40% off all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the planet's surface. There are 160 billion red dwarfs in the Milky Way, so this means there must be tens of billions of super-Earths (planets with masses between one and ten times that of Earth) in our galaxy.

An artist's impression of a sunset seen from the super-Earth Gliese 667 Cc is pictured above. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right.

The international team of researchers has been making observations with the HARPS spectrograph on the 3.6-meter telescope at ESO's La Silla Observatory in Chile. The HARPS team surveyed a sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths were found, including two inside the habitable zones of Gliese 581 and Gliese 667 C respectively.

By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team found that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%. As there are many red dwarf stars close to our Sun the new estimate means that there are probably about 100 super-Earth planets in the habitable zones around stars in the neighborhood of the Sun at distances less than about 30 light-years.

Xavier Bonfils (IPAG, Observatoire des Sciences de l'Univers de Grenoble, France), the leader of the team, says, "Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet. Because red dwarfs are so common - there are about 160 billion of them in the Milky Way - this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone."

The astronomers say Red dwarfs may bombard some of its super-Earth planets with X-rays or ultraviolet radiation, which would make them more hostile to life.

Stephane Udry (Geneva Observatory and member of the team) says, "The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the Sun. But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely."

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