An international team of astronomers working with the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile has found that there are tens of billions of rocky planets in the Milky Way galaxy alone, and probably about one hundred in the Sun’s immediate neighborhood.
An artist's impression of a sunset seen from the super-Earth Gliese 667 Cc (ESO / L. Calçada)
The HARPS team has been searching for exoplanets orbiting red dwarf stars in the Milky Way. These stars are faint and cool compared to the Sun, but very common and long-lived, and therefore account for 80% of all the stars in our galaxy.
“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,” explained Dr. Xavier Bonfils of the Observatory for Science and the Universe in Grenoble, France, a leader of the team. “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 HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. They found nine super-Earths, including two inside the habitable zones of Gliese 581 and Gliese 667 C respectively. The astronomers could estimate how heavy the planets were and how far from their stars they orbited.
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 has been able to work out how common different sorts of planets are around red dwarfs. They find that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%. The study will be published in the journal Astronomy & Astrophysics.
On the other hand, more massive planets, similar to Jupiter and Saturn in our Solar System, are found to be rare around red dwarfs. Less than 12% of red dwarfs are expected to have giant planets with masses between 100 and 1000 times that of the Earth.
As there are many red dwarf stars close to the Sun the new estimate means that there are probably about one hundred super-Earth planets in the habitable zones around stars in the neighborhood of the Sun at distances less than about 30 light years.
“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,” said Dr. Stephane Udry of the Geneva Observatory, a member of the team. “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.”
“Now that we know that there are many super-Earths around nearby red dwarfs we need to identify more of them using both HARPS and future instruments,” added Dr. Xavier Delfosse, a member of the team. “Some of these planets are expected to pass in front of their parent star as they orbit – this will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life.”
