Planets by the Billions

New calculations suggest that there may be as many as 60 billion habitable planets just in our own Milk Way galaxy, maybe more. At least 100 of these planets may be as close as 30 light-years away.

Researchers at the European Southern Observatory (ESO) carefully sampled 102 red dwarf stars in the southern skies. Red dwarfs are very common in our galaxy. Based on observations and calculations, the ESO team estimates that approximately 40% of the red dwarf stars are orbited by planets. What’s more, these planets are in what astronomers call the “habitable zone,” meaning they are neither too close nor too far from their sun. In particular, it means that the temperature may be right for liquid water to exist on the planet’s surface.

The ESO project is the work of an international team using observations with the HARPS spectrograph on the 3.6-metre telescope at ESO's La Silla Observatory in Chile. It is published in the March 28 issue of Astronomy & Astrophysics.

Caption: This artist's impression shows a sunset seen from the super-Earth Gliese 667 Cc. 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. Astronomers have estimated that there are tens of billions of such rocky worlds orbiting faint red dwarf stars in the Milky Way alone. Credit: ESO/L. Calçada. Usage Restrictions: None

"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," says Xavier Bonfils in a press release issued by ESO.

"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," according to Bonfils.

"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," says Stephane Udry, another member of the ESO team.

Red dwarfs, however, may pose a special challenge to life. According to Udry, "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." In the more technical language of the scientific publication, “The main difference from earth are a significantly higher mass and a different stellar environment, which potentially can have caused divergent evolutions.”

All the more tantalizing, of course. As one of the ESO scientists puts it, "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. 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," concludes Xavier Delfosse.

The article, "The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample", by Bonfils et al. appears in Astronomy & Astrophysics on March 28.