Billions and billions?

An artist’s impression shows a sunset seen from the super-Earth Gliese 667Cc. The brightest star in the sky is the red dwarf Gliese 667C, which is part of a triple-star system. The other two more distant stars, Gliese 667A and B, appear in the sky also to the right.
(Courtesy: ESO/L Calçada)

By Tushna Commissariat

The field of exoplanetary research has been abuzz over the past year as the first exoplanet – called Gliese 581d – was found within the habitable zone of a star last May and three others were found by March this year. Most of these are super-Earth planets – exoplanets that are two to 10 times more massive than the Earth.

Today, new results from the European Southern Observatory’s (ESO) High Accuracy Radial velocity Planet Searcher (HARPS) instrument reveal that rocky super-Earths might abound in our galaxy, orbiting within the habitable zones around faint red stars. The international team of researchers claims that there may be tens of billions of such planets in the Milky Way alone, and probably about 100 in the Sun’s immediate neighbourhood.

The work is described in two papers here and here that are currently available on the arXiv preprint server, one of which will soon be published in the journal Astronomy & Astrophysics. This is the first direct measurement of the frequency of super-Earths around red dwarfs – faint and cool stars compared with the Sun that are common and long-lived – as they account for 80% of the stars in the Milky Way.

“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 from the University of Grenoble, France, who is leader of the HARPS team. “Because red dwarfs are so common – there are about 160 billion of them in the Milky Way – this leads us to the astonishing conclusion that there are tens of billions of these planets in our galaxy alone,” he says.

The HARPS team surveyed a carefully chosen sample of 102 red-dwarf stars in the southern skies over a six-year period. Over this time the researchers found nine super-Earths, of which two reside inside the habitable zones of Gliese 581 and Gliese 667C, respectively. The astronomers were able to estimate how heavy the planets are and how far from their stars they orbit.

From a study of all the data, including observations of stars that did not have planets, the team says that the frequency of occurrence of super-Earths in the habitable zone is 41%, in a range from 28% to 95%.

In addition, the researchers say that more massive planets – gas giants like Jupiter and Saturn in our solar system – are rarely found orbiting red dwarfs, with less than 12% of them having such giant planets in their systems. All of this means that there is the exciting possibility of 100 super-Earth planets in close vicinity to us, given that there are many red-dwarf stars close to the solar system at distances of 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,” says Stéphane Udry from the Geneva Observatory, who is also 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 so make life there less likely.”

One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667Cc. This is the second planet in this triple-star system and it orbits almost exactly in the centre of the habitable zone of its parent star. Although more than four times heavier than the Earth, it is considered to be the Earth’s closest twin found so far and almost certainly has the right conditions for the existence of liquid water on its surface.

“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, which would open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life,” concludes Xavier Delfosse, one of the authors of the papers.