Billions of Habitable Worlds Likely in the Milky Way

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Could there be ‘tens of billions’ of habitable worlds in our own galaxy? That’s the results from a new study that searched for rocky planets in the habitable zones around red dwarf stars. An international team of astronomers using ESO’s HARPS spectrograph now estimates that there are tens of billions of such planets in the Milky Way galaxy, with probably about one hundred in the Sun’s immediate neighborhood, less than 30 light years away.

“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,” said Xavier Bonfils, from IPAG, Observatoire des Sciences de l’Univers de Grenoble, France, and the 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.”

This is the first direct estimate of the number of smaller, rocky planets around red dwarf stars. Add this to another recent finding which suggested that every star in our night sky has at least one planet circling it — which didn’t include red dwarf stars – and our galaxy could be teeming with worlds.

This team used the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile to search for exoplanets orbiting the most common kind of star in the Milky Way — red dwarf stars (also known as M dwarfs). 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 the Milky Way.

The HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths (planets with masses between one and ten times that of Earth) 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 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%.

Bonfils and his team also found that rocky planets were far more common than massive gas giants like Jupiter and Saturn. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1000 times that of the Earth).

However, the rocky worlds orbiting red dwarfs wouldn’t necessarily be a good place to spend your first exo-vacation – or for harboring life.

“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 Stéphane Udry from the Geneva Observatory and 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.”

New Exoplanet Discovered

A new exoplanet was discovered in this HARPS survey of red dwarfs: Gliese 667 Cc. This is the second planet in this triple star system and seems to be situated close to the center of the habitable zone. Although this planet is more than four times heavier than the Earth it is the closest twin to Earth 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 planet inside the habitable zone of a red dwarf discovered during this HARPS survey, after Gliese 581d was announced in 2007 and confirmed in 2009.

“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,” said Xavier Delfosse, another 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.”

Research papers: Bonfils et al. and Delfosse et al.

Source: ESO