Billions of super-Earths “out there”

Artist's impression of a planet circling a red dwarf star

Artist's impression of a planet circling the star Gliese 581. Astronomers estimate there could be tens of billions of "super-Earth" planets in our galaxy.

ROCKY PLANETS NOT MUCH BIGGER THAN EARTH are very common in the habitable zones around faint red stars, say astronomers.

The habitable zone is the distance from a star where it is neither too hot nor too cold for liquid water to exist on the surface of a rocky planet.

The international team used a “planet finder” instrument to estimate that there are tens of billions of such planets in the Milky Way galaxy alone, and probably about 100 in the Sun’s immediate neighbourhood.

This is the first direct measurement of the frequency of super-Earths around red dwarfs, which account for 80% of the stars in the Milky Way.

This first direct estimate of the number of light planets circling red dwarf stars used observations made with the HARPS spectrograph on the 3.6-metre telescope at the European Southern Observatory’s La Silla Observatory in Chile.

Super-Earths abound

The HARPS team has been searching 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.

“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone…,” says Xavier Bonfils (IPAG, Observatoire des Sciences de l’Univers de Grenoble, France), the leader of the team.

Diagram showing the habitable zone for small, medium and large stars.

Diagram showing the habitable zone (green area) varies depending on the size and temperature of the star. Too close in (red area) and it's too hot; too far out (blue area) and it's too cold.

“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. 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 stars 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 in red dwarf systems.

They find that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%.

On the other hand, more massive planets, similar to Jupiter and Saturn in our Solar System, are found to be rare in red dwarf systems. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1,000 times that of the Earth).

In the zone

As there are many red dwarf stars close to the Sun the new estimate means that there are probably about 100 super-Earth planets in the habitable zones around stars in the neighbourhood 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 [a red dwarf] star than the Earth is to the Sun,” says Stéphane Udry (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.”

One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667 Cc. This is the second planet in this triple-star system and it seems to be situated close to the centre of the habitable zone.

Artist’s impression of a sunset seen from the super-Earth Gliese 667 Cc

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.

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.

Gliese 667 Cc 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,” concludes Xavier Delfosse, another member of the team.

Some of these planets are expected to pass in front of, or transit, their parent star as they orbit, and astronomers can use these transits to learn more about the planets’ atmospheres and look for signs of life.

Adapted from information issued by ESO / L. Calçada.

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