27 June 2013

A nearby star has a record-breaking three super-Earths lying in the habitable zone where liquid water could exist according to an international team of astronomers led by Guillem Anglada-Escude, University of Goettingen, Germany, and Mikko Tuomi, University of Hertfordshire, UK.

New observations reveal three super-Earths

New observations of the star, known as Gliese 667C, have been combined with existing data to reveal a system with at least six planets. Three of these planets are confirmed to be super-Earths — planets more massive than Earth, but less massive than planets like Uranus or Neptune — that are within their star’s habitable zone, a thin shell around a star in which water may be present in liquid form if conditions are right and making them possible candidates to support life. This is the first time that three such planets have been spotted orbiting in this zone in the same system - completely filling up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist at the right distance to it.

Mikko Tuomi said: “We knew that the star had three planets from previous studies, so we wanted to see whether there were any more. By adding some new observations and revisiting existing data we were able to confirm these three and confidently reveal several more. Finding three low-mass planets in the star’s habitable zone is very exciting!”

In the Sun’s neighbourhood

Gliese 667C is a very well-studied star and is part of a triple star system known as Gliese 667 in the constellation of Scorpius. At 22 light-years away, this is quite close to us — within the Sun’s neighbourhood — and much closer than the star systems investigated using telescopes such as the planet-hunting Kepler space telescope.

Previous studies of Gliese 667C had found that the star hosts three planets with one of them in the habitable zone. The team added new observations from HARPS (the High Accuracy Radial velocity Planet Searcher) at ESO (European Southern Observatory) 3.6 metre in Chile, along with data from ESO's Very Large Telescope, the W.M. Keck Observatory and the Magellan Telescopes, to the already existing picture.

“The number of potentially habitable planets in our galaxy is much greater if we can expect to find several of them around each low-mass star — instead of looking at ten stars to look for a single potentially habitable planet, we now know we can look at just one star and find several of them,” adds co-author Rory Barnes (University of Washington, USA).

Habitable zone much closer than our Sun

Compact systems around Sun-like stars have been found to be abundant in the Milky Way. Around such stars, planets orbiting close to the parent star are very hot and are unlikely to be habitable. But this is not true for cooler and dimmer stars such as Gliese 667C. In this case the habitable zone lies entirely within an orbit the size of Mercury's, much closer in than for our Sun.

The Gliese 667C system is the first example of a system where such a low-mass star is seen to host several potentially rocky planets in the habitable zone.

This is analogous to our Solar System where Venus orbits close to the inner edge of the habitable zone, Earth in the middle and Mars close to the outer edge. The precise extent of the habitable zone depends on many factors.

Anglada-Escudé concludes: “These new results highlight how valuable it can be to re-analyse data in this way and combine results from different teams on different telescopes.”

Image credit: ESO/M. Kornmesser. Artist’s impression shows the view from the exoplanet Gliese 667Cd looking towards the planet’s parent star (Gliese 667C).