9 November 2012

Super-Earth found in habitable zone

A new super-Earth has been found that could have liquid water and therefore support life.
Super Earth exoplanet habitable zone

This artist’s impression shows HD40307g in the foreground (on the left), with its host star HD40307 and two other planets in the system on the right. Credit: J. Pinfield, for the RoPACS network at the University of Hertfordshire

LONDON: A new super-Earth has been found that could have liquid water and therefore support life.

The new planet is one of six orbiting a dwarf star approximately 42 light years away, and is the only one within the habitable zone where an Earth-like climate could exist.

This is “the detection of the first super-Earth planet in the stellar habitable zone of a rather Sun-like star,” said astronomer Mikko Tuomi, from the University of Hertfordshire in Britain, who is the lead author of the paper published in the journal Astronomy & Astrophysics.

Six super-Earth system

The research team used pioneering software called HARPS-TERRA to re-analyse data previously collected by the HARPS (High Accuracy Radial velocity Planet Searcher) instrument at ESO’s la Silla Observatory in Chile.

They discovered that what was originally thought to be a three-planet system orbiting the star HD 40307, was in fact a six-planet system. Remarkably, all of these planets are super-Earths and as such have a mass between one and 10 times that of Earth. The absence of any massive planets within the system means that the signals from the new planets were easier to detect using the new software.

The star itself is a dwarf and smaller and less luminous than our Sun, but it is of a similar age and stable. The liquid-water habitable zone therefore occurs much closer than it does in our own Solar System, at a distance of between 0.43AU and 0.85AU (1AU, or astronomical unit, is the distance from the Earth to the Sun).

Surface conditions

The inner five planets are all too close to be in this region and the researchers indicate they are most likely Neptune-like proto-gas giants, although it is possible some may be hot super-Earths with rock based composition. The increased effect of stellar gravity indicates that they would be tidally locked – where the same side faces the star at all times, generating extremes of temperature.

It is the sixth and outermost planet, known simply as HD 40307g that is of key interest. It has mass approximately seven times that of Earth and an orbital period or ‘year’ of 200 days. At a distance of about 0.6AU from the star, it sits comfortably within the habitable zone, even allowing for variations within its orbit. In addition, the researchers believe that conditions are right for it to be rotating on its own axis creating periods of day and night – vital for a stable atmosphere similar to Earth’s to evolve.

Though situated in an ideal place, it is not yet known what conditions are like there. “The temperature is likely similar or slightly lower than we have on Earth,” explains Tuomi. The planet is estimated to receive only 62% of the radiation that we receive from the Sun. “However, we do not know anything about the composition of HD 40307g – it could have a rocky surface or even be covered by a giant ocean.”

With current detection methods it is not possible to confirm the physical and geochemical properties of the planets found. “We will likely have to wait until there are next-generation space telescopes capable of direct imaging,” said Tuomi.

Although not the first planet observed in the habitable zone of a Sun-like star – in 2011 NASA identified Kepler 22b (600 light years from Earth) – it is by far the closest. The researchers hope, therefore, it would be the first to benefit from direct imaging technology when it comes into place.


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