A team of American planet hunters announced the discovery last week of an Earth-like planet in orbit around the nearby red dwarf star Gliese 581. The planet has got the astrobiology community excited because it fulfils several criteria thought necessary for a world to be hospitable to extraterrestrial life.
Weighing in at just 3.1 earth masses, the planet, unimaginatively named Gl581g, is the third lightest planet discovered orbiting a star other than our Sun. It is outdone only by one of its five sister planets, Gl581e, which orbits so close to their parent star that any atmosphere it might once have had has now boiled off into space, and a tiny irradiated rock in orbit around the pulsar PSR 1257+12.
Gl581g’s low mass makes it highly likely that it is a rocky, Earth-like body rather than a gas giant like Jupiter, Saturn and the majority of other extrasolar planets discovered to date. Whilst the mass is low, it is still high enough to allow the world to retain an atmosphere, which is another prerequisite for habitability. Furthermore, the team put the orbital distance of the new planet at 0.146 AU (where 1 AU is the orbital distance of the Earth around the Sun), which is slap-bang in the middle of the star’s so called ‘habitable zone’, defined as the range of orbital radii within which the moderate temperature allows water to exist in liquid form. As well as this stellar habitable zone, there is also a galactic habitable zone, which places limits on the position of a potentially habitable planet’s parent star in the galaxy. Too near the centre, and any planets that form will be sterilised by the flux of high energy radiation from the galactic centre. Too far away, and there are insufficient heavy elements to form the complex molecules that make up life as we know it, not to mention the rocky planets themselves. Until now, only one planet in the Universe was known to have all these factors ‘just right’ to support life, but this new discovery, if confirmed, would bring that figure up to two.
“Our findings offer a very compelling case for a potentially habitable planet,” said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz who led the team along with Paul Butler from the Carnegie Institute of Washington. “The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.”
Gl581g was discovered by the radial velocity or ‘RV’ method, which involves searching for the tiny forward-backward wobble of a star’s motion induced by the gravitational attraction of its daughter planets. The wobble manifests itself as a shifting of the dark absorption lines in the star’s spectrum, first to the blue (short wavelength) as the star is pulled towards us, then to the red (long wavelength) end as the star is pulled away. This phenomenon, ubiquitous in astronomy, is called Doppler shifting, and is the process responsible for the change in pitch of an ambulance siren as it speeds first towards and then away from an observer on the pavement. Vogt and Butler’s team studied the spectrum of the star over a period of 11 years, and by plotting the precise radial velocity implied by the star’s Doppler shift against time, they were able to infer the existence of the ‘Super-Earth’ and its five sisters.
The paper is due to be published in the Astrophysical Journal, and is available now as a preprint from arXiv.org, or from: http://www.nsf.gov//news/longurl.cfm?id=206. Gl581g data on the exoplanet encyclopedia: http://exoplanet.eu/planet.php?p1=Gl+581&p2=g.