The newly-discovered planet is exciting because it has a number of attributes that are similar to those that astronomers believe are needed to exist to support life, and indicates that these types of planets might be common beyond our Solar System.

The small, cool planet — called in typical astronomical terms ‘OGLE-2005-BLG-390Lb’ — orbits a star in the inner Milky Way, weighs in at around five times the mass of Earth and is one of the least massive discovered so far. It orbits its parent star at a distance three times the distance of Earth from the Sun, and one orbit takes 11 Earth years.

The planet was identified by astronomers from 32 institutions in 12 countries working on a collaborative project, and included the Director of the Research School of Astronomy and Astrophysics (RSAA) at ANU, Professor Penny Sackett. The results of their research are published in the latest edition of Nature.

“This tremendously exciting discovery provides tantalising information about the potential of our galaxy to house another Earth-like planet, where conditions are conducive to some form of life,” Professor Sackett said.  “We’re getting closer to discovering an Earth-twin.”

According to the researchers, the new planet’s parent star (the Earth’s parent star is the Sun) is most likely an M dwarf star, the most common type of star in our galaxy. Current theory predicts that planets forming around such stars are Earth-sized to Neptune-sized, and have a restricted orbit size of under 10 AU – the Earth orbits its star, the Sun, at a distance of 1 AU.

Unlike the Jovians — large, gaseous Jupiter-like planets, some orbiting close to their parent star — which make up most of the 170 planets discovered outside of the Earth’s Solar System so far, the new planet is much smaller, and is likely to be made of rock and ice.

Its parent star is also diminutive, only 0.2 times as massive as the Sun, which means that it emits less heat, putting the surface temperature of the newly-discovered planet at a chilly -225 degrees Celsius. The star is about 23,000 light-years away from us.

The new planet was found using the microlensing technique, which has the potential to detect much smaller planets than those that can be found using other techniques.

Microlensing occurs when a star passes in front of a background star. The gravitational field of the foreground star acts as a lens, bending and focussing the light of the background star.

As the foreground star passes across, the light from the background star brightens and then fades in a very characteristic manner. Planets orbiting the lensing, foreground star produce secondary bumps on the light curve. Analysis of the light curve gives the planet-to-star mass ratio, the planet-star separation and the geometry of the lensing system.

The Optical Gravitational Lensing Experiment (OGLE) team, based in Poland, started recording the microlensing event in early July. A network of lensing teams around the world capable of keeping the event under constant observation sprang into action, with telescopes in Chile, New Zealand, Australia, South Africa, Hawaii and the Canary Islands monitoring the event. Most of the measurements were made by the international ‘PLANET’ collaboration, co-founded just over 10 years ago by Professor Sackett and now led by Dr J-P Beaulieu from the Institut d’Astrophysique de Paris.        

The geographical location of Australia allowed Australian researchers to record key data for this record-breaking event. In August, the tell-tale planetary bump appeared and observations from Perth Observatory and Canopus Observatory in Tasmania were vital in tying down the peak and the shape of the stellar and planetary curves.

“This demonstrates that the microlensing technique can detect low-mass, solar system-like planets. The discovery of an analogue of the Earth is within our reach,” Dr John Greenhill, from the University of Tasmania, said.

Professor Sackett, PLANET’s principal investigator for several years, said: “This amazing discovery required worldwide cooperation and teamwork. It's delightful to see the crucial role played by the original Australian team.

“Some things are worth popping the cork on a bottle of good champs,” Professor Sackett added. “This is definitely one of them.”

ANU contact:
Professor Penny Sackett
Research School of Astronomy and Astrophysics, ANU and
Co-founder of the PLANET team
T: +61 2 6125 0266
E: director@mso.anu.edu.au

Other Australian contacts:
Dr John Greenhill
University of Tasmania
School of Mathematics and Physics
T: +61 3 6226 2429
E: John.Greenhill@utas.edu.au

Dr Andrew Williams
Perth Observatory
T: +61 8 9293 8255
E: andrew@physics.uwa.edu.au

Further information:

ANU Media Office
Jane O’Dwyer (02) 6125 5001 / 0416 249 231 / Jane.ODwyer@anu.edu.au
Amanda Morgan (02) 6125 5575 / 0416 249 245 / Amanda.Morgan@anu.edu.au