'More planets than stars' in galaxy

UNIVERSITY OF TASMANIA

“It seems very likely that there are more Earth-mass planets than stars in our galaxy.” Image: adventtr/iStockphoto New research has found planets around stars are the rule rather than the exception – there are more planets in the Galaxy than there are stars. An international team which includes five astronomers from the University of Tasmania has made the discovery. The team searched for exoplanets (planets outside the Solar System) using gravitational microlensing, a little-known technique for planet-finding. Gravitational microlensing can detect planets over a much wider range of masses and distances from their parent stars (stars that give the planets light and warmth) than other methods. In six years of observations, the Probing Lensing Anomalies NETwork (PLANET) and the Optical Gravitational Lensing Experiment (OGLE) searches discovered three exoplanets on their own and seven more in co-operation with other survey teams.  Although the number of planets that were detected is small, statistical analysis of the microlensing process shows that this is a truly impressive haul. To detect these planets, astronomers have either hit a jackpot despite huge odds against them, or planets are so abundant in the Galaxy that their discovery is almost inevitable. Dr Arnaud Cassan, from the Institut d’Astrophysique de Paris, is lead author of the research paper titled One or more bound planets per Milky Way star from microlensing observations. He said it was also discovered that lighter planets, such as super Earths (planets with mass 3-10 times the mass of the Earth) or those of about Neptune size, are much more common than heavier ones. Dr Daniel Kubas, co-lead author of the paper, said we used to think Earth was at the centre of the Universe and planets were rare. “But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way,” he said. Dr John Greenhill, from the UTAS School of Maths and Physics, was the UTAS team leader on the research project. "Our analysis shows that Earth-sized and smaller planets are even more common than suspected,” said Dr Greenhill. “It seems very likely that there are more Earth-mass planets than stars in our galaxy.” The UTAS observations used the Mt Canopus one-metre telescope in the Meehan Ranges near Hobart. The Institut d’Astrophysique de Paris and several other institutions around the world helped make the observations. The research paper, One or more bound planets per Milky Way star from microlensing observations, has appeared in the journal Nature. Editor's Note: Original news release can be found here.