February 20, 2008 With upwards of 100 billion stars in our own Milky Way and at least that number of galaxies in the observable universe, the odds have long pointed to the likely existence of planets beyond our own solar system. The first discovery of such an extra-solar planet to receive subsequent confirmation took place in 1988 and two decades later, as detection techniques and equipment continue to improve, that number is now approaching 300. Now news that Astronomers from the University of St Andrews have found a planetary system some 5,000 light years away that bears "striking similarities" to our Solar system.

Dr Martin Dominik and Professor Keith Horne made the find as part of an international team using a world-wide net of telescopes.

Labeled (not very poetically) OGLE-2006-BLG-109Lb and OGLE-2006-BLG-109Lc, the two new planets are similar to Jupiter and Saturn and orbit a star half the mass of the Sun. Both the ratio between the two masses of the detected giant planets (close to 3:1) and the ratio between their orbital radii (1:2) are remarkably similar to those of Jupiter and Saturn.

Dr Dominik, Royal Society University Research Fellow at the St Andrews' School of Physics & Astronomy, said, "Our gravitational microlensing technique is currently best suited for studying extra-solar planets that resemble the gas giants of the Solar System. This system of planets bears a remarkable similarity to our Solar System, with the ratios of their properties remarkably close to those of Jupiter and Saturn.

The research also points to the likelihood of smaller planets being found alongside gas-giants.

"Our finding also suggests that lonesome gas-giant planets are the exception and planetary systems are the rule. It seems that planets do not like being lonely hearts."

"The lack of further planetary signals makes it plausible that the more massive of the two planets - OGLE-2006-BLG-109Lb - is the innermost giant planet orbiting its parent star, leaving room for terrestrial planets (like Mercury, Venus, Earth, and Mars) to reside inside its orbit. Moreover planets taking the role of Uranus and Neptune, respectively, could also be present," explained Dr Dominik.

We may not have long to wait before the existence of planets similar to our own is confirmed according to Professor Keith Horne: "While most planetary systems around other stars substantially differ from the Solar system, a series of recent detections have brought us closer and closer to home. Sooner rather than later, someone can be expected to discover an Earth-mass planet orbiting a star other than the Sun - and it could be us. In fact, with our ARTEMiS (Automated Robotic Terrestrial Exoplanet Microlensing Search) expert system, even less massive planets could bedetected," he said.

Dominik and Horne also contributed to the discovery of an Earth-like planet in early 2006 using the same gravitational microlensing technique.http://octans.st-andrews.ac.uk/ARTEMiS/Microlensing_freefrom.html

Noel McKeegan

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