An international group of astronomers led by Dr. Jean-Philippe Beaulieu (Institut d'Astrophysique de Paris) and Dr. Martin Dominik (University of St Andrews) are about to continue their hunt for extrasolar planets with an enhanced world-wide telescope network in May this year.

They are hoping to secure the firm evidence for the existence of Earth-mass planets orbiting stars other than the Sun, which has so far eluded astronomers. Dr Dominik will describe the project, known as PLANET (Probing Lensing Anomalies NETwork), at the Royal Astronomical Society National Astronomy Meeting at the Open University on Thursday.

Recent scientific research shows that the existence of life on other worlds is a realistic scenario. By measuring the periodic variation of the radial velocity of stars induced by an orbiting planet, astronomers have so far detected over 100 planets but all of them are large, similar to Jupiter and Saturn in our solar system, and environmental conditions suitable for life do not exist on such gas giant planets.

The only technique currently capable of detecting planets similar to Earth makes use of the phenomenon called "galactic microlensing." In a microlensing event, a star temporarily appears brighter than it really is because another astronomical body is passing between it and observers on Earth; the gravitational field of the intervening object affects the starlight in a way similar to a lens.

If the intervening object is a star, it causes a characteristic signal that lasts about a month. Any planets orbiting this star can produce significant deviations in the signal, lasting days for giant planets down to hours for Earth-mass planets. The probability of this happening is between 1.5% and 20% depending on the mass of the planet.

The PLANET campaign performs nearly-continuous round-the-clock high-precision monitoring of ongoing microlensing events, sampling the lightcurve at intervals that may be as little as few minutes with a world-wide network of telescopes. The backbone of the network is formed by the Danish 1.54-m telescope at the European Southern Observatory at La Silla (Chile), the Canopus Observatory 1.0-m telescope (Tasmania, Australia), the Perth 0.6-m telescope (Western Australia), and the Boyden 1.5-m telescope (South Africa), which is supplemented by some other telescopes.

PLANET will share information and some resources with the microlensing campaign performed with RoboNet, a UK robotic telescope network comprised of the Liverpool 2.0-m (Canary Islands, Spain) and the two Faulkes 2.0-m telescopes (Hawaii and Australia).

From the 500-700 microlensing events announced annually by the survey campaigns OGLE and MOA that monitor tens of millions of stars on a daily basis, PLANET focuses on up to 75 events that are selected as most suitable candidates for the detection of planets around the intervening lens star. "If 20% of these stars are surrounded by planets, 10-15 giant planets and 1 or 2 terrestrial planets are expected to reveal their existence over three years of operation", Dr. Dominik said.

While PLANET might detect a second Earth, its typical expected distance would be 20,000 light years - much too far to think of establishing any contact!