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Scientists 'microlens' Earth-like exoplanet

Smallest one to date

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An international colloboration of astronomers has used a microlensing technique to spot an Earth-like exoplanet just five times bigger than our own piece of rock - the smallest such body discovered so far, and only the third pinpointed with microlensing from a list of around 160 known exoplanets.

The snappily-named OGLE-2005-BLG-390Lb is orbiting a red dwarf "roughly five times less massive than our Sun", 25,000 light years from Earth, close to the centre of our own Milky Way galaxy. Its orbital period is ten years and it lies around three times further from its parent as we do from the Sun.

The microlensing technique uses the "gravity of a dim, intervening star" as a "giant natural telescope, magnifying a more distant star, which then temporarily looks brighter", as a UK Physics & Astronomy Research Council statement explains. It also says: "A small 'defect' in the brightening reveals the existence of a planet around the lens star. The planet is not directly 'seen', or even the star that it's orbiting, but its presence can be deduced from the effect of its gravity."

Three separate microlensing outfits were involved in the OGLE-2005-BLG-390Lb project: PLANET/RoboNet, OGLE, and MOA, which involved 73 collaborators affiliated with 32 institutions in 12 countries (Australia, Austria, Chile, Denmark, France, Germany, Japan, New Zealand, Poland, South Africa, UK and the US).

MOA collaboration member, Dr Nicholas Rattenbury of Jodrell Bank Observatory, noted: "The chance that an observed background star is sufficiently magnified at a given time is only about one in a million. However, with more than 100 million stars routinely monitored by the OGLE and MOA surveys, there are about 120 ongoing events at any one time where the lens star can be probed for surrounding planets - leading to the prospect of seeing many more low mass objects."

The astronomers did, however, hit paydirt with OGLE-2005-BLG-390Lb. PLANET co-leader Dr Martin Dominik, of the University of St Andrews, explained: "We first saw the usual brightening reaching a peak magnification on July 31, 2005, after which the event started to fade back symmetrically. On August 10, however, there was a small 'flash' lasting about half a day. By succeeding in catching this anomaly with two of the telescopes of our network and with careful monitoring, we were able to conclude that the lens star is accompanied by a low-mass planet."

Straying from the pure science for a moment, the statement allows itself the indulgence of imagining how OGLE-2005-BLG-390Lb might be: "Its relatively cool parent star and large orbit implies that the likely surface temperature of the planet is 220 degrees centigrade below zero, far too cold for liquid water, which would be a requirement for the development of life.

"It is likely to have a thin atmosphere, like Earth, but its rocky surface is probably buried deep beneath frozen oceans. It may therefore more closely resemble a more massive version of Pluto, rather than the inner rocky planets like Earth and Venus." ®

More background on the microlensing collaboration

[From the Particle Physics & Astronomy Research Council press release]

In order to catch and characterise a planet, round-the-clock high-precision monitoring of ongoing microlensing events is required, which is achieved by the PLANET network of 1m-class telescopes consisting of the ESO 1.54m Danish at La Silla (Chile), the Canopus Observatory 1.0m (Hobart, Tasmania, Australia), the Perth 0.6m (Bickley, Western Australia), the Boyden 1.5m (South Africa), and the SAAO 1.0m (Sutherland, South Africa). PLANET operates a common campaign with RoboNet, the UK operated network of 2m-class, fully robotic telescopes currently comprising the Liverpool Telescope (Roque de Los Muchachos, La Palma, Spain) and the Faulkes Telescope North (Haleakala, Hawaii).

The other two microlensing planets have masses of a few times that of Jupiter, where the detection of one of them, OGLE-2005-BLG-071Lb, involved PLANET/RoboNet data taken in April 2005. The low number of detections for planets of Jupiter-mass or above, despite being easier to detect, indicates that gas giant planets are rare around red dwarfs. All three detected planets have red dwarf parent stars, which are very dim but also very common and therefore favoured by the microlensing searches that rely on the gravity, rather than the light, of the lens star. The discovery of a sub-Neptune mass object already as the third one detected via microlensing is a strong hint that these objects in contrast are quite common. In fact, "core-accretion" planet-formation simulations predict a large fractional abundance of planets with masses below 10 Earth masses, with orbits of up to 10 AU (Astronomical Unit). By coincidence, these orbital separations match well the range preferred by microlensing, making it an ideal technique for studying this population down to Earth mass.

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