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Astronomy and space

Astronomy and space

Gravitational lensing brings extrasolar planets into focus

10 Jun 2004

Astronomers have used gravitational microlensing to detect a cool planet orbiting a star some 15,000 light-years away

Figure 2

A major breakthrough in the search for new worlds beyond our solar system happened recently with the discovery of a planet using a technique known as gravitational lensing. The new planet is about 1.5 times the mass of Jupiter, and is about half way between the Sun and the centre of the Milky Way. This makes it the most distant extrasolar planet detected by astronomers to date.

The discovery, made by Ian Bond and co-workers in the MOA and OGLE collaborations, represents the debut of a new and faster technique for discovering cool planets that orbit stars at large distances from the Sun (Astrophys. J. at press). Most significant of all, however, is the unique capability of gravitational lensing to discover Earth-like planets from the ground.

Cool planet detector

Today the encyclopedia of extrasolar planets contains about 120 gas giants with masses between about 0.5 and 10 Jupiter masses. Almost all of these were found using the “Doppler wobble” method in which the gravitational pull of a large planet swings the star around the centre of mass of the star–planet system (see “Extrasolar planets”). This wobble introduces periodic Doppler shifts of a few tens of metres per second in the light from the star, and such shifts can now be routinely detected by large telescopes equipped with high-precision spectrographs. A handful of extrasolar planets have also been found using the “transit method”: if the orbit of the planet is such that it passes directly between the host star and the Earth, the planet can be detected by measuring the tiny amount of starlight it blocks as it passes in front of the host star.

However, both the Doppler and transit methods are only sensitive to large planets that orbit relatively close to their parent stars. Smaller and cooler planets that are similar to the Earth escape discovery because their Doppler signatures are small and because they take several years to orbit their host star. On the other hand, gravitational lensing – in which light from a distant star is bent by a massive intervening object – can reveal small, cool planets without having to wait for them to complete an orbit.

The bending of starlight by a massive object is one of the most important predictions of general relativity. In particular, Einstein predicted that the Sun would cause a grazing light ray from a distant star to be deflected by 1.7 arcseconds – a prediction that was famously confirmed by Eddington’s measurements during the total solar eclipse of 1919. Einstein later showed that more distant stars can act as gravitational lenses, and today astronomers routinely use this technique to “weigh” a distant object by simply looking at the effect it has on light.

However, gravitational lenses are imperfect because the rays that pass closest to the lensing mass are deflected more than rays passing further away. This spherical aberration means that an observer looking at a background star through a gravitational lens sees two magnified and distorted images on opposite sides of the lensing star. When the background star, the lens star and the Earth are all perfectly aligned, the two images expand to form an “Einstein ring”.

Stars in our galaxy typically bend light from more distant stars by only a few milliarcseconds, which means that it is not possible to resolve the two images with conventional telescopes. However, the observed brightness of the distant star displays a symmetric rise and fall over a few weeks as the lens star slides past the line of sight. It is this magnification that allows extrasolar planets to be detected (figure 1).

Gravitational microlensing

Planets close to the lens star act like smaller gravitational lenses that can briefly increase or decrease the magnification of the lens. A cool planet in the “lensing zone” – which is typically between 1.5 and 6 times the Earth-Sun distance – can therefore be detected without having to wait for it to complete its orbit. Furthermore, both the duration and probability of planetary-lensing events scale as the square root of the mass of the planet, which means that the technique is also sensitive to low-mass planets.

Large planets like Jupiter, for example, have a 10% probability of being in the right place to act as lenses for a few days, while Earth-mass planets have a 1% probability of alignment and only act as lenses for a few hours. Unlike other methods, the magnification signal in a microlensing event can be large even though the planet is small. However, the finite angular sizes of the source stars mean that gravitational microlensing is not sensitive to planets smaller than Earth.

The MOA (Microlensing Observations in Astrophysics) and OGLE (Optical Gravitational Lensing Experiment) teams used small dedicated telescopes in Chile and New Zealand to scan rich star fields at the centre of our galaxy, and identified over 500 gravitational-lensing events during 2003. Ian Bond of Edinburgh University identified the new planet by noticing a sudden and unexpected increase in the brightness of one of the lensed stars. The signature of the planet was a pair of spikes in the brightness curve: the first occurs when the lensing effect of the planet causes two new images of the source star to appear, and the second spike – which appeared seven days later – happens when the extra images merge and disappear.

The main challenge in planet searches based on microlensing is to detect the brief and rare planet-lensing anomalies while scanning vast areas of the sky. Moreover, to be able to detect a Jupiter-sized planet the light curves must be measured at least twice per day – and at least twice per hour for Earth-sized planets. Bond’s vigilance enabled him to spot the planet anomaly in its early stages, and therefore to step-up the observational campaign before it was too late.

Now that gravitational lensing has secured its first new planet, we can expect the pace of discovery of extrasolar planets to increase as teams of astronomers join forces to build a network of dedicated observing facilities. The 1.3 m OGLE telescope at the Las Campanas Observatory in Chile and the MOA 0.6 m telescope at the Mount John observatory in New Zealand find 500-700 microlensing events each year. In addition, the PLANET and microFUN collaborations operate a series of smaller telescopes in Israel, South Africa, Chile and Australia to provide 24 hour coverage of the most promising lensing events. The RoboNet experiment in the UK has joined the quest this month, linking three 2 m robotic telescopes – the Liverpool Telescope on La Palma in the Canary Islands and the two Faulkes telescopes in Hawaii and Australia.

Beyond the excitement of discovering new worlds, the scientific goal of this work is to measure the abundance and mass distribution of cool planets with masses similar to the Earth and above. If cool Earth-like planets turn out to be relatively abundant, gravitational microlensing could uncover them within 3-5 years.

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