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Looking through a Gravitational Lens

Alan Gilmore

Astronomers are watching how light "bends" to spot unseen planets and pulsating stars, with a New Zealand-Japanese team reporting success in using the technique to find a planet smaller than usual and further away from its parent star.

At the recent Royal Astronomical Society of New Zealand conference, researcher Philip Yock demonstrated how gravitational lensing can reveal low-mass planets at around one astronomical unit (i.e. Earth's distance or 150 million kilometres) from their parent stars. Most extra-solar planets detected to date have been large Jupiter-mass objects only a fraction of an AU from their star.

If a star, or any compact mass, crosses our line of sight to a distant star then the far star's light is focused at us by the gravity of the nearer "lensing" object. When both stars are exactly aligned, the distant one appears as a ring around the closer. This "Einstein ring" is about 2 AU in radius at the lensing star (far too small to be seen). A planet near that distance perturbs the ring and causes a "bump" on the normally symmetrical brightness peak produced by the crossing star.

One such bump observed by the NZ-Japanese MOA team indicates a planet in a mass range from Earth to twice Neptune's -- definitely lighter than Saturn or Jupiter. Its distance from the star is between 0.6 and 2.8 AU, equivalent to somewhere between Venus and the asteroid belt in our system.

Yock noted that detailed mathematical modelling of these events is not a simple job, with one group using a "Gigablaster" system consisting of 200 PCs operating in parallel.

For some years several gravitational lensing projects have been searching for compact dim objects: brown, red and white dwarfs, neutron stars and small black holes; anything that might contribute to the galaxy's unexplained excess gravity. Lensing results now rule out brown and white dwarfs and other objects in the planet- or solar-mass range. Future observing plans include continuous monitoring of the galactic bulge from the south pole and possibly from space.

Alan Gilmore works at the Mt John Observatory in Tekapo