In 1915, Einstein predicted that gravity should bend light (more than that predicted by the gravitational theory of Newton), and observations of the 1919 Total Solar Eclipse proved it. Stars almost directly behind the Sun during a solar eclipse showed exactly the position change predicted.
The gravity of distant stars bends light as well, distorting and magnifying the sky behind them, but this effect is only visible if there is another star almost perfectly in line with the 'lens' star. Finding these near-perfect 'gravitational lenses' has only been possible since the mid-90's. Now, large telescopes run by the Polish-American OGLE (Optical Gravitational Lensing Experiment) team and New Zealand-Japanese MOA (Microlensing Observations in Astrophysics) collaboration, find hundreds of these events every season after monitoring many millions of stars.
The diagram (above) illustrates a typical microlensing event in the Milky Way. The source star is a bright (giant class) star in the bulge of the Milky Way, far enough above or below the disk to be visible past the dark dust clouds in the plane of the main disk of the galaxy. The lens star is a dim (main-sequence) star like our sun, part-way in between us and the source star, inside the disk itself. The lensing of the source star manifests itself by the brightening then dimming back to its average brightness. This change in brightness is dependent not only on lensing but also the relative motion of the source and lens stars; before the event the two stars are unaligned, then at the peak lensing brightness they are aligned, then as they separate the source star returns to its average brightness.
While the gravity of a single star generates a perfect lens effect, two or more masses give a distorted magnification pattern (as in the accompanying diagram), like projecting light through an oddly-shaped lump of glass. This can range from a large effect (like looking through the base of a wine glass), all the way down to small-scale changes, like a small flaw in an otherwise perfect lens. Although the nearby star (whose gravity is bending the light) is often so dim that it's not visible at all, the exact pattern of 'distortion' seen in the brightness of the background star can be used to calculate the masses and separation of the elements of the gravitational lens. Detection of a small distortion in lensing can indicate the presence of a plant as in the detection of the"Earth-like" planet "OGLE-2005-BLG-390Lb".
The PLANET network
In order to characterize microlensing events, nearly-continuous round-the-clock high-precision monitoring is required. This is achieved by the PLANET (Probing Lensing Anomalies NETwork) network of 1m-class telescopes:
- The Perth-Lowell Automated 0.6m Telescope at Perth Observatory (Bickley, Western Australia)
- The Boyden 1.5m telescope (South Africa)
- The SAAO 1.0m (Sutherland, South Africa)
- The Danish 1.54m at the ESO LaSilla (Chile)
- The Canopus Observatory 1.0m (Hobart, Tasmania, Australia)
Perth Observatory plays an essential role in the PLANET collaboration because of our position on the map. Detecting short-lived fluctuations like the one caused by this planet requires observations every hour or so, 24 hours a day. Observations from Perth Observatory (and Canopus Observatory in Tasmania) fill the large time-zone gap between Chile and South Africa in the Southern Hemisphere.
Since 2005, PLANET has been collaborating with RoboNet, a UK-operated robotic telescope network consisting of the Liverpool 2.0m (Roque de Los Muchachos, La Palma, Spain) and the Faulkes North 2.0m (Haleakala, Hawaii, USA). This network will be enhanced by the Faulkes South 2.0m (Siding Springs, Australia) from 2006.
