The first exoplanet - that is a planet in a distant solar system - was discovered eight years ago. It was identified due to the effect that the central star together with its planet circulates around the common centre of gravity, and this can be observed by a spectral shift of the "sun". However, this method allows only to identify relatively large planets - at least by terrestric telescopes.
Gravitational lensing makes it possible to identify exoplanets even with the size of the earth. The idea is as follows:
- From the total light emitted by a remote source only a very small amount can be seen by an observer on earth.
- However, if a distant sun passes by between the source and the observer, the sun acts as a lens, which bundles the light and thus the source appears brighter. This temporary change in brightness has the form of a bell.
- If the lens is orbited by a planet, this shows up in an additional spike on the brightness graph.
The first planet discovered via gravitational lensing in 2003 had one-and-a-half the mass of Jupiter. A second planet was identified this year, and one can expect that with further improvement of observation methods several exoplanets will be discovered within the next years.