A chance alignment between two stars may have allowed astronomers to detect the first Earth-sized planet found outside our solar system.

In June 1998 observations made at the Mount Stromlo Observatory in Australia and the Mount John Observatory in New Zealand detected a star near the centre of our galaxy becoming abnormally bright.

The brightening occurred because of the so-called gravitational lensing effect. This happens when another star passes directly between the distant star and ourselves.

Its gravity acts like a lens bending and amplifying the light from the distant object. The observatories were looking for just such an event.

Some 20 new planets have been discovered in recent years

By studying the way the distant object's light rises and falls, astronomers could determine the distant star appeared to be larger than our own Sun.

A detailed analysis of the data suggests the light-curve can only be explained if it has a planet orbiting it.

If so then it must be the smallest planet discovered orbiting another star - possibly as small as the Earth.

Different techniques

In the past few years astronomers have discovered about 20 planets orbiting some of the nearest stars to our Sun using different techniques.

All of them however have been massive and are probably more like the gas giant Jupiter than the small, rocky Earth.

This new planetary system would be unique in not having any gas giant planets. A detailed analysis has been submitted for publication in the Astrophysical Journal.