For the first time astronomers have seen a star move in the sky under the influence of the gravity of a planetary companion.

The star is called Upsilon Andromedae, it is about 45 light years away and remarkably like our Sun. It hit the headlines earlier this year as being the first star, other than our Sun, to be known to have more than one planet orbiting it. It has three.

In recent years about 20 "exoplanets" have been detected circling other stars using the so-called radial velocity technique.

As a planet orbits its parent star, it causes it to wobble slightly. This motion can be detected using the most sensitive instruments as a shift in the star's spectrum. It appears as a regular motion first towards and then away from the Earth.

Star catalogue

The latest observations are the first time that a planet has been detected by observing the motion of the star from side to side on the sky.

The observations were made by the Hipparcos satellite launched in 1989. It has compiled one of the most accurate star catalogues ever made, charting the positions of 100,000 stars more accurately then ever before.

Previous observations showed that the outermost planet of Upsilon Andromedae circles the star in 1,269 days. A close examination of the Hipparcos data showed that the star wobbles from side to side at this periodicity.

The new observations allow the mass of the planet to be determined with unprecedented accuracy. It turns out to be about 10 Jupiter masses, with an uncertainty of 5 Jupiter masses.