Page last updated at 14:31 GMT, Monday, 1 June 2009 15:31 UK

Distant world circles tiny star

VB 10b and star (NASA/JPL-Caltech)
The planet might have about the same girth as its star

A distant "sun" residing in the constellation Aquila has become the smallest star known to host a planet.

The discovery of a Jupiter-like "exoplanet" orbiting the star VB 10 is the first to be made using the astrometry method.

Astrometry is based on measuring small changes in a star's position.

At one-twelfth the mass of the Sun, VB 10 is tiny; though the star is more massive than its planet, it would have about the same girth, experts say.

Astrometry has long been proposed as a tool for finding other planets, but this is the method's first "catch".

The results are to be published in an upcoming edition of the Astrophysical Journal.

Using astrometry to find exoplanets involves measuring the precise motions of a star on the sky as an unseen planet tugs the star back and forth. It is best suited to finding planets with large orbits around their parent stars.

But the method requires very precise measurements over long periods of time.

Gas giant

The newfound planet, VB 10b, is a gas giant with a mass six times that of Jupiter that lies 20 light-years away. Scientists think the planet's own internal heat would give it an Earth-like temperature.

Lead author Steven Pravdo, from Nasa's Jet Propulsion Laboratory in Pasadena, US, commented: "We found a Jupiter-like planet at around the same relative place as our Jupiter, only around a much smaller star.

Hale telescope
Researchers used an instrument attached to the Hale telescope

"It's possible this star also has inner rocky planets. And since more than seven out of 10 stars are small like this one, this could mean planets are more common than we thought."

If there are other planets there, this solar system could be a miniature, scaled-down version of our own.

The discovery was the outcome of meticulous, intermittent observations of 30 stars.

Two to six times a year, for the past 12 years, Dr Pravdo and Stuart Shaklan, also from JPL, have bolted their Stellar Planet Survey instrument on to the Palomar Observatory's 5m Hale telescope to search for planets.

Planet hunt

The instrument, which has a 16-megapixel charge-coupled device (CCD) can detect minuscule changes in the positions of stars.

VB 10b causes its star to wobble a small fraction of a degree. Detecting this wobble is equivalent to measuring the width of a human hair from about 3km away.

In wider use as planet-hunting techniques are the radial velocity and the transit methods.

Like astrometry, radial velocity detects the "wobble" of a star, but it measures Doppler shifts in the star's light caused by its motion towards and away from us.

The transit method looks for dips in a star's brightness as orbiting planets pass by and block the light.

Nasa's Kepler space telescope, which was launched on 6 March, will use the transit method to search for Earth-like worlds around stars similar to the Sun.

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