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Last Updated: Monday, 14 February, 2005, 14:27 GMT
Smallest extra-solar planet found
Artist's impression of extrasolar planetary system
The finding suggests Earth-mass planets may be relatively common
US astronomers say they have found the smallest planet orbiting outside our Solar System to date.

The new world, which is about one fifth the size of Pluto, is the fourth planet to be discovered orbiting around a pulsar called PSR B1257+12.

A pulsar is a spinning neutron star producing powerful beams of radiation.

The new planet is orbiting inside a large cloud of hot, charged gas that surrounds the pulsar and is some 1,500 light-years away from Earth.

Details of the work were announced at an astronomy meeting held in Aspen, US.

Pulsars are formed from the collapsed cores of so-called "supergiant" stars that have exploded.

The discovery was made by Alex Wolszczan of Pennsylvania State University, US, and Maciej Konacki of the California Institute of Technology (Caltech).

The orbit of the new planet is close to the average distance from our Sun to the asteroid belt.

The orbits of three planets discovered orbiting the same pulsar in 1992 were almost in exact proportion to the spacings between Mercury, Venus and Earth in our Solar System.

Wolszczan and Konacki say the new planet could mark the fringes of the pulsar's planetary system, just like Pluto marks the edge of ours.

The new planet was discovered with the Arecibo radio telescope in Puerto Rico.

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