London, Nov 1 (IANS): An international team of astronomers has found a gas giant the size of Jupiter orbiting a star half the size of the Sun, a discovery that challenges theories of planet formation which state that a planet of this size could not be formed by such a small star.

This unusual planet, NGTS-1b, is the largest planet compared to the size of its companion star ever discovered in the universe, according to the study to be published in an upcoming issue of the journal Monthly Notices of the Royal Astronomical Society.

"The discovery of NGTS-1b was a complete surprise to us -- such massive planets were not thought to exist around such small stars," said lead author of the research Daniel Bayliss from the University of Warwick in England.

According to existing theories of planet formation, small stars can readily form rocky planets but do not gather enough material together to form Jupiter-sized planets.

The planet NGTS-1b which is six hundred light years away from Earth is a hot Jupiter, at least as large as the Jupiter in our solar system, but with around 20 per cent less mass.

It is very close to its star -- just three per cent of the distance between Earth and the Sun - and orbits the star every 2.6 days, meaning a year on NGTS-1b lasts two and a half days.

The temperature on the gassy planet is approximately 530 degrees Celsius, or 800 kelvin.

The researchers spotted the planet using the Next-Generation Transit Survey (NGTS) - a wide-field observing facility made of a compact ensemble of telescopes, designed to search for transiting planets on bright stars - run by the Universities of Warwick, Leicester, Cambridge, Queen's University Belfast, Observatoire de Genève, DLR Berlin and Universidad de Chile.

The planet orbits a red M-dwarf -- the most common type of star in the universe, leading to the possibility that there could be more of these planets waiting to be found by the NGTS survey.