A “MONSTER” planet that should be too big for its star is forcing astronomers to rethink their theories.

The Jupiter-sized world, NGTS-1b, is the biggest planet compared with the size of its parent star ever discovered.

It orbits a small red dwarf star half the size of the Sun 600 light years away.

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Dr Daniel Bayliss, from the University of Warwick, who led the team of astronomers, said: “The discovery of NGTS-1b was a complete surprise to us. Such massive planets were not thought to exist around such small stars.

“We are already challenging the received wisdom of how planets form. Our challenge is to find how common these types of planets are in the galaxy.”

NGTS-1b was spotted using a robotic array of telescopes in Chile’s Atacama desert designed to search for exoplanets passing in front of their parent star.

The “hot Jupiter” gas giant is very close to its star, just three per cent of the distance between the Earth and the Sun, and makes one orbit every 2.6 days. It has a surface temperature of around 530C.

A report on the discovery is due in the journal Monthly Notices of the Royal Astronomical Society.