But the presumed "gas giant" has a much lower mass than Jupiter - the biggest planet in our Solar System - making it of extremely low density.

Details of the work are to appear in the Astrophysical Journal.

The new exoplanet, called TrES-4, is located in the constellation of Hercules and was discovered by a team working on the Transatlantic Exoplanet Survey (TrES).

TrES-4 circles the star GSC02620-00648, which lies about 1,435 light-years away from Earth. Being only about seven million km (4.5 million miles) from its parent star, the planet is also very hot, about 1,327C (1,600 K; 2,300F).

Because of the relatively weak pull exerted by TrES-4 on its upper atmosphere, some of the atmosphere probably escapes in a curved comet-like tail.

"TrES-4 is the largest known exoplanet," said lead author Georgi Mandushev, from the Lowell Observatory in Flagstaff, US.

It is so big, in fact, that its size is difficult to explain using current theories about superheated giant planets.

"We continue to be surprised by how relatively large these giant planets can be," says Francis O'Donovan, a graduate student in astronomy at the California Institute of Technology (Caltech) which operates one of the TrES telescopes.

"But if we can explain the sizes of these bloated planets in their harsh environments, it may help us better understand our own Solar System planets and their formation."

Its density of 0.2 grams per cubic centimetre is so low that the planet would, in theory, float on water.

By definition, a transiting planet passes directly between the Earth and the star, blocking some of the star's light and causing a slight drop in its brightness.