The planet has a large radius but a low density

The "transiting" planet - meaning one that passes in front of its parent star as seen from Earth - is about 70% larger than Jupiter.

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).

We continue to be surprised by how relatively large these giant planets can be Francis O'Donovan, Caltech

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.

Surprising size

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.

"TrES-4 blocks off about 1% of the light of the star as it passes in front of it," said Dr Mandushev.

"With our telescopes and observing techniques, we can measure this tiny drop in the star's brightness and deduce the presence of a planet there."

Planet TrES-4 makes a complete revolution around its parent star every 3.55 days, so a year on this planet is shorter than a week on Earth.

The TrES is a network of three 10cm telescopes in Arizona, California and the Canary Islands.

In order to accurately measure the size of the TrES-4 planet, astronomers used the 0.8m telescope at the Lowell Observatory in Arizona, the 1.2m telescope at the Whipple Observatory, also in Arizona, and the 10m Keck telescope in Hawaii.