Largest known exoplanet a big mystery for astronomers

An international team of astronomers say they have discovered the largest known planet orbiting another star.

The planet, called TrES-4, is a gas giant about 70 per cent larger than Jupiter but far less massive. It was discovered using a network of small telescopes called the Trans-Atlantic Exoplanet Survey (TrES), located in Arizona, California and the Canary Islands.

An exoplanet is an extrasolar planet, or one that lies outside our solar system.

Georgi Mandushev, the Lowell Observatory astronomer and lead author of a paper to appear in the Astrophysical Journal, said the planet has some unusual characteristics that make it hard to classify.

One measure where the planet is unique is its density.

"Its mean density is only about 0.2 grams per cubic centimetre, or about the density of balsa wood," said Mandushev in a statement. "And because of the planet's relatively weak pull on its upper atmosphere, some of the atmosphere probably escapes in a comet-like tail."

"TrES-4 appears to be something of a theoretical problem," said Edward Dunham, a Lowell Observatory instrument scientist. "It is larger relative to its mass than current models of superheated giant planets can presently explain."

The new planet is also intriguing because it orbits so closely to its parent star, a massive star on its way to becoming a red giant. The new planet orbits about seven million km from its star, or about a 10th of the distance from the sun to the first planet in our solar system, Mercury.

As a result, the planet is very hot, with a temperature of about 1,300 C. Its close proximity also means it orbits its star once every 3.55 Earth days.

The exoplanet was discovered transiting — or passing between its parent star and the Earth — at a distance of some 1,400 light years away in the constellation Hercules.

When transiting planets pass in front of a star they block some of the star's light that would otherwise reach Earth. Charting these slight dips in brightness can help astronomers detect the presence of transiting planets.

Once a planet has been observed, scientists can deduce other properties such as its size, mass and proximity to the star.