Tranny planet gives astronomers a show

Astronomers have found an extrasolar planet that is perfectly positioned to allow a decent analysis of its atmosphere.

Most extra-solar planets are detected because they cause a wobble in their star's motion. This planet, orbiting a sun-like star around 750 light years from Earth, was detected because its orbital plane takes it directly between us and its star.

This very rare geometry, called transiting, means scientists are in with a shot of learning about it in unprecedented detail. Only ten planets that pass between their star and Earth have been discovered. By measuring how much of the star's light it blocks as it transits, astronomers can deduce the planet's size, mass and orbit.

The configuration also means astronomers can filter out the planet's own infrared signal, and therefore learn about its temperature and chemical properties.

The astronomers, led by Francis O'Donovan at Caltech, were working with a network of reasonably small telescopes called the Trans-Atlantic Exoplanet Survey (TrES), according to New Scientist. This is the second planet they have discovered.

O'Donovan told the magazine: "It is only for the nearby transiting planets that we can precisely measure the size and mass of the planet, and hence study its composition. That makes each new transiting planet an exciting find."

The planet is a gas giant about 1.3 times as massive as Jupiter. It is in a very tight orbit - about ten times closer to its star than Mercury is to our sun, which means that its orbital period is also very short: it circles its star once every 2.5 days.

The next step is to bring NASA's Spitzer infrared space telescope to bear on the problem. It will be able to distinguish the planet's infrared signal from the background of the star.

Because the view from Earth is of the planet on a diagonal transit, spotting any variations in surface temperature should be relatively easy, the team says. This means the astronomers will be able to infer things about the planet's weather systems: if the temperature is even, there must be strong winds distributing the heat across the planet.

In a couple of years the team hopes to use the Kepler observatory (scheduled for a 2008 launch) to determine whether the planet is alone, or if the star has a more populous solar system. It might also be able to identify moons orbiting the planet. ®