This opens up an exciting new phase of extrasolar planet exploration

David Charbonneau, California Institute of Technology

The unique observations show it is now possible for Hubble and other telescopes to search for the chemical signature of life beyond Earth.

"This opens up an exciting new phase of extrasolar planet exploration, where we can begin to compare and contrast the atmospheres of planets around other stars," said David Charbonneau of the California Institute of Technology, US.

The unnamed planet orbits a yellow, Sun-like star called HD 209458, which lies 150 light-years away in the constellation of Pegasus.

Ideal target

The planet was discovered in 1999 because of its slight gravitational tug on its parent star. It is estimated to be 70% the mass of Jupiter, or 220 times more massive than Earth.

Charbonneau explains how the study was carried out

Astronomers discovered later that the tilt of the planet's orbit means it passes in front of the star - as viewed from Earth - making it unique among all the approximately 80 extrasolar planets found to date.

In fact, the transit of the planet across the face of the star had been detected years earlier by the Hipparcos satellite but was not recognized for what it was.

As the planet moves in front of the HD 209458, it causes the star to dim very slightly. Observations by Hubble and ground-based telescopes confirm the planet is primarily gaseous, rather than liquid or solid, meaning it is a gas giant, like Jupiter and Saturn.

Filtered starlight

It is an ideal target for repeat observations because it transits the star every 3.5 days - the time it takes the planet to orbit the star at a distance of 6.4 million kilometres (four million miles).

This close proximity heats the planet's atmosphere to a 1,100 deg Celsius (2,000 degrees Fahrenheit).

The planet's atmosphere was probed when it passed in front of its parent star, allowing astronomers for the first time ever to see light from a star filtered through an exoplanet's gases.

They saw less sodium than predicted for the Jupiter-class planet, leading to one interpretation that high-altitude clouds in the alien atmosphere may be blocking some of the light.

Life gases

The observations, made with the Hubble Space Telescope, were not tuned to look for gases expected in a life-sustaining atmosphere, which astronomers consider improbable for a planet as hot as this.

Nevertheless, this unique observing technique opens a new phase in the exploration of exoplanets, or extrasolar planets as they are also called. Such observations could provide the first direct evidence for life beyond Earth by measuring unusual abundances of atmospheric gases associated with life.

The team plans to look at the planet again with Hubble in other colours of the HD 209458's spectrum to see which are filtered by the alien world's atmosphere. They hope eventually to detect methane, water vapour, potassium and other chemicals in the planet's atmosphere.

Once other transiting giants are found in the next few years, the team expects to characterise chemical differences among the atmospheres of exoplanets.

A Nasa simulation of the planet around HD 209458