Hubble probes planet around star

By Dr David Whitehouse
BBC News Online science editor

An artist's impression of the planet

The first direct detection of oxygen and carbon in the atmosphere of a planet outside our Solar System has been made using the Hubble Telescope.

Astronomers say the planet - called HD 209458b - orbits a yellow, Sun-like star and is situated 150 light-years away in the constellation of Pegasus.

It is a gas-giant world like Jupiter and scientists can see that it is being slowly destroyed by its parent star.

An international team has sent its results to the Astrophysical Journal.

The Hubble observations were made in October and November 2003.

They were carried out by astronomers from the University of Arizona and the Jet Propulsion Laboratory in the US; York University in Canada; and the Paris Observatory and the Geneva Observatory in Europe.

Ideal target

The remarkable planet was first identified in 1999 because of its slight gravitational tug on its parent star.

It is estimated to be 70% of the mass of Jupiter, or 220 times more massive than Earth.

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 very rare among the approximately 120 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 HD 209458, it causes the star to dim very slightly.

Analysis of changes in the light from the system during the eclipse can reveal the chemical composition of the planet's atmosphere.

It is an ideal target for repeat observations because the transits occur 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).

Scorched world

The close proximity to its parent star heats the planet's atmosphere to 1,100 deg Celsius (2,000 deg Fahrenheit), turning it into what astronomers call a "hot Jupiter".

HD 209458: The planet passes in front as viewed from Earth

In 2001, the planet's atmosphere was probed 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.

Subsequent Hubble observations have detected a plume of hydrogen billowing out behind the planet as the gas is driven away by the intense heat of the parent star.

Astronomers are planning to subject the planet to further scrutiny. They intend to search for evidence of water vapour as well as heavy metals in its atmosphere.