Astronomers announced Tuesday that they have discovered evidence for an atmosphere around an extrasolar planet, the first time an atmosphere around any planet outside our solar system has been detected.

Using a novel yet not unprecedented technique, astronomers using the Hubble Space Telescope said they found traces of the element sodium that could only come from the atmosphere of a gas giant planet orbiting the star HD 209458, 150 light-years from Earth.

This is an artist's impression of the gas-giant planet orbiting the yellow, Sun-like star HD 209458. Illustration Credit: G. Bacon (STScI/AVL)

The discovery was made possible by the fact that the planet, HD 209458 b, passes directly between the Earth and the star every orbit. During these passes, known as transits, a small amount of the light from the star is blocked. An even smaller amount of light would pass through the planet's atmosphere, if any existed, and would carry the spectral signature of the components of the planet's atmosphere.

To test this, a team of astronomers led by David Charbonneau of Caltech and Tim Brown of National Center for Atmospheric Research observed several transits using the Space Telescope Imaging Spectrograph instrument on Hubble. They then compared the spectra taken during those transits with those taken at other times. They found traces of atomic sodium in the spectra of the transits that could have only come from an atmosphere around the planet.

Sodium is not a major component of the atmosphere: astronomers believe it makes up only a few parts per million of the atmosphere. However, sodium acts like a tracer gas that is easy for Hubble's spectrograph to detect. ³The sodium tells us that there really is an atmosphere there,² said Charbonneau.

Models of gas giant planets, like Jupiter and HD 209458 b, predict that sodium would exist in the atmosphere, but the Hubble data detected only about half the sodium predicted by the model. The difference, said Charbonneau, could be caused by either a high cloud deck that essentially blocks the rest of the atmosphere from view or reaction of sodium with other atmospheric components to form more complex molecules. Additional observations in the coming months and years will be needed to determine which explanation is correct.

The discovery was not entirely unexpected. Since its discovery in 1999 astronomers had assumed that HD 209458 b was a gas giant with an extensive atmosphere. With a mass 70 percent that of Jupiter, astronomers had classified it as one of about a dozen ³hot Jupiters²: large exoplanets that orbit very close to their stars. In the case of HD 209458 b, it is less than 7 million kilometers from its star, completing one orbit in just three and a half days.

In addition, astronomers had estimated the size of the planet based on previous transit observations by measuring the fraction of light from the star it blocked. Those studies showed that the planet's radius is a third larger than Jupiter's. This combination of mass and radius led astronomers to conclude that the planet had to be a gas giant, a conclusion supported by the Hubble data released Tuesday. Anne Kinney, director of the astronomy and physics division of NASA's office of space sciences called the Hubble conclusions "profound, but not surprising."

More important that the discovery of the atmosphere around HD 209458 b is the validation of this technique as a way to study the atmospheres of exoplanets. "This opens up an exciting new phase of extrasolar planet exploration, where we can begin to compare and contrast the atmosphere of planets around other stars," said Charbonneau.

Even though Hubble and its instruments predate the era of exoplanet discoveries, the telescope is particularly well-suited for this type of work because it is above the Earth's atmosphere, which makes it difficult to sort out absorption from beyond the atmosphere with absorption by terrestrial elements. "I never thought Hubble would be able to do it," said Bruce Margon, associate director for science at the Space Telescope Science Institute.

While this is the first time this technique has been used to probe the atmospheres of exoplanets, it is not the first time that a transit has been used to study planetary atmospheres. The first evidence of an atmosphere around any other planet came from observations of the transit of Venus across the disk of the Sun in the 18th century, when astronomers noticed a shimmering effect caused by light being refracted by the planet's atmosphere.

To date HD 209458 b is the only exoplanet for which transits have been observed. However, as exoplanet searches ramp up, astronomers expect to find more. Charbonneau said that, on average, about one in ten hot Jupiters should make transits. Planets farther away from their stars are less likely to make transits since there is less margin for error for a transit to take place.

Eventually, astronomers hope to be able to directly observe exoplanets and obtain their spectra, allowing them to study all planets and not just those visible by transits. However, the orbiting observatories capable of perform these observations, such as NASA's Terrestrial Planet Finder and ESA's Darwin, will not be ready until well into the next decade.