David Aguilar
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Christine Pulliam
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CfA Press Release
 Release No.: 01-12
For Release: November 27, 2001

An Atmosphere is Detected on a Distant World Orbiting Another Star

Cambridge, MA - Astronomers have made the first direct detection of the atmosphere of a planet orbiting a star outside our solar system and have obtained the first information about its chemical composition. Their unique observations demonstrate that it is possible to measure the chemical makeup of extrasolar planet atmospheres and potentially to search for chemical markers of life beyond Earth. The discovery was made by David Charbonneau of the California Institute of Technology (Pasadena, California) and the Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts), Timothy Brown of the National Center for Atmospheric Research (Boulder, Colorado), Robert Noyes of the Harvard-Smithsonian Center for Astrophysics, and Ronald Gilliland of the Space Telescope Science Institute (Baltimore, Maryland). The scientists used NASA's Hubble Space Telescope to detect the atmosphere of a planet as it passed in front of the star HD 209458, a sun-like star about 150 light years away in the constellation Pegasus.

"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," says Charbonneau.

The planet orbiting HD 209458 was first detected in 1999 by David Latham of the Harvard-Smithsonian Center for Astrophysics and colleagues, as well as independently by another group, because of the "wobble" it induced on its parent star. Charbonneau (then a Harvard astronomy graduate student working with Noyes) then made follow-up observations together with Brown, which established that the planet actually passes in front of the star during its orbit. Charbonneau, Brown, Noyes, and Gilliland then turned to the Hubble Space Telescope because its unique location outside the Earth's atmosphere permitted detection of the very subtle signature of gases in the planet's atmosphere, which appear during the time the planet is in front of the star.

Based on the earlier observations, the planet was strongly believed to be a "gas giant," very similar to Jupiter. That is, like Jupiter it must be composed largely of gases rather than solid rocks like the Earth. But in contrast to Jupiter, it orbits extremely close to its parent star-only about 4 million miles above the star's surface. Because the planet is so close to the star, its atmosphere is calculated to be much hotter than the Earth's-about 2000 degrees Fahrenheit (1100 degrees Centigrade).

Theoretical calculations predict that such a hot planetary atmosphere should show the signature of chemical constituents such as sodium, when starlight shines through the atmosphere during the planet's passage in front of the star. The team used the Hubble Space Telescope Imaging Spectrograph to search for these signatures, and successfully detected the presence of sodium gases in the planet's atmosphere.

The observations were not tuned to look for gases expected in a life-sustaining atmosphere (which is improbable for a planet as hot as the one observed). Nevertheless, this unique observing technique opens a new phase in the exploration of exoplanets, say astronomers. Such observations could potentially provide the first direct evidence for life beyond Earth by measuring unusual abundances of atmospheric gases caused by the presence of living organisms.

The team next plans to look at HD 209458 again with Hubble in other colors of the star's spectrum to seek other atmospheric constituents. They hope eventually to detect methane, water vapor, potassium and other chemicals in the planet's atmosphere. Once other transiting giants are found in the next few years, the team expects to characterize chemical differences among the atmospheres of these planets.

These anticipated findings would ultimately help astronomers better understand a bizarre class of extrasolar planets discovered in recent years that are dubbed "hot Jupiters." They are the size of Jupiter but orbit closer to their stars than the tiny innermost planet Mercury in our solar system. While Mercury is a scorched rock with essentially no atmosphere, these planets have enough gravity to hold onto their atmospheres, though some are hot enough to melt copper.

Conventional theory is that these giant planets could not have been born so close to their stars. Gravitational interactions with other planetary bodies or gravitational forces in a circumstellar disk must have carried these giants via spiraling orbits precariously close to their stars from their birthplace farther out.

Proposed moderate-sized US and European space telescopes could allow for the detection of many much smaller Earth-like planets by transit techniques within the next decade. The chances for detection will be more challenging, since detecting a planet orbiting at an Earth-like distance will mean a much tighter orbital alignment is needed for a transit. And the transits would be much less frequent for planets with an orbital period of a year, rather than days. Eventually, study of the atmosphere of these Earth-like planets will require meticulous measurements by future larger space telescopes.

The Space Telescope Science Institue is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international co-operation between NASA and the European Space Agency. The Harvard-Smithsonian Center for Astrophysics is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. The National Center for Atmospheric Research's primary sponsor is the National Science Foundation.

Images associated with this release are available at http://oposite.stsci.edu/pubinfo/PR/2001/38/

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
(617) 495-7462

Christine Lafon, Harvard-Smithsonian Center for Astrophysics
(617) 495-7463

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