The planet orbits a yellow, Sun-like star called HD 209458, a seventh-magnitude star (visible in an amateur telescope) that lies 150 light-years away in the autumn constellation Pegasus. Its atmospheric composition was probed when the planet passed in front of its parent star, allowing astronomers for the first time ever to see light from the star filtered through the planet's atmosphere.
Lead investigator David Charbonneau of the California Institute of Technology, Pasadena, and the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.; Timothy Brown of the National Center for Atmospheric Research, Boulder, Colo.; and colleagues used Hubble's spectrometer (the Space Telescope Imaging Spectrograph, or STIS) to detect the presence of sodium in the planet's atmosphere.
"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 astronomers actually saw less sodium than predicted for the Jupiter-class planet, leading to one interpretation that high-altitude clouds in the alien atmosphere may have blocked some of the light. The team's findings are to be published in the Astrophysical Journal.
The Hubble observation was 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, or extrasolar planets, 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 planet was discovered in 1999 through its slight gravitational tug on the star. The planet was estimated to be 70 percent the mass of the giant planet Jupiter, or 220 times more massive than Earth. Subsequently, astronomers discovered that the tilt of the planet's orbit makes it pass in front of the star -- relative to our line-of-sight from Earth -- making it unique among all the approximately 80 extrasolar planets discovered to date. As the planet passes in front of the star, it causes the star to dim very slightly for the duration of the transit. Transit observations by Hubble and ground-based telescopes confirmed that the planet is primarily gaseous, rather than liquid or solid, meaning that the planet is a gas giant, like Jupiter and Saturn.
The planet is an ideal target for repeat observations because it transits the star every 3.5 days -- which is the extremely short time it takes the planet to whirl around the star at a distance of merely four million miles from the star's surface. This close proximity heats the planet's atmosphere to a torrid 2,000 degrees Fahrenheit (1,100 degrees Celsius).
Observations of four separate transits were made by Hubble in search of direct evidence of an atmosphere. During each transit a small fraction of the star's light on its way to Earth passed though the planet's atmosphere. When the color of the light was analyzed by STIS, the telltale "fingerprint" of sodium was detected. Though the star also has sodium in its outer layers, STIS precisely measured the added influence of sodium in the planet's atmosphere.
The team, including Robert Noyes of the Harvard-Smithsonian Center for Astrophysics and Ronald Gilliland of the Space Telescope Science Institute in Baltimore, plans to look at HD 209458 again with Hubble in other colors of the star's spectrum to see which are filtered by the planet's atmosphere. 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.
The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), 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 (ESA). The National Center for Atmospheric Research is sponsored primarily by the National Science Foundation.
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