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Alien Atmospheres

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Alien Atmospheres


Marshall Space Flight Center


see captionNovember 27, 2001: Astronomers using the orbiting Hubble Space Telescope have made the first direct detection and chemical analysis of the atmosphere of a planet outside our solar system. Their unique observations show it is possible to measure the chemical makeup of extra-solar planetary atmospheres -- and potentially to search for chemical markers of life far beyond Earth.

Right: An artist's impression of a gas-giant planet orbiting the yellow Sun-like star HD 209458. Credit: G. Bacon, STScI. [more]

The Jupiter-sized planet orbits a yellow, Sun-like star called HD 209458 that lies 150 light-years away in the 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.

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"This opens up an exciting new phase of extra-solar planet exploration," says Caltech's David Charbonneau. "[Now] we can begin to compare and contrast the atmospheres of planets around other stars."

It was Charbonneau, along with Timothy Brown (NCAR) and colleagues, who used Hubble's "STIS" spectrometer to detect the presence of sodium in the planet's atmosphere. The astronomers actually saw less sodium than expected for this Jupiter-class planet, leading to one interpretation that high-altitude clouds in the alien atmosphere might have blocked some of the light.

The Hubble observation was not tuned to look for gases that would likely appear in a life-sustaining atmosphere -- which were improbable anyway for a planet as hot as this one. Nevertheless, this unique observing technique opens a new phase in the exploration of extra-solar 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.

see captionLeft: HD 209458 is a 7th magnitude star in the constellation Pegasus, visible through binoculars or amateur telescopes. The planet itself is invisible but the star is easy to find in Northern skies after sunset. Sky charts: northern star map; detailed map of Pegasus.

The planet was discovered in 1999 as a result of its slight gravitational tug on the star. It is estimated to be about 70 percent the mass of the giant planet Jupiter, or 220 times more massive than Earth.

Subsequently, astronomers found that the tilt of the planet's orbit makes it pass in front of the star, as seen from Earth. It that way it is unique among all the extra-solar planets discovered to date (there are approximately 80 of them). 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 it 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 go around the star at a distance of merely four million miles (6.4 million km) from the stellar surface. This close proximity heats the planet's atmosphere to a torrid 2,000 degrees Fahrenheit (1,100 degrees Celsius).

see captionAstronomers call such planets "hot Jupiters." They are more or less 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 airless rock, hot Jupiters have enough gravity to hold onto their atmospheres, though some are hot enough to melt copper.

Using Hubble, astronomers observed four separate transits of the planet across HD 209458, searching for direct evidence of an atmosphere. And they found it!

Above: As seen from Earth, a gas giant planet passes in front of the Sun-like star HD 209458 every 3.5 Earth days -- and absorbs a fraction of the light from the parent star. [more]

During each transit a small fraction of the star's light on its way to Earth passed though the planet's atmosphere -- and was absorbed. When the color of the light was analyzed, 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, which included Robert Noyes of the Harvard-Smithsonian Center for Astrophysics and Ronald Gilliland of the Space Telescope Science Institute, 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.

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Above: Sodium in the atmosphere of the planet filters out light from its parent star. Image Credit: A. Feild, STScI. [more illustrations]

Once other transiting giant planets are found in the next few years (it's just a matter of time, say researchers) the team expects to characterize their chemical differences. Just imagine, comparative planetology -- outside the solar system!

Not long ago astronomers couldn't be certain that extra-solar planets existed at all. Now they're being found at a rapid pace and, thanks to Hubble, we can begin to study their properties. This is truly an exciting moment in astronomy. And what's next? Only time -- and more observations -- will tell.

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.

Web Links

Hubble Space Telescope -- home page

Animations -- watch a giant planet pass between Earth and the Sun-like star HD 209458

More Illustrations -- from the Space Telescope Science Institute. -- astronomers have discovered more than 70 planets outside our own solar system. Learn more about them at this site from the University of California.


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