Hubble Space Telescope Discerns Planet's Atmosphere


Astronomers for the first time have observed the atmosphere of a planet circling a distant star, opening the possibility of more comprehensive searches for the chemical markers of life on distant planets.

Using the Hubble Space Telescope, a team headed by David Charbonneau of Caltech and Timothy Brown of the National Center for Atmospheric Research in Boulder, Colo., were able to study the atmosphere of a planet circling a star about 150 light-years away. The atmosphere contains sodium, a common element.

The team does not expect to find signs of life because the planet has a surface temperature of more than 2,000 degrees. But they hope the technique they used eventually could allow observers on Earth to detect gases such as methane, carbon dioxide and water vapor--indicators of life--in the atmosphere of other distant planets.

The planet, circling a star called HD 209458, is one of more than 80 planets circling other stars that have now been detected.

The unnamed planet is 70% as massive as Jupiter--about 220 times the size of Earth--and orbits only 4 million miles from the surface of its yellow, sun-like star. Because it is so close, its year is only about 3.5 Earth days.

In its orbit, the planet regularly passes across the face of the star as seen from Earth, dimming the star's light. That fact, plus the planet's quick orbit, makes it an ideal target for measurements.

Charbonneau and Brown used a spectroscope on Hubble to measure slight differences in the light emitted by the star when it is unobscured and the light emitted as the planet passes in front of it. When the planet passes in front of the star, the sodium in the planet's atmosphere absorbs some of the star's light, leaving a distinctive signature in the light spectrum.

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