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November 27, 2001 01:00 PM (EST)

News Release Number: STScI-2001-38

Hubble Makes First Direct Measurements of Atmosphere on World Around another Star

A Space Science Update Release

November 27, 2001: Astronomers using the Hubble Space Telescope have made the first direct detection of the atmosphere of a planet orbiting a star outside our solar system. Their unique observations demonstrate that it is possible with Hubble and other telescopes to measure the chemical makeup of alien planet atmospheres and to potentially search for the chemical markers of life beyond Earth. The planet orbits a yellow, Sun-like star called HD 209458, located 150 light-years away in the constellation Pegasus.

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Q & A: Understanding the Discovery

  1. 1. How did they detect the atmosphere?

  2. Astronomers discovered the atmosphere by watching how starlight dimmed slightly when the planet crossed in front of its star, an event known as a transit. During the transit, a small amount of starlight passed through the planet's atmosphere on its way to Earth. Hubble's spectrograph collected the light and dispersed it into the colors of the spectrum, which yielded clues about the atmosphere's chemical makeup. When astronomers analyzed the spectrum, they found the telltale "fingerprint" of sodium.

  3. 2. What does sodium reveal about the planet?

  4. Astronomers expected to find sodium in the planet's atmosphere. Discovering sodium does not mean that life exists on the alien planet. In fact, astronomers don't think the planet can sustain life. It is a Jupiter-sized planet made up mostly of gas and is 20 times closer to its star than the Earth is to the Sun. The planet is so close to its star that its atmosphere is heated to a torrid 2000 degrees Fahrenheit (1100 degrees Celsius).

    The astronomers, however, actually found less sodium than scientists had predicted for a Jupiter-class planet, leading to one interpretation that high-altitude clouds in the alien atmosphere may have blocked some of the starlight. The astronomers discovered the sodium by analyzing the starlight that passed through the planet's atmosphere.

  5. 3. What are the conditions for life?

  6. A key ingredient for life as we know it is oxygen. The most suitable planets for life, where oxygen may be abundant, are small, rocky planets like Earth that orbit at comfortable distances away from their stars. Finding these planets and probing their atmospheres for signs of life is beyond the scope of current telescopes and detection techniques, including the transit method used in this Hubble observation. So far, astronomers have been successful at discovering a parade of alien planets. But all of them are Jupiter-sized giants that are much larger than Earth. Some of them orbit perilously close to their stars, like the planet whirling around the star HD 209458.

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Science Credit: NASA, D. Charbonneau (Caltech & CfA), T. Brown (NCAR), R. Noyes (CfA) and R. Gilliland (STScI)

Illustration Credit: G. Bacon (STScI/AVL)