Tales of: The hunt for life on other worlds

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Tales of ...
The hunt for life on other worlds
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Methane gas rises from rotting garbage on Earth and droplets of liquid methane rain down on Saturn’s moon Titan. Even the planets Neptune and Uranus have an abundance of methane in their atmospheres. That’s why they appear blue-green.

Methane is a common chemical in our solar system, generated by living organisms and natural, non-living things.

So why are astronomers delighted to find methane in the atmosphere of a faraway planet orbiting another star? The detection was made with the Near-Infrared Camera and Multi-Object Spectrometer aboard NASA’s Hubble Space Telescope. The Jupiter-sized planet, called HD 189733b, is too hot to sustain life because it is too close to its parent star. The gas-giant planet completes an orbit in just over two days. Life as we know it could not exist on a planet whose atmosphere is a scorching 1,700 degrees Fahrenheit, about the temperature it takes to melt silver.

But astronomers say this detection offers hope that they will one day be able to probe the atmospheres of cooler, more hospitable worlds. Finding methane in the atmosphere of HD 189733b demonstrates that astronomers can successfully use spectroscopy to detect organic molecules on planets around other stars. Spectroscopy splits light into its component colors to reveal the “fingerprints” of various chemicals.

“This observation is one of the first steps in the search for life on another planet,” says astrophysicist Marc Kuchner of NASA Goddard Space Flight Center’s Exoplanets and Stellar Astrophysics Laboratory. “We need to study the chemistry in a planet’s atmosphere in order to determine whether the planet could harbor life.”

As the HD 189733b observation shows, organic chemicals like methane can be produced by chemical processes that do not require life. On Titan, Saturn’s largest moon, the abundance of methane may have been generated by a geological process between water and rock deep inside the moon. Even on Earth, not all the methane is the byproduct of life processes. A small amount of the gas is produced by volcanism.

Discovering methane on an Earth-like world in a habitable zone around a star — a location where liquid water could exist on the planet’s surface — would be promising, Kuchner says. (Continued >>)

Organic molecule detected on an extrasolar planet
Artist's conception of extrasolar planet and graphic showing that some starlight is absorbed by the planet's atmosphere
Top: An artist's conception of HD 189733b, a distant planet orbiting a distant star;
Bottom: The planet's atmosphere contains methane, which absorbs some of the light from the star the planet orbits. This results in a characteristic spectroscopic pattern.


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Tales of: The hunt for life on other worlds