The basic molecules required for life as we know it have been detected in a second hot gas planet beyond our solar system.
The planet, which orbits a sun-like star about 150 light-years away in the constellation Pegasus, is not habitable but it has the same chemistry that, if found around a rocky planet in the future, could indicate a world that might support life or the presence of life.
"It's the second planet outside our solar system in which water, methane and carbon dioxide have been found, which are potentially important for biological processes in habitable planets," said researcher Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Detecting organic compounds in two exoplanets now raises the possibility that it will become commonplace to find planets with molecules that may be tied to life."
The first planet in which organic molecules were detected was HD 189733b, a hot, Jupiter-sized planet. The discovery was made by Swain and his colleagues in December 2008.
Swain's team used data from two of NASA's orbiting Great Observatories, the Hubble Space Telescope and Spitzer Space Telescope, to study another hot planet bigger than Jupiter, HD 209458b, and characterize it as the second known exoplanet with organic signatures.
The detections were made through spectroscopy, which splits light into its components to reveal the distinctive spectral signatures of different chemicals. Data from Hubble's near-infrared camera and multi-object spectrometer revealed the presence of the molecules, and data from Spitzer's photometer and infrared spectrometer measured their amounts.
"This demonstrates that we can detect the molecules that matter for life processes," Swain said.
Astronomers can now begin comparing the two planetary atmospheres for differences and similarities. For example, the relative amounts of water and carbon dioxide in the two planets is similar, but HD 209458b shows a greater abundance of methane than HD 189733b.
"The high methane abundance is telling us something," Swain said. "It could mean there was something special about the formation of this planet."
Other large, hot Jupiter-type planets can be characterized and compared in the same way. The techniques used are also similar for those that will be required to shortlist rocky Earth-like planets where the signatures of organic chemicals might indicate the presence of life.
Rocky worlds are expected to be found by NASA's Kepler mission, which launched earlier this year, but astronomers believe we are a decade or so away from being able to detect any chemical signs of life on such a body.
If and when such Earth-like planets are found in the future, "the detection of organic compounds will not necessarily mean there's life on a planet, because there are other ways to generate such molecules," Swain said. "If we detect organic chemicals on a rocky, Earth-like planet, we will want to understand enough about the planet to rule out non-life processes that could have led to those chemicals being there."
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