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Planetary News: Extrasolar Planets (2008)Methane Found in Exoplanet's Atmosphere May Point the Way to Detection of LifeBy Amir AlexanderMarch 20, 2008
The dream of detecting another "Earth" out in the depths of space just got one step closer to realization. In an article published in the latest issue of Nature, an international team of planetary scientists announced that they had detected the presence of the organic molecule methane in the atmosphere of an extrasolar planet. While organic substances are common in our solar system, this is the first time they have been detected in a world orbiting a faraway star. "This is a crucial stepping stone to eventually characterizing prebiotic molecules on planets where life could exist" said team leader Mark Swain of NASA's Jet Propulsion Laboratory (JPL) in Pasadena. Organic molecules are essential for the existence of life, and on Earth methane is closely associated with life's byproducts, including oceans and wetland environments, and livestock waste. This, however, is certainly not the case on HD 189733b. "The planet's atmosphere is far too hot for even the hardiest life to survive" said Giovanna Tinetti of University College, London, who co-authored the article with Swain and Gautam Vasisht of JPL; "it's highly unlikely that cows could survive there!" In fact, conditions on the planet are so hostile to life as we know it, that they practically rule it out. Designated HD 189733b, the planet is an enormous gas giant 62 light years away, 15% more massive than Jupiter and orbiting a Sun-like star at a distance of only 5,000 kilometers (3.000 miles) – 30,000 times closer than Earth is to the Sun. As a result the planet’s average temperature is a scorching 900 degrees Kelvin (1600 degrees Fahrenheit), making the presence of water-based life unthinkable. Nevertheless, the fact that organics on a faraway planet can be detected from Earth suggests that it may soon be possible to find such substances on worlds more hospitable to life. The planet HD 189733b had been in the news as recently as last summer, when researchers used the Spitzer Space Telescope to detect water vapor in its atmosphere. This time Swain and his colleagues used Spitzer's "cousin," the Hubble Space Telescope, to reveal the telltale signs of the presence of methane. In both cases the researchers relied on the fact that HD 189733b is one of a small group of extrasolar planets that "transit" between their star and the Earth, periodically blocking out part of the star's light. Whereas most exoplanets are known solely through the wobble they cause their star by their gravitational pull, the fact that HD 189733b also transits provides scientists with additional valuable information about it. In particular, the degree to which the star dims during a transit is a strong indication of the planet’s diameter. Significantly, however, a star’s dimming during a planetary transit is dependent on another crucial factor: how much of the starlight is absorbed by the planet’s atmosphere? This is especially the case when the planet in question is a gas giant, which means that its atmosphere encompasses a large portion of its total volume. If the atmosphere blocks out much of the light from the star then it will appear dimmer and, by inference, the planet will appear larger; but if the atmosphere blocks out only a small portion of the light, then the star will appear brighter and the planet’s diameter will be thought smaller. The crucial point for Swain and his colleagues is that the atmosphere’s degree of light-absorption is not uniform across the spectrum. Depending on the composition of gasses in the atmosphere, the planet will block more of the star’s light at certain wavelengths and less of its light at others. If the planet appears “bigger” at a certain frequency, that meant that it is absorbing more light, whereas if it appears “smaller” it is absorbing less. By going through the spectrum and registering the apparent size of the planet, researchers can determine how much light the planet absorbs at each wavelength. Since each gas has its own signature wavelengths in which it absorbs the most light, the planet's absorption spectrum provides a strong indication as to which gasses are present in its atmosphere.
Last summer a team led by Tinetti used transit measurements by the Spitzer Space Telescope to determine HD 189733b's absorption spectrum in the in the infrared range. They then showed that the peaks of absorption closely corresponded to the characteristics of water vapor, indicating that it was almost certainly present in the atmosphere. This time Swain and his collaborators used Hubble to closely track the planet's transits and determine its absorption spectrum in the sub-infrared range. Like Tinetti's group before them, Swain's team found strong evidence that water vapor was present in HD 189733b's atmosphere. But they found something else as well: the best match for the planet's absorption spectrum in the sub-infrared range is attained if the atmosphere is assumed to contain methane. "Since a steep change in absorption occurs at 2.2 microns" they wrote in the Nature paper, "the observations decisively show that methane is present in addition to water." In particular, they wrote, the best model fitting the observations has a water concentration of 5 parts in 10,000, and a methane concentration of 5 parts in 100,000. "Water alone could not explain all the spectral features observed" said Tinetti. According to Swain the detection is a milestone on the road to finding true signs of life. "This observation," said Swain, "is proof that spectroscopy can eventually be done on a cooler and potentially habitable Earth-sized planet." The ultimate goal, he added, is to identify such molecules in planets orbiting in the "habitable zones" of their stars, where liquid water is stable. On such worlds, where conditions may resemble those on Earth, it just might be the case that methane and other organic molecules are indeed the telltale signs of extraterrestrial life. Did you like this article? Send
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