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 |  Subject: Stuff of Life (but Not Life Itself) Is Detected on a Distant Planet  Wed Mar 19, 2008 9:14 pm | |
| Astronomers reported Wednesday that they had made the first detection of an organic molecule, methane, in the atmosphere of a planet outside our solar system and had confirmed the presence of water there, clearing the way for a bright future of inspecting the galaxy for livable planets, for the chemical stuff of life, or even for life itself.
Under the right conditions, water can combine with organic chemicals like methane to make amino acids, the building blocks of life as we know it. While the presence of these chemicals was not a big surprise and while the planet in question — in the constellation Vulpecula — is too hot and massive for living creatures, the result left astronomers elated at their improving powers of celestial discernment.
“The big news is that we were able to do this at all,” said Mark Swain of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., the lead author of the study, being reported Thursday in the journal Nature. Other members of the team, which used the Hubble Space Telescope, were Gautam Vasisht of the propulsion lab and Giovanna Tinetti of University College London.
The work, they said, represents a shift from barely detecting the existence of so-called exoplanets to probing them chemically.
“We are able to start studying the conditions and chemistry of exoplanet atmospheres,” Dr. Swain said at a news conference on Wednesday. “That’s a very exciting development.”
David Charbonneau, an astronomer at the Harvard-Smithsonian Center for Astrophysics who was not part of the team, called the detection “both persuasive and important.”
Sara Seager, a planetary theorist at M.I.T., called it “another great day for exoplanets,” and a “tipping point” for the study of their detailed properties, though she cautioned that the findings still needed to be duplicated.
“Hubble was never been designed to make measurements like this,” she said. “This is pushing the telescope to its limits.”
She said she was looking forward to the day when the experiment would be repeated on Earth-like planets with the much more powerful James Webb Space Telescope, set to be launched in 2013. In that case, she said, the existence of methane and water would be indicative that the planet was habitable.
The planet in question, known as HD 189733b, is definitely not a candidate abode for life. It is a suffocatingly hot ball about the mass of Jupiter orbiting only about 3 million miles from a star slightly smaller than the Sun. The temperature is a toasty 1,700 degrees Fahrenheit, enough to melt silver.
The Vulpecula planet was in the news a year ago after astronomers using the Spitzer Space Telescope tried and failed to find signs of water there and on another planet — a surprise since all the theoretical models predicted it should be there in abundance. But that was a year ago, a long time in the world of exoplanets, where an avalanche of data in the last decade has produced a series of milestones and some 270 new planets.
This planet, like every exoplanet discovered to date, is too dim and close to its parent star to be seen directly, but conveniently for astronomers it is one of a few dozen which passes directly in front of and behind its parent star in the course of an orbit, a geometrical quirk that Dr. Swain and his colleagues were able to exploit.
They watched with Hubble and an instrument known as Nicmos (for Near Infrared Camera and Multi-Object Spectrometer) as the star slipped behind the planet and thus backlit its atmosphere. Gases in the planet’s atmosphere absorbed the starlight in a band of wavelengths characteristic of methane, causing dips in the combined spectrum of star and planet.
A similar dip also occurred at another band, characteristic of water vapor, resolving a controversy that had been simmering for the last year. Asked why the previous measurements had failed, Dr. Swain and others explained that the Spitzer measurements had been made during the opposite part of the cycle, when the planet goes behind its star and was thus being seen face on. From this angle, structural features in the atmosphere, like an inversion layer, could mask the presence of water.
Adam Burrows, a theorist from Princeton University, said, “A temperature inversion can change the spectrum of the atmosphere without changing its composition.” But all these planets, he continued, have to have water.
“If you don’t see it you have to have a really good reason you don’t,” he said.
Indeed, later last year, Dr. Tinetti used the Spitzer telescope to make crude spectral measurements of light coming through the planet’s atmosphere while it was eclipsing its sun and found evidence of water absorption. Dr. Swain said those measurements fitted perfectly with the abundance of water on the Vulpecula planet derived from his observations.
One lingering puzzle, he said, is why they did not detect carbon monoxide in the planet’s atmosphere. The models, he said, suggest that at high temperatures that molecule is more likely to form than methane, which predominates in colder regions.
Dr. Burrows, theorized, however, that if the planet was tidally locked — with one side always facing its sun and being roasted while the other faces away and freezes — “the hot side would have more carbon monoxide, the cold one more methane.”
During the transits observed by Hubble, he pointed out, the starlight passes through the dividing line, or terminator, between the hot side and the cold side, where fierce winds might be blowing redistributing heat and chemical species around the planet.
But nobody really knows how chemistry, climate and cosmic history are manifested on these planets. Dr. Swain said he hoped to perform similar measurements on a half dozen other so-called transiting planets that are within reach.
Dr. Burrows said, “A lot of other shoes are about to drop in this subject.”
But time is of the essence. Hubble will have four more years if its scheduled refurbishment by astronauts goes well this August, but the other warhorse of the effort, the Spitzer, has only a year to go before it runs out of the cryogenics that keep its infrared detectors cold and sensitive.
“People are frantic to get as much data as they can in short term,” Dr. Burrows said.
http://www.nytimes.com/2008/03/20/science/space/20planetw.html |
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