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Astronomers catch first light from alien planet
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  • PARIS (AFP) Mar 22, 2005
    Little more than a decade after they first divined the existence of planets orbiting other stars, astronomers report another breakthrough -- the first detection of infrared light from an alien world.

    So far, a total of 145 planets beyond our solar system -- known as exoplanets -- have been spotted since 1994, according to a tally compiled by NASA's Jet Propulsion Laboratory (JPL).

    Nearly all of these planets have been discovered indirectly. In other words, they are not directly visible but their existence can be determined thanks to a wobble in stellar light when they pass in front of their star.

    But getting a photograph of an exoplanet and then trying to extracting useful data from the image is a major headache.

    A star is 10,000 times brighter and many hundreds of times bigger than a planet, and this sets astronomers a challenge equivalent to that of trying to pick out a moth that flits in front of a huge searchlight.

    Two teams of astronomers believe they have devised a novel way of getting around this problem.

    In a study to be published on Wednesday on the web site of the British journal Nature, one team describes how it used the orbiting US Spitzer Space Telescope to get a direct infrared image of an exoplanet, rather than attempt to get an image visible to the naked eye.

    Their target is HD 209458b, a planet that is slightly larger than our own Jupiter, and which was previously spotted by the "wobble" technique.

    HD 209458b is one of a category of so-called "hot Jupiters" -- gassy giant planets that orbit so close to their star that their surface is extremely hot and their atmosphere may even get stripped away by the stellar blast.

    In this case, HD 209458b swings around its star at an orbit of just 0.05 of an astronomical unit, or a mere one-twentieth of the distance between Earth and the Sun.

    HD209458b is so hot that as it passes behind the star, the amount of infrared light coming from the area drops slightly.

    That drop can be extrapolated into the planet's temperature, which the astronomers calculate to be a blistering 850 C (1,562 F).

    The authors caution that this estimate may have to be modified if it emerges, for instance, the planet's atmosphere has water molecules, which would slightly skew the infrared reading.

    The team is led by Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

    A parallel breakthrough on exoplanet discovery is also announced by a team led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics.

    It used the Spitzer's Infrared Array Camera to observer an exoplanet dubbed TrES-1. The study will appear in a forthcoming issue of the US-published Astrophysical Journal.

    The infrared technique, while still in its infancy, offers an exciting advance in studying these very distant and enigmatic worlds, said Charbonneau.

    It means that researchers can, for the first time, directly measure and compare such planetary characteristics as colour, reflectivity and temperature.

    One of the drivers for exoplanet research is the hope, one day, of finding a sister planet to Earth -- although the chances of ever reaching it with today's primitive chemical rockets are negligible.




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