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Planetary News: Extrasolar Planets (2005)

Scientists Measure the Light of Distant Planets

By Amir Alexander
March 22, 2005

Your first paragraph only.

Thanks to the Spitzer Space Telescope, this situation has now changed. Two new studies using the Spitzer to study extrasolar planets have managed to isolate the light of two different extrasolar planets in the infrared range of the spectrum. “It’s fantastic” said David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, lead author of one of the studies; “we’ve been hunting for this light for almost 10 years.”

The two groups, headed by Charbonneau and Drake Deming of NASA’s Goddard Space Center respectively, made their discoveries by observing known planets whose orbit takes them exactly between their home star and the Earth. These planets “transit” in front of their stars, as seen from Earth, and also disappear behind their stars in what is known as a “secondary transit.” Only seven extrasolar planets that answer to this unique geometry are known, and only two of these orbit Sun-like stars and are therefore suitable for study with the Spitzer Space Telescope. These two are known as HD 209458b and TrES-1.

Since both planets have been studied in detail, Charbonneau, Deming, and their teams knew their orbits and periods with great accuracy. Using the Spitzer they measured the system’s average infrared radiation, and then compared it with the same wavelength radiation during the secondary transit. If their predictions proved correct, they would detect a slight dip in the infrared radiation during the secondary transit, as the heat emanating from the planet itself would be blocked by the star. The difference between the system’s “normal” infrared radiation and the radiation during the secondary transit represents the infrared radiation that belongs only to the planet itself.

The scientists’ predictions proved true: when the results were in, the infrared radiation from both planets showed a distinct dip during their secondary transit behind their stars. Furthermore, the level of infrared radiation from the planets showed them to be glowing hot – at least 1000 degrees Kelvin (1340 degrees Fahrenheit)! This is not surprising, since both planets are “Hot Jupiters” – giant planets orbiting very close to their stars. Nevertheless, this is the first direct indication that “Hot Jupiters” are indeed very hot.

The study of HD 209458b and TrES-1, explained Charbonneau, could not be accomplished in visible light, but only in the infrared range, which is the Spitzer’s specialty. “In visible light,” he said, “the glare of the star completely overwhelms the glimmer of light reflected by the planet.” “In infrared, the star-planet contrast is more favorable because the planet emits its own light.”

By measuring a planet’s infrared spectrum scientists hope to learn more about it than its temperature. Future measurements by the Spitzer of HD 209458b and TrES-1 in a range of infrared wavelengths, they hope, will provide information about the planets’ atmospheric conditions as well. This type of information, explained Alan Boss of the Carnegie Institute, will help determine conditions on the planets’ surface, and provide a crucial link between the search for extrasolar planets and Astrobiology – the study of life in the universe. “2005,” said Boss, “ will be remembered as the year in which we actually detected light from a planet.”