WASHINGTON, D.C., 22 March 2005. NASA's Spitzer Space Telescope captured the light, for the first time, from two known planets orbiting stars other than our sun. The findings mark the beginning of a new age in planetary science, in which extrasolar planets can be directly measured and compared.
All confirmed extrasolar planets, including the two recently observed by Spitzer, have been indirectly discovered. They were discovered mainly by the "wobble" technique and more recently, the "transit" technique. In the first method, a planet is detected by the gravitational tug it exerts on its parent star, which makes the star wobble. In the second, a planet's presence is inferred when it passes in front of its star, causing the star to dim, or blink. Both strategies use visible-light telescopes and reveal the mass and size of planets.
"Spitzer has provided us with a powerful new tool for learning about the temperatures, atmospheres and orbits of planets hundreds of light-years from Earth," said Dr. Drake Deming of NASA's Goddard Space Flight Center (GSFC), Greenbelt, Md. Deming is lead author of a new study on one of the planets.
"It's fantastic," said Dr. David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., lead author of a separate study on a different planet. "We've been hunting for this light for almost 10 years, ever since extrasolar planets were first discovered."
In the new studies, Spitzer directly observed the warm infrared glows of two previously detected "hot Jupiter" planets, designated HD 209458b and TrES- 1. Hot Jupiters are extrasolar gas giants that zip closely around their parent stars. From their toasty orbits, they soak up ample starlight and shine brightly in infrared wavelengths.
To distinguish this planet glow from the fiery hot stars, the astronomers used a simple trick. First, they used Spitzer to collect the total infrared light from both the stars and planets. Then, when the planets dipped behind the stars as part of their regular orbit, the astronomers measured the infrared light coming from just the stars. This pinpointed exactly how much infrared light belonged to the planets.
"In visible light, the glare of the star completely overwhelms the glimmer of light reflected by the planet," Charbonneau said. "In infrared, the star- planet contrast is more favorable because the planet emits its own light."
The Spitzer data told the astronomers both planets are at least a steaming 1,000 Kelvin (1340 Fahrenheit). These findings confirm hot Jupiters are indeed hot. Upcoming Spitzer observations, using a range of infrared wavelengths, are expected to provide more information about the planets' winds and atmospheric compositions.
Spitzer is ideally suited for studying extrasolar planets, known to transit stars the size of our sun, out to distances of 500 light-years. Of the seven known transiting planets, only the two mentioned here meet those criteria. As more are discovered, Spitzer will be able to collect their light, a bonus for the observatory, considering it was not originally designed to see extrasolar planets. NASA's future Terrestrial Planet Finder coronagraph, set to launch in 2016, will directly image extrasolar planets as small as Earth.
TrES-1 was found via the transit method in 2004 as part of the NASA-funded Trans-Atlantic Exoplanet Survey, a ground-based telescope program established in part by Charbonneau.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center, Pasadena, Calif.
The Deming paper appears today in Nature's online publication; the Charbonneau paper will be published in an upcoming issue of the Astrophysical Journal. For artist's concepts and additional information about the Spitzer Space Telescope on the Internet, see www.spitzer.caltech.edu/Media. And for more information, see www.nasa.gov.