First Light from Extrasolar Planets

Dip in brightness as prospective planet transits in front of parent star Credit: ESO

Most of the 150 known extrasolar planets are discovered and studied through techniques such as finding the telltale wobble of a star tugged by an orbiting planet, or the "blink" of a star as a planet passes in front of it. Now for the first time scientists have observed an extrasolar planet through the light it emits in the infrared.

"I feel we’ve been blind and have just been given sight," commented co-author of the study, Dr. Sara Seager of the Carnegie Institution. "Detecting light from these other worlds is very exciting. It opens a whole new window on these objects. It’s the beginning of our ability to study their temperature, and composition," she added.

The study, published in the March 23 on-line edition of Nature, used measurements from NASA’s Spitzer Space Telescope, an infrared observatory launched in August 2003.

The planet, HD 209458b, is a so-called hot Jupiter–a massive gaseous world that orbits very closely to its parent star in only 3.5 days. It has not yet been possible to see these planets in the visible part of the spectrum because the light from the star vastly outshines that from the planet. However in the infrared, the planets show up more brightly than they do at visible wavelengths, making them detectable.

As Seager explained: "This planet was discovered indirectly in 1999 and was later found to transit its star–the star dims as the planet moves in front of it during the course of the planet’s orbit. With Spitzer, we first measured the combined light of the planet and star just before the planet went out of sight. Then when the planet was out of view, we measured how much energy the star emitted on its own. The difference between those readings told us how much the planet emitted."

"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.

The results of the measurements agreed with models created to determine how much infrared radiation hot Jupiters are likely to emit. HD 209458b was found to be a scorching 1,574 F (1130 K), confirming that hot Jupiters are in fact intensely baked by their stars. This finding refutes a hypothesis that the planet has an elliptical orbit due to the influence of another planet in the system. Such an orbit would have heated the planet by means of a varying tide, but this heating mechanism is now ruled out. The finding doesn’t rule out other planets, only those in certain positions.

"Our Spitzer observations are the first direct measurements of light from confirmed extrasolar planets," says Joseph Harrington, a senior research associate in the Center for Radiophysics and Space Research at Cornell University, Ithaca, N.Y.

Star field
Estimates suggest that up to a quarter of all stars have planets.
Credit: NASA/ STScI/ ESA

Another Spitzer study, led by Dr. David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, also detected infrared light from a planet, TrES-1, using the same technique, making two infrared detections of hot Jupiters. That research will be published in an upcoming issue of The Astrophysical Journal. "It’s fantastic," said Charbonneau. "We’ve been hunting for this light for almost 10 years, ever since extrasolar planets were first discovered."

"This first detection of light from two confirmed extrasolar planets is another major milestone along the way to the ultimate goal of finding Earth-like planets and examining their atmospheres for signs of life," said Alan Boss, a star and planet formation theorist at Carnegie’s Department of Terrestrial Magnetism who advises NASA about the search for extrasolar planets. "This detection means that we are succeeding in the effort to combine astronomy and biology into the new field of astrobiology, which seeks to determine if life has originated and evolved elsewhere in the universe."

"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 scientists got an added bonus in the Nature study. Researchers had thought that the seemingly bloated HD 209458b, with its particularly large radius, might have been stretched out from tidal tugs from the star due to an elongated orbit caused by gravitational interactions from yet another undetected planet. However, this scenario was ruled out because researchers found the orbit to be circular. "This finding adds to the growing number of mysteries that so many of these extrasolar planets seem to exhibit," mused Seager.

Astronomers can deduce a great deal by observing the variations in a star’s light and by observing its spectrum. Based on spectral type, HD 209458b is orbiting a star very much like our sun. TrES-1’s star is smaller and cooler. Both planets are closer to their stars than is Mercury to our sun, and both complete one orbit about every three days. Tidal forces cause their rotation to match that time, so that they always turn the same face to their stars, just as the moon always has the same face to Earth.

The temperature measurement of HD 209458b has a possible error of plus or minus about 150 Kelvin, while for TrES-1 it is plus or minus 50 Kelvin.

Gas giants like Jupiter become very hot when they are formed, as the gas that makes them up is compressed by gravity. Jupiter cooled over time, and now has a temperature at its cloud tops of 123 Kelvin, or 150 degrees Celsius below zero. "Hot Jupiters," by contrast, receive heat from their stars and remain hot. It is this heat that makes these planets a choice for infrared detection.

Spitzer Space Telescope, the fourth and final element in NASA’s family of Great Observatories.
Credit: Ball Aerospace & Technologies Corp., 2003


Researchers on the paper are Drake Deming, Goddard Space Flight Center; Sara Seager, Carnegie Institution; L. Jeremy Richardson, Goddard Space Flight Center; and Joseph Harrington, Cornell University. Co-authors of the Astrophysical Journal paper are Lori E. Allen, S. Thomas Megeath, Alessandro Sozzetti, David W. Latham and Guillermo Torres of the Harvard-Smithsonian; Roi Alonso of the Instituto de Astrofffisica de Canarias, Spain; Timothy M. Brown of the High Altitude Observatory, NationalCenter for Atmospheric Research, Boulder, Colo.; Ronald L. Gilliland of the Space Telescope Science Institute, Baltimore, Md.; Georgi Mandushev of Lowell Observatory, Flagstaff, Ariz.; and Francis T. O’Donovan of the University of Pittsburgh. The research was supported by NASA, NASA’s Origins of Solar Systems program, and the NASA Astrobiology Institute.




Related Web Pages

Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Darwin Mission
Space Interferometry Mission
Voyager: Beyond the Great Beyond
Fire and Ice
Beyond Pluto: Ice Planet