Spitzer first to crack open light of far away worlds
Wed Feb 21, 2007 at 18:57 UTC
Spitzer Space Telescope has captured for the first time enough light from planets outside our solar system, known as exoplanets, to identify signatures of molecules in their atmospheres. The landmark achievement is a significant step toward being able to detect life on rocky exoplanets and comes years before astronomers had anticipated.
"This is an amazing surprise," said Spitzer project scientist Michael Werner of NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "We had no idea when we designed Spitzer that it would make such a dramatic step in characterizing exoplanets."
Spitzer, a space-based infrared telescope, obtained the detailed data, called spectra, for two different gas exoplanets: HD 189733b is 370 trillion miles away in the constellation Vulpecula, and HD 209458b is 904 trillion miles away in the constellation Pegasus.
Just as a prism disperses sunlight into a rainbow, Spitzer uses an instrument called a spectrograph to reveal a spectrum by splitting light from an object into different wavelengths. The process uncovers "fingerprints" of chemicals making up the object. The exoplanets Spitzer observed are known as "hot Jupiters" because they are gaseous like Jupiter but orbit much closer to their stars.
The data indicate the two planets are drier and cloudier than predicted. Theorists thought hot Jupiters would have lots of water in their atmospheres, but were surprised when none was found around HD 209458b or HD 189733b. In addition, one of the planets, HD 209458b, showed hints of tiny sand grains, called silicates, in its atmosphere. This could mean the water is present in the planet's atmosphere but hidden under high, dusty clouds unlike anything seen around planets in our own solar system.
"The theorists' heads were spinning when they saw the data," said Jeremy Richardson of NASA's Goddard Space Flight Center, Greenbelt, Md.
"It is virtually impossible for water, in the form of vapor, to be absent from the planet, so it must be hidden, probably by the dusty cloud layer we detected in our spectrum," he said. Richardson is lead author of a paper appearing in the Feb. 22 issue of Nature that describes a spectrum for HD 209458b.
A team led by Carl Grillmair of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, Calif., captured the spectrum of HD 189733b. A team led by Mark R. Swain of JPL focused on the same planet in the Richardson study and came up with similar results. Grillmair's results will be published in the Astrophysical Journal Letters. Swain's findings have been submitted to the Astrophysical Journal Letters.
Image Credit: NASA/ JPL-Caltech/J. Richardson (GSFC)
This infrared data from NASA's Spitzer Space Telescope -- called a spectrum -- tells astronomers that a distant gas planet, a so-called "hot Jupiter" called HD 209458b, might be smothered with high clouds. It is one of the first spectra of an alien world.
Astronomers using Spitzer's spectrograph were able to obtain infrared spectra for two so-called "transiting" hot-Jupiter planets using the "secondary eclipse" technique. In this method, the spectrograph first collects the combined infrared light from the planet plus its star, then, as the planet is eclipsed by the star, the infrared light of just the star. Subtracting the latter from the former reveals the planet's own rainbow of infrared colors.
When astronomers first saw the infrared spectrum above, they were shocked. It doesn't look anything like what theorists had predicted. For example, theorists thought there'd be signatures of water in the wavelength ranges of 8 to 9 microns. The fact that water is not detected might indicate that it is hidden under a thick blanket of high, dry clouds.
In addition, the spectrum shows signs of silicate dust -- tiny grains of sand -- in the wavelength range of 9 to 10 microns. This suggests that the planet's skies could be filled with high clouds of dust unlike anything seen in our own solar system.
There is also an unidentified molecular signature at 7.78 microns. Future observations using Spitzer's spectrograph should be able to determine the nature of the mysterious feature.
This spectrum was produced by Dr. Jeremy Richardson of NASA's Goddard Space Flight Center, Greenbelt, Md. and his colleagues. The data were taken by Spitzer's infrared spectrograph on July 6 and 13, 2005.
Image Credit: NASA/JPL-Caltech/M. Swain (JPL/Caltech)
This spectrum was produced by Dr. Mark R. Swain of NASA's Jet Propulsion Laboratory in Pasadena, Calif., using a complex set of mathematical tools. It was derived using two different methods, both of which led to the same result. The data were taken on July 6 and 13, 2005, by Dr. Jeremy Richardson of NASA's Goddard Space Flight Center and his team using Spitzer's infrared spectrograph.
Image Credit: NASA/JPL-Caltech/C. Grillmair (SSC/Caltech)
Astronomers were perplexed when they first saw the infrared spectrum above. It doesn't look anything like what theorists had predicted. Theorists thought the spectra of hot, Jupiter-like planets like this one would be filled with the signatures of molecules in the planets' atmospheres. But the spectrum doesn't show any molecules, and is instead what astronomers call "flat." For example, theorists thought there'd be a strong signature of water in the form of a big drop in the wavelength range between 7 and 10 microns. The fact that water is not detected may indicate that it is hidden underneath a thick blanket of high, dry clouds. The average brightness of the spectrum is also a bit lower than theoretical predictions, suggesting that very high winds are rapidly moving the terrific heat of the noonday sun from the day side of HD 189733b to the night side.
This spectrum was produced by Dr. Carl Grillmair of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, Calif., and his colleagues. The data were taken by Spitzer's infrared spectrograph on November 22, 2006.
Image Credit: NASA/JPL-Caltech/R. Hurt (SSC)
This diagram illustrates how astronomers using NASA's Spitzer Space Telescope can capture the elusive spectra of hot-Jupiter planets. Spectra are an object's light spread apart into its basic components, or wavelengths. By dissecting light in this way, scientists can sort through it and uncover clues about the composition of the object giving off the light.
To obtain a spectrum for an object, one first needs to capture its light. Hot-Jupiter planets are so close to their stars that even the most powerful telescopes can't distinguish their light from the light of their much brighter stars.
But, there are a few planetary systems that allow astronomers to measure the light from just the planet by using a clever technique. Such "transiting" systems are oriented in such a way that, from our vantage point, the planets' orbits are seen edge-on and cross directly in front of and behind their stars.
In this technique, known as the secondary eclipse method, changes in the total infrared light from a star system are measured as its planet transits behind the star, vanishing from our Earthly point of view. The dip in observed light can then be attributed to the planet alone.
To capture a spectrum of the planet, Spitzer must observe the system twice. It takes a spectrum of the star together with the planet (first panel), then, as the planet disappears from view, a spectrum of just the star (second panel). By subtracting the star's spectrum from the combined spectrum of the star plus the planet, it is able to get the spectrum for just the planet (third panel).
This ground-breaking technique was used by Spitzer to obtain the first-ever spectra of two planets beyond our solar system, HD 209458b and HD 189733b. The results suggest that the hot planets are socked in with dry clouds high up in the planet's stratospheres. In addition, HD 209458b showed hints of silicates, indicating those high clouds might be made of very fine sand-like particles.
"With these new observations, we are refining the tools that we will one day need to find life elsewhere if it exists," said Swain. "It's sort of like a dress rehearsal."
Spitzer teased out spectra from the feeble light of the two planets through the "secondary eclipse" technique. In this method, the telescope monitors a planet as it transits, or circles behind its star, temporarily disappearing from view.
By measuring the dip in infrared light that occurred when the planets disappeared, Spitzer's spectrograph was able to obtain spectra of the planets alone. The technique will work only in infrared wavelengths, where the planet is brighter than in visible wavelengths and stands out better next to the overwhelming glare of its star.
In previous observations of HD 209458b, NASA's Hubble Space Telescope measured changes in the light from the star, not the planet, as the planet passed in front. Those observations revealed individual elements, such as sodium, oxygen, carbon and hydrogen, which bounce around the very top of the planet.
"When we first set out to make these observations, they were considered high risk because not many people thought they would work," said Grillmair. "But Spitzer has turned out to be superbly designed and more than up to the task."
Jet Propulsion Laboratory News Release
