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Planetary News: Extrasolar Planets (2007)Spitzer Captures the Light from Dry Distant WorldsBy Amir Alexander21 February 2007
Contrary to the expectations of planetary scientists, the atmospheres of gas giant planets orbiting distant stars appear to contain no water. So say three teams of scientists who used the Spitzer space telescope to capture the light spectrum of two known extrasolar planets. Scientists had long assumed that such planets contain substantial amounts of water, which would be present as vapor in their atmosphere. But the spectrums obtained by Spitzer show no water at all. This, according to Jeremy Richardson of NASA Goddard Space Center who led one of the teams, is a major surprise. “The theorists’ heads were spinning when they saw this result” he said. The fact that Spitzer can detect light spectra from extrasolar planets at all is a surprising accomplishment in itself. The telescope was designed years before the first extrasolar planets were first found, and was intended for deep space observations, not for highly specialized work of detecting planetary spectra. But in 2005 Spitzer was first to isolate the light coming from an extrasolar planet, and now, for the first time, the light data it gathered from two extrasolar planets is sufficiently clear that it is possible to analyze part of their spectrum. “We had no idea when we designed Spitzer that it would make such a dramatic step in characterizing exoplanets” said Spitzer project scientist Michael Werner of the Jet Propulsion Laboratory. Even with Spitzer’s highly sensitive spectrometer, isolating the light coming from a dim planet orbiting in the immediate vicinity of a bright star is a daunting challenge. It is “like trying to see a firefly next to an airport searchlight from several miles away” explained Carl Grillmair of the Spitzer Science Center, who led one of research teams. To accomplish this, scientists focused the telescope on two of the 14 extrasolar planets that are known to pass directly in front of their star when observed from Earth. Such “transiting” planets are recognized by the slight dimming of their star as they pass before it. At the opposite end of their orbit, transiting planets disappear behind their home star, in an event known as a “secondary transit.” To detect the light signal from the planet alone, scientists first used Spitzer to measure the combined spectrum of the star and planet when the planet was close by the star. They then took a second measurement of the star alone, when the planet disappeared behind it during a secondary transit. The difference between the two spectra consists of the planet’s contribution of the combined spectrum, and therefore consists only of light from the planet. That is the light whose analysis has got theoreticians scratching their heads.
One team, led by Grillmair, focused on a planet known as HD 189733b, 62 light-years away, and two teams, led by Richardson and Mark Swain of the Jet Propulsion Laboratory, observed a planet designated HD 209458b, 153 light-years away. Both planets are “Hot Jupiters,” gas giants that orbit in the very shadow of their star, with HD 189733b completing each orbit in 2.2 days, and HD 209458b circling its star once every 3.5 days. But regardless of the planet they observed and the methods they used, all three teams arrived at the same conclusion: the light spectra from the two planets shows no evidence of water vapor in the atmosphere. “We’re getting our first sniffs of air from an alien world” reflected David Charbonneau of the Harvard Smithsonian Center for Astrophysics, a member of Grillmair’s team, “and what we found surprised us.” “Or more accurately what we DIDN”T find surprised us.” “we expected to see common molecules like water, methane, or carbon dioxide” added Grillmair. “But . . . the spectrum was flat, with no molecular fingerprints that we could detect. The most fundamental thing we predicted was wrong.” The fact that water and other common atmospheric components were not detected in the planets’ spectrum does not mean that they are not there cautioned Richardson. “It is virtually impossible for water, in the form of vapor, to be absent from the planet” he argued, and therefore it must somehow be hidden. The spectrum of planet HD 209458b, he pointed out, contained hints of silicate molecules. On Earth, such molecules are usually present as sand or in rocks, but on a Hot Jupiter, where the average temperature is a searing 1100 degrees Kelvin, they would exist as tiny dust grains floating in the atmosphere. If clouds of silicate dust do indeed envelope the surface of these distant giants then they would certainly block out the signals from water vapor and other atmospheric components. “These worlds are blacker than any planet in our solar system” said Charbonneau. An atmospheric coat of silicate dust would explain why this is so.
It is, however, far too early to jump to conclusions about the meaning of the unexpected results. Many more observations will be needed before scientists will feel confident that they understand the puzzling data. But Alan Boss of the Carnegie Institution in Washington, who was not a member of the research teams, is not surprised by the plethora of open questions. In the field of extrasolar planets, he pointed out, discoveries have consistently outstripped theories. “The current score is Observers 200, Theorists 0” he quipped. But there was no dispute about the importance of the importance of Spiter’s discoveries. The same methods that are currently used to study the atmospheres of steaming gar giants, noted Boss, will one day be used to search for the signs of life on temperate rocky planets such as our own. The new results, said Boss, start us on the road of “following the water” that could lead to the discovery of life on faraway worlds. |
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