Water Mysteriously Absent from Extrasolar Planets' Atmospheres

Contrary to predictions, two planets orbiting distant stars show no signs of water and other simple compounds; dark clouds or haze may hide them
extrasolar planet

For the first time, telescopes have captured the light spectra emitted directly from planets outside of our solar system. Researchers trained the infrared-sensitive Spitzer Space Telescope on two extrasolar gas giant planets, called HD 209458 b and HD 189733 b.

The atmospheres are most notable for what they lack: "We find no evidence for water in the spectrum, and all the theorists will tell you that there should be water (in the form of vapor) in the atmosphere[s] of these planets," says astrophysicist Jeremy Richardson of the NASA Goddard Space Flight Center, a member of a team that analyzed HD 209458 b. A second group measured the spectrum of HD 189733 b, which is about 60 light-years away, and a third one reanalyzed the data from the former, which resides in the constellation Pegasus, more than 150 light-years distant.

Both planets' spectra are missing water along with carbon monoxide and methane—at least some of which researchers had expected to find. Richardson's group did identify what seem to be clouds of fine dust high in HD 209458 b's atmosphere, suggesting that dark clouds could be obscuring light from the missing constituents in at least one case. "Our idea is there is a really thick bank of clouds high in the atmosphere," he says, "so you just can't see deep enough.

The two planets are members of a large class of extrasolar planets known as "hot Jupiters" because they are gaseous like Jupiter but orbit closely to their stars, giving them high temperatures of 1,000 to 2,000 kelvins. Based on the inferred quantities of elements in the planets and their temperatures, researchers had deduced that certain compounds, including water, should form, Richardson says.

As seen from Earth, both planets travel in front of and behind their stars during their orbits, which last only a few days. To make their measurements, two separate groups had Spitzer's infrared detector take a steady bead on the planets' stars while the planets were eclipsed by them. The researchers determined the planets' spectra by subtracting the light emitted by each star from the combined light radiated by the star and planet, leaving just the planet's light spectrum.

Prior observations of extrasolar planets had detected elements by analyzing light passing through their atmospheres, but Richardson says this is the first measurement of the energy coming directly from the planets at many different wavelengths. Researchers can get clues about the types of chemicals in the atmosphere by analyzing the brightness of wavelengths emanating from it, because all chemicals absorb light of characteristic wavelengths.

"The spectra certainly seem to lack water. These are very important observations," says Jonathan Fortney of NASA Ames Research Center, who has modeled extrasolar atmospheres but was not involved in any of the new reports.

In a paper published online February 21 by Nature, Richardson's group reports that HD 209458 b's spectrum does suggest the presence of clouds of perhaps 10-micron-wide particles of dustlike silicate, or silicon oxide. According to Fortney, "We know silicate clouds affect the spectra of brown dwarfs at similar atmospheric temperatures."

But Fortney notes that the two planets differ in temperature by 200 K, which suggests clouds of the same composition are not present in both. Because the planets are so close to their suns, the star shine may burn off a haze that also obscures light, he says. Another possibility, he adds, is that the atmospheres might have uniform temperatures all the way down, which would flatten out their light spectra because molecules would not transfer energy as easily.

The study of HD 189733 b is slated for publication in The Astrophysical Journal Letters, where a team has also submitted the second report on HD 209458 b.

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