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Down to Earth: Technique Lets Ground-Based Telescopes Parse Exoplanet Atmospheres

A new study shows promise for Earthbound observatories in identifying molecules in planetary atmospheres outside the solar system
HD 189733b

ESA, NASA, G. Tinetti (University College London, UK & ESA) and M. Kornmesser (ESA/Hubble)

In the 15 years since the first planet orbiting a sunlike star outside our solar system was conclusively discovered, astronomers have compiled a vast and diverse menagerie of such so-called exoplanets. Of the more than 400 now known, many are large—10 times the mass of Jupiter or more—and a precious few are small, just a few times Earth's mass. Little is known about these faraway worlds beyond bulk properties such as their orbital periods, estimated masses and, on relatively rare occasions, their diameters.

As instrumentation and observation techniques have become more sophisticated over the past few years, exoplanet researchers have begun probing the nature of these bodies more deeply, even identifying specific molecules in their atmospheres. Someday, detailed observations of exoplanet atmospheres might allow astronomers to pinpoint worlds where life seems to have taken hold, producing, for instance, otherwise unexplainable cocktails of oxygen and methane.

But although many planets are first discovered using telescopes on the ground, most of the spectroscopic measurements that have afforded astronomers a glimpse at their atmospheres have come from space-based observatories such as the Hubble and the Spitzer space telescopes, which operate outside the obscuring veil of Earth's own atmosphere. (Telescope spectrographs break down the light coming from a celestial object into its individual wavelengths, allowing the identification of atoms or molecules by their unique emission or absorption properties.)

In a paper published in the February 4 issue of Nature, a team of researchers from the U.S., England and Germany demonstrates the ability of moderate-size telescopes on the ground to identify the chemical fingerprints in exoplanet atmospheres. (Scientific American is part of Nature Publishing Group.) With observation time on the few active orbiting observatories at a premium, an effective ground-based approach would allow exoplanet characterization to proceed more quickly, and on larger telescopes, to boot.

Mark Swain, an astronomer at the NASA Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and his colleagues used a three-meter telescope in Hawaii to take infrared spectra of the exoplanet HD 189733 b, a relatively nearby exoplanet some 63 light-years distant that was discovered at a French observatory in 2005. HD 189733 b was already known from space-borne observations to harbor several specific molecules in its atmosphere: water, methane, carbon dioxide and carbon monoxide. After painstaking work to cancel out the effects of Earth's atmosphere, Swain and his colleagues found that their ground-based spectra matched up well with those from space where they overlapped. What is more, the observations from the ground turned up a curious feature outside the space-based coverage.

"The big hurdle was trying to figure out how to remove the effects of Earth's atmosphere," Swain says, adding that honing the calibration method took about three years. Study co-author Pieter Deroo, also an astronomer at JPL, says that he and his colleagues needed to find a very faint exoplanetary signal in a data set dominated by Earth's atmospheric effects. "What we did basically is say, let's assume first that everything we see is due to the Earth's atmosphere," Deroo says. "That's a really good assumption, because the Earth's atmosphere is producing signals 500 times larger than the exoplanet is. And so we clean it out, and then we search for correlations in the residuals."

The team's technique could be used on other telescopes, Swain says, opening up much larger instruments for use than those available in space. (Even the relatively modest telescope Swain and his co-authors used is larger than Hubble, whose primary mirror is 2.4 meters across.) "I think the prospects from the ground to do spectra of many exoplanets are excellent," Swain says.

Carl Grillmair, an astrophysicist at the California Institute of Technology, calls the new research "an impressive and potentially very exciting result" that will likely motivate a number of follow-up observations on larger telescopes. "While ground-based telescopes will always have to contend with the obscuration and instability in our own atmosphere, development of successful observing techniques like this will enable us to throw much larger apertures at the problem than we will ever have available in space," Grillmair says.

Beyond the wavelength coverage of the space-borne observatories, the spectra taken in Hawaii even turned up a curious new feature—a boost in emissions of unknown origin at wavelengths of about 3.3 microns. (A micron is one millionth of a meter.) "It was totally unexpected," Swain says. "There's a big spike at 3.3 microns in the spectrum. Well, it wasn't supposed to look anything like that."

Although the emission remains a mystery, it may arise from the fluorescence of atmospheric methane, a phenomenon witnessed in our own solar system. "It's going to take some more testing and more observations and more modeling to figure out what in the world is going on," Swain says. "It's a real puzzle." Grillmair notes that, in conjunction with earlier observations, the mysterious spike in the spectra "is beginning to paint a picture of a much more complex and dynamic atmosphere than we had previously considered."

For Swain and his co-authors, testing their approach on a planet whose properties were already well studied was crucial to establishing the validity of the ground-based observations. Without corroborating data from space, Swain says, "I think we wouldn't have believed our answer, and I don't think anybody else would have either."

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