In distant planet's fluorescent glow, a new way to look for life

For the first time, astronomers detected the fluorescent signature of an exoplanet's atmosphere. Fluorescence can give scientists clues about processes that might make a planet habitable for life.

For the first time, astronomers have detected gas glowing like a fluorescent bulb in the atmosphere of a planet orbiting a distant star.

Researchers have had glimpses of the atmospheres of so-called exoplanets before, but never in this way. The fluorescent signature of HD 189733b, a Jupiter-size planet in the faint constellation of Vulpecula, could help astronomers explore exoplanets in more detail – including ways that help them in the search for life elsewhere in the galaxy.

HD 189733b is no candidate for life. But the techniques used to study fluorescence in the highest reaches of its atmosphere will help scientists tease out information that, on more Earth-like planets, could be crucial to life. These include the presence and strength of a planet's magnetic field and processes driving changes in the planet's atmosphere, researchers say.

Moreover, the work can be done with modest-size ground-based telescopes, not just mountaintop behemoths. As a result, more telescopes can be employed to try to uncover the processes at work on exoplanets, accelerating the pace of discovery, says Mark Swain, an astronomer who led a team of researchers conducting the measurements.

The team's work "is tremendously exciting," says Seth Redfield, an astronomer at Wesleyan University in Middletown, Conn. So little is known about individual exoplanets that any new window on these distant worlds is important, he continues.

A peculiar planetary glow

HD 189733b is orbiting a sun-like star some 63 light-years away. It's slightly larger than Jupiter and has 13 percent more mass. But unlike Jupiter, HD 189733b orbits far closer to its "sun" than Jupiter does. At a distance of only about 2.8 million miles, the planet completes one orbit in 2.2 Earth days. The puts it squarely into the category of "hot Jupiters."

It was discovered in 2005, and since then astronomers have used ground- and space-based telescopes to detect several gases in the planet's atmosphere, including sodium, water vapor, carbon dioxide, and methane.

They did this by studying the planet as it swung in front of and behind its star. In this way, they used the star's light as well as light coming from the planet's day side before it moved behind the star to probe the atmosphere's composition.

But such work can be tough slogging, and it's hard to get time on space-based telescopes. A long line of astronomers is waiting for time to do a broad range of research projects on these expensive observatories.

Dr. Swain and his team wanted to see if ground-based telescopes could fill that gap. They did it by refining the calibration of a well-used spectrometer bolted to the back of NASA's Infrared Telescope Facility atop Hawaii's Mauna Kea volcano.

What fluorescence can tell us

The discovery of fluorescence – from methane in the atmosphere – "was a complete surprise," says Swain, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. In fact, the methane signature in the planet's fluorescence was stronger than the one gathered by Hubble and Spitzer.

The phenomenon has been observed before in the atmospheres of Saturn, its moon Titan, and Jupiter, Dr. Redfield says. But finding it on exoplanets is an important addition to astronomers' toolkits.

Methane itself is of keen interest. Methane on Earth comes from geologic and atmospheric processes. But it also comes from biological sources – from bacteria and termites to dairy cows. Thus, if methane is found in unexpectedly large concentrations in the atmosphere of an Earth-like planet at the right distance from its host star, it could suggest that the planet harbors life.

Moreover, fluorescing molecules can help scientists probe an exoplanet's magnetic field, which could help determine if a planet would even be hospitable to life. Strong magnetic fields can serve as a barrier against cosmic radiation.

Already, Swain is talking about using the new technique to target known super-Earths – planets with a greater mass than Earth but smaller than that of our solar system’s gas giants.

“An immediate goal for using this technique is to more fully characterize the atmosphere of this and other exoplanets, including detection of organic and possibly prebiotic molecules” like those that preceded the evolution of life on Earth, said Swain. “We’re ready to undertake that task.”

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