Methane Glows in Alien Planet’s Atmosphere
Artist's impression of the Jupiter-sized extrasolar planet HD 189733b, now known to have methane and water.
Credit: ESA, NASA and G. Tinetti (University College London, UK & ESA)

The glow of methane has been detected in the atmosphere of Jupiter-sized alien planet orbiting close to its parent star.

Because the signature of glowing methane, which might be triggered in a similar way to Earth's auroras, is so strong, it could help scientists better understand the atmospheres of exoplanets, if it turns out to be a common feature among them.

The detection was also made from a ground-based telescope and not space-based one, suggesting that many more detailed measurements of exoplanet atmospheres will be made in the coming years, possibly even the signatures of biological activity, researchers said.

Methane is belched out by certain kinds of microbes on Earth (as well as by big animals, such as cows), and scientists think this is one form that potential alien life could take. (Methane is also created through geophysical and chemical processes on Earth that have nothing to do with life.)

"That's not where we are today, but that's where we're going," said Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, Calif., who led the team that made the methane discovery.

Glowing methane

Scientists detected this particular signature of methane in the atmosphere of an extrasolar planet dubbed HD 189733b, which was one of the first exoplanets to have its atmosphere "sniffed" my spectrometers, which measure the range of light given off by a particular object and show the light signatures that are peculiar to different elements and molecules.

?Water vapor, carbon dioxide, and methane have already been detected in HD 189733b's atmosphere, though that first methane detection had a different signature than the new one.

The new detection seems to be from the fluorescence of methane in the atmosphere of the planet. (An Earth analogue to this phenomenon would be something like the aurora borealis, Swain said.)

The finding, detailed in the Feb. 4 issue of the journal Nature, was unexpected if not a total surprise, as similar signatures have been seen in the atmospheres of bodies in our own solar system.

"It's not particularly surprising since we have seen fluorescent methane in Jupiter, Saturn and even Titan," said Seth Redfield of Wesleyan University in Middletown, Conn., who was not involved with the finding, but has previously made detections of sodium in the same exoplanet's atmosphere. Redfield wrote an opinion article about the new discovery in the same issue of Nature.

Understanding atmospheres

For the atmospheric signatures collected for exoplanets so far, astronomers have assumed that heat is what is causing the emission of various atmospheric constituents, as most are so-called hot Jupiters, which orbit very close to their stars and are bathed in large amounts of stellar radiation.

But heat can't explain the fluorescence of methane. "The light is being generated by something other than heat," Swain told SPACE.com.

But the energy source driving the emission is still a mystery.

"We don't know the answer for that today," Swain said, but he added that two possible sources where collisions with photons or charged particles from the stellar wind. ?The solar wind from our sun is not known to cause methane fluorescence in any planets in our solar system.

But the fluorescence does tell astronomers something about the atmosphere of the planet: that the part where the fluorescence is happening is likely "very tenuous layers in the atmosphere of the planet," Swain said.

This is because fluorescence is what in physics is called a non-locally thermodynamic equilibrium process.

So in an atmosphere that is in locally thermodynamic equilibrium, energy moves between particles primarily through collisions ? this can happen because the atmosphere is thick and the molecules are relatively close together. This is the case in the lower portions of Earth's atmosphere.

But when the atmosphere thins out, its molecules can become far enough apart that the time between collisions is long enough that energy can get to molecules through other means. A similar process occurs in the upper portions of Earth's atmosphere, where things like the solar wind can collide with particles ? this is what creates the auroras that flash over Earth's poles.

So it's possible that the signature of methane fluorescence from HD 189733b is coming from a different part of the atmosphere than the previous methane signature, though Swain cautions that it will take more observations and new atmospheric models to really characterize the exoplanet's atmosphere.

Future detections

Swain and his team are already at work looking for this fluorescent signature in other exoplanets. If it turns out to be a common feature, "it could change how detectable these exoplanets are," because the signature is strong and unique, Swain said.

Redfield said the finding is exciting because it adds to the list of known exoplanet atmospheric components, which are building up at a time when "we're just getting use to finding exoplanets." In a decade, exoplanet atmosphere detections will likely be as routine as exoplanet detections now are, he said.

Making more detections of methane in particular could be helpful because it can be a by-product of biological processes. Building a better understanding of what kinds of methane are out there and where in exoplanet atmospheres the gas occurs could help scientists determine which signatures are most likely to be related to alien biology.

"This is one step on a much longer journey," Redfield said.

The finding is also exciting, both Swain and Redfield said, because it was made with a relatively modest-sized ground-based telescope, NASA's Infrared Telescope Facility (IRTF) in Hawaii, whereas most other atmospheric detections were made with space-based telescopes, such as Hubble and the Spitzer Space Telescope.

The detections of atmospheres from the Earth's surface can only be made in particular wavelengths of light that aren't absorbed or scattered by the Earth's atmosphere, but ground-based detections are an important complement to space-based ones because ground-based telescopes are much bigger ? while Hubble is 2.4-m telescope, the Keck telescopes (also in Hawaii) are 10 meters in diameter. This means that atmospheres could be observed with more detail or at fainter objects.

This capability "is going to prove really, really critical to understanding these exoplanet atmospheres," Redfield said.

Swain already has plans to use some bigger Earth-based telescopes in the future.

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