Methane Glows in Alien Planet’s Atmosphere

Key Organic Molecule Detected at Extrasolar Planet
Artist's impression of the Jupiter-sized extrasolar planet HD 189733b, now known to have methane and water.
(Image: © ESA, NASA and G. Tinetti (University College London, UK & ESA))

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

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

The detection was also made from a ground-based telescopeand not space-based one, suggesting that many more detailed measurements of exoplanetatmospheres will be made in the coming years, possibly even the signaturesof 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 oneform 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'regoing," 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 inthe atmosphere of an extrasolar planet dubbed HD 189733b, which was one of the firstexoplanets to have its atmosphere "sniffed" my spectrometers,which measure the range of light given off by a particular object and show thelight signatures that are peculiar to different elements and molecules.

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

The new detection seems to be from the fluorescence ofmethane in the atmosphere of the planet. (An Earth analogue to this phenomenonwould be something like the auroraborealis, Swain said.)

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

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

Understanding atmospheres

For the atmospheric signatures collected for exoplanets sofar, astronomers have assumed that heat is what is causing the emission ofvarious atmospheric constituents, as most are so-called hotJupiters, which orbit very close to their stars and are bathed in largeamounts of stellar radiation.

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

But the energy source driving the emission is still amystery.

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

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

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

So in an atmosphere that is in locally thermodynamicequilibrium, energy moves between particles primarily through collisions ? thiscan happen because the atmosphere is thick and the molecules are relativelyclose together. This is the case in the lower portions of Earth's atmosphere.

But when the atmosphere thins out, its molecules can becomefar enough apart that the time between collisions is long enough that energycan get to molecules through other means. A similar process occurs in the upperportions of Earth's atmosphere, where things like the solar wind can collidewith particles ? this is what creates the auroras that flash over Earth'spoles.

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

Future detections

Swain and his team are already at work looking for thisfluorescent signature in other exoplanets. If it turns out to be a commonfeature, "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 thelist of known exoplanet atmospheric components, which are building up at a timewhen "we're just getting use to finding exoplanets." In a decade,exoplanet atmosphere detections will likely be as routine as exoplanet detectionsnow are, he said.

Making more detections of methane in particular could behelpful because it can be a by-product of biological processes. Building abetter understanding of what kinds of methane are out there and where inexoplanet atmospheres the gas occurs could help scientists determine whichsignatures are most likely to be related to alienbiology.

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

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 otheratmospheric detections were made with space-based telescopes, such as Hubbleand the Spitzer Space Telescope.

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

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

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

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