Living in the Universe

Methane on a Distant World

March 28, 2008 on 8:00 am | In Exoplanets | Comments Off

Fourth Week of March 2008

Last week saw the exciting detection of the molecule methane for the first time in the atmosphere of a planet outside the solar system. The finding comes from the extrasolar system called HD 189733. It’s a system that’s been in the news before because the star has a gaseous hot Jupiter locked in a tight orbit around it.  They are both 63 light years away. According to the team, who are from NASA’s Jet Propulsion Lab and the University College of London, the observations decisively show that methane is present, in addition to water. The same team reported last year that they’d identified water vapor in the atmosphere of this planet using a similar technique.

This story conjures up thoughts of cow farts on Alpha Centauri, but although we earthlings associate methane with gassy cows and ruminants, it is a common and perfectly non-biological constituent of other atmospheres in the solar system, including Mars and Titan, as well as the gas giants Jupiter, Saturn, Uranus, and Neptune. There’s been a debate in the past few years over the presence of methane in the thin Martian atmosphere and whether that methane could point to microbial life under the surface. But the methane on Mars is at such a low concentration—a few parts per billion—that it doesn’t decisively indicate biological metabolism at work. It could easily come from geological processes. Researchers believe that methane and water will be common constituents of planetary atmospheres outside the solar system; nonetheless, it’s an important ingredient.

Methane as a tracer is part of a larger debate over biomarkers. Which atmospheric tracers are most likely to indicate biology? Measuring the relative abundance of elements in an atmosphere allows researchers to infer details about how the planet has formed and its weather patterns. The planet and star HD 189733 present an exciting opportunity because it’s one of the few extrasolar planets suited to such measurements.  It’s a transiting exoplanet, which means it crosses in front of its parent star, and because it’s on such a tight orbit it does so every 2.2 days. And because it’s a big planet, Jupiter-sized, it blocks two percent of the parent star’s light each time it does so.

The eclipse technique relies on the fact that the planetary atmosphere is backlit by the star. Every molecule absorbs light most strongly at particular wavelengths so by measuring the amount of light blocked at different wavelengths in the planetary atmosphere, researchers can infer its composition.  There’s extra absorption at the particular wavelengths corresponding to methane when the planet is in transit.  Researchers reported their results in the journal Nature. They saw the telltale patterns of both methane and water. The instrument used was NICMOS, the Near Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope, which was the same instrument they’d used to detect water the years before.

We might wonder if this technique can find interesting gases in planets more like Earth, but the technique is difficult. Jupiter, remember, is ten times larger than the Earth so if an Earth-like planet passes in front of its star it blocks ten squared, or a hundred times, less light.  In other words the technique would have to be a hundred times more sensitive to find methane in an Earth-like planet, assuming we can find Earth-like planets.

In an accompanying commentary by planetary scientist Adam Showman, who works across the street at the Lunar and Planetary Lab, the implication is that a third constituent carbon monoxide is waiting to be found in the atmosphere of this planet. Planets are presumed to form from the same material as stars but Showman notes that the intensity of the methane absorption implies that the planet has a low methane-to-hydrogen ratio, no more than five parts per hundred thousand, which is only ten percent of its parent star. The scorching temperature of this planet, around a thousand Kelvin or thirteen hundred degrees Fahrenheit, may cause the carbon in its atmosphere to prefer joining oxygen as carbon monoxide instead of forming methane. “Finding the carbon monoxide and mapping its distribution with that of methane will illuminate the planet’s exotic weather patterns,” according to Showman. He says, “These are exciting times for studies of extrasolar planets.  Researchers are finally moving beyond simply discovering them to truly characterizing them as worlds.”

This is slow and painstaking research, but over the next decade we can anticipate that dozens of extrasolar planets will have spectral diagnostics and we’ll begin to truly understand the nature of their atmospheres. And that is an important step along the road to using biomarkers to detect microbial life on those planets.