Faraway planet lacks methane gas, violating everything we know about atmospheres

A Neptune-sized exoplanet thirty-three light-years away is defying all our expectations of what planetary atmospheres should look like. The planet has 7,000 times less methane gas than expected, but it's rich in carbon monoxides...and we have no idea why.

The planet, designated GJ 436b, should be absolutely dominated by methane gas, and yet analysis reveals there's almost none of that gas in its atmosphere. All four of our solar system's gas giants are rich in methane, and that's not by accident. Any planet rich in hydrogen and carbon will have a lot of methane - or, as it's known chemically, HC4 - because it's the simplest hydrocarbon compound. We expected a gas giant, particularly one so close to us, to have much the same chemical composition, which makes this such a weird result.

But surely some missing methane isn't that big of a deal, is it? Well, according to chief researcher Joseph Harrington, it's completely shocking:

"Actually, it blew our minds. This planet's atmosphere could have some sort of alien chemistry going on. We just don't know yet."

The researchers do have at least one theory - the planet's star is interfering with the natural production of methane, breaking it apart and recombining it into something else:

UV radiation from the planet's star could be converting the methane into polymers like ethylene," says Harrington. "If you put plastic wrap out in the sun, the UV radiation breaks down the carbon bonds in the plastic, causing it to deteriorate as the long carbon chains break. We propose a similar process on GJ 436b, but there hydrogen atoms split off from methane and let the remnants stick together to make ethylene (C2</subH4)."

Although methane has a reputation on Earth as being a harmful, poisonous gas, its presence on rocky planets like Earth can be a key sign of life, particularly when coupled with signs of oxygen. As such, astronomers had hoped to search rocky exoplanets for signs of methane gas, one of the easier gases to detect at from many light-years away.

But that was built on the assumption that faraway atmospheres follow the same rules as those of the planets in our solar system. If that's not the case - and GJ 436b certainly suggests that could be the case - then our search for other Earths just got a whole lot more complicated.

[Science @ NASA]