NASA's Spitzer Space Telescope has found evidence that a hot, Neptune-sized planet orbiting a star beyond our sun lacks methane -- an ingredient common to many planets in our own solar system.
The planet illustrated here, called GJ 436b is about 980 degrees Fahrenheit (527 degrees Celsius) – it was expected to have methane but Spitzer's observations showed it does not.
A big, hot alien planet with almost no methane in its atmosphere continues to stump astronomers, who have long thought the gas was a common feature of such worlds.
In our solar system, the gas giants Jupiter, Saturn, Uranus and Neptune are all rich in methane. But when astronomers trained NASA's Spitzer Space Telescope on the extrasolar planet GJ 436b, a Neptune-sized world around a star 33 light-years from Earth, they found almost none of the stuff.
"Methane should be abundant on a planet of this temperature and size, but we found 7,000 times less methane than what the models predict," said Kevin Stevenson of the University of Central Florida in a new NASA statement released Monday. Stevenson was lead author of a study on the planet published in April.
Methane is the simplest hydrocarbon, made up of four hydrogen atoms attached to a single carbon. It forms easily where hydrogen and carbon are common, as in the atmospheres of our solar system's gas giants. So the researchers expected to find a lot of it when they analyzed the spectrum of the similar GJ 436b.
But they were surprised to find the alien world's atmosphere to be full of carbon monoxide instead. [The Strangest Alien Planets]
"Actually, it blew our minds," said principal investigator and co-author Joseph Harrington, also of UCF.
Alien chemistry on gas giant
There are several possibilities for what's going on with GJ 436b, the researchers said. For example, ultraviolet radiation could be breaking the planet's methane apart into ethylene or some other polymer.
"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," Harrington said. "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."
The researchers also speculate that strong vertical winds in the planet's atmosphere might be sweeping up material from deep, hot layers where carbon monoxide is abundant, replacing methane with the other gas. The planet has an average temperature of about 980 degrees Fahrenheit (527 degrees Celsius).
Or it could be something else entirely, researchers said.
"This planet's atmosphere could have some sort of alien chemistry going on," Harrington said. "We just don't know yet."
Methane in space, on Earth
Gas giant planets are not the only worlds that contain methane in their atmospheres. The gas is common on Earth and has been spotted on Mars. For Mars and the gas giants, the gas is not an indicator of life since it can form through geophysical and chemical processes.
On Earth, methane can be a sign of life since it can form naturally bubbling up from the bottom of swamps as organic matter decays or as a byproduct from cows, goats or other lifeforms.
Methane's link to life on our planet has made it a sought-after gas on other planets, including distant Earth-sized planets, since its presence could be a potential sign of biological activity.
But the lack of methane in a gas-giant world like GJ 436b, which was expected to have the compound in spades, is still a mystery to be solved, researchers said.
"GJ 436b is telling us something important," Harrington said. "We're not in Kansas anymore."
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