A Mystery of Missing Methane
'This Planet Tastes Funny,' According to Spitzer Telescope
“It’s a big puzzle,” said Kevin Stevenson, a planetary sciences graduate student at the University of Central Florida in Orlando, lead author of a study in the journal Nature. “Models tell us that the carbon in this planet should be in the form of methane. Theorists are going to be quite busy trying to figure this one out.”
The discovery brings astronomers one step closer to probing the atmospheres of distant planets the size of Earth. The methane-free planet, called GJ 436b, is about the size of Neptune, making it the smallest distant planet that any telescope has successfully “tasted,” or analyzed. Eventually, a larger space telescope could use the same kind of technique to search smaller, Earth-like worlds for methane and other chemical signs of life, such as water, oxygen and carbon dioxide.
“Ultimately, we want to find biosignatures on a small, rocky world. Oxygen, especially with even a little methane, would tell us that we humans might not be alone,” said Stevenson.
Methane is present on our life-bearing planet, manufactured primarily by microbes living in cows and soaking in waterlogged rice fields. All of the giant planets in our solar system have methane too, despite their lack of cows. Neptune is blue because of this chemical, which absorbs red light. Methane is a common ingredient of relatively cool bodies, including “failed” stars, which are called brown dwarfs.
In fact, any world with the common atmospheric mix of hydrogen, carbon and oxygen, and a temperature up to 1,000 Kelvin (1,340 degrees Fahrenheit) is expected to have a large amount of methane and a small amount of carbon monoxide. The carbon should “prefer” to be in the form of methane at these temperatures.
At 800 Kelvin (or 980 degrees Fahrenheit), GJ 436b is supposed to have abundant methane and little carbon monoxide. Spitzer observations have shown the opposite. The space telescope has captured the planet’s light in six infrared wavelengths, showing evidence for carbon monoxide but not methane.
“We’re scratching our heads,” said Harrington. “But what this does tell us is that there is room for improvement in our models. Now we have actual data on faraway planets that will teach us what’s really going on in their atmospheres.”
GJ 436b is located 33 light-years away in the constellation Leo, the Lion. It rides in a tight, 2.64-day orbit around its small star, an “M-dwarf” much cooler than our Sun. The planet transits, or crosses in front of, its star as viewed from Earth.
“The Spitzer technique is being pushed to smaller, cooler planets more like our Earth than the previously studied hot Jupiters,” said Charles Beichman, director of NASA’s Exoplanet Science Institute at NASA’s Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena, Calif. “In coming years, we can expect that a space telescope could characterize the atmosphere of a rocky planet a few times the size of the Earth. Such a planet might show signposts of life.”