NASA’s Spitzer Space Telescope has discovered something odd about a distant planet—it lacks methane, an ingredient common to many of the planets in our solar system.
“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 that appeared April 22 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.
“In this case, we expected to find methane not because of the presence of life, but because of the planet’s chemistry. This type of planet should have cooked up methane. It’s like dipping bread into beaten eggs, frying it, and getting oatmeal in the end,” said Joseph Harrington of the University of Central Florida, the principal investigator of the research.
On Planet Earth, methane is commonly known to us as the “natural gas” that we burn daily in our stoves and heaters. It is piped from underground deposits, usually found in conjunction with petroleum, but it is also being formed today—mainly by microbes living in cows or in boggy places such as 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 planetary 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 1,340 degrees Fahrenheit is expected to have a large amount of methane and a small amount of carbon monoxide. According to our knowledge of chemistry, the carbon should “prefer” to be in the form of methane at these temperatures.
With a temperature of 980 degrees Fahrenheit, planet GJ 436b should have abundant methane and very 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 from us 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.
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