NASA’s Spitzer Space Telescope has discovered something odd about the atmosphere of a Neptune-size planet orbiting a distant star it lacks methane, a common ingredient for many planets in our solar system.

The discovery was made after Spitzer captured the extrasolar planet’s light in six infrared wavelengths, allowing researchers to analyze the components of the exoplanet’s atmosphere.

The results are puzzling because existing models suggest that carbon present in the exoplanet should be in the form of methane, said study lead author Kevin Stevenson, a doctoral student at the University of Central Florida in Orlando.

“The observations are quite telling,” Stevenson said. “The ball is in the theorists’ court now. They will have to improve their models, taking into account the disequilibrium processes that could account for what is happening. The current models are a very good first step in determining the atmospheres of these planets, but now we need to go a step further.”

The hot, methane-free planet, called GJ 436b, is about the size of Neptune, making it the smallest alien planet whose atmosphere has been successfully analyzed by any telescope. The smallest known exoplanet today is a distant, rocky world called Gliese 581e, which is located about 20.5 light-years from Earth.

GJ 436b is 33 light-years away. The planet rides in a tight, 2.64-day orbit around its small star, an M-dwarf class star that is much cooler than our sun.

The study’s findings will help move astronomers one step closer to probing and characterizing the atmospheres of distant planets the size of Earth.

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,” Stevenson said. “Oxygen, especially with even a little methane, would tell us that we humans might not be alone.”

All of the giant planets in our solar system have methane in their atmospheres. On Earth, methane is manufactured primarily by microbes living in cows and soaking in waterlogged rice fields.

Neptune is blue because of this chemical, which absorbs red light. Methane is a common ingredient of relatively cool bodies, including brown dwarfs, which are cool, dim substars.

In fact, any planet with the common atmospheric mix of hydrogen, carbon and oxygen and a temperature of up to 727 degrees Celsius is expected to have a large amount of methane and a small amount of carbon monoxide. That is because under these temperatures, any carbon present should be chemically favored to be in the form of methane.

“A lot of the larger planets and brown dwarfs are thought to have similar atmospheric behavior,” said Joseph Harrington, an associate professor at the University of Central Florida and the study’s principal investigator. “Brown dwarfs pretty much all follow a fairly straight-forward atmospheric chemistry that is not difficult to predict. Many theorists have applied these models to hot exoplanets, but in this case it doesn’t work.”

With a temperature of 527 degrees Celsius, GJ 436b should have abundant methane and less carbon monoxide. Yet Spitzer observations have detected the opposite.

The infrared wavelengths captured by the space telescope show evidence for carbon monoxide but not methane.

“What this does tell us is that there is room for improvement in our models,” Harrington explained. “The lesson here is that planets really do have individual personalities.”

Spitzer was able to detect the faint glow of GJ 436b by watching it slip behind its star, an event that is known as a secondary eclipse. As the planet disappears from sight, the total light observed from the star system drops. This reduction is measured to find the brightness of the planet at various wavelengths. This technique was first used by Spitzer in 2005 and has since been used to measure the atmospheric components of several Jupiter-size exoplanets, or so-called hot Jupiters.

“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 the Jet Propulsion Laboratory and the California Institute of Technology, both located 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,” he added.

This research was performed before Spitzer ran out of its liquid coolant in May 2009, officially beginning its so-called warm mission.