The U.S. space agency says data from its Spitzer Space Telescope revealed a distant planet that lacks methane in its atmosphere.

"It's a big puzzle," said Kevin Stevenson, a planetary sciences graduate student at the University of Central Florida, lead author of a study focusing on the planet. "Models tell us 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."

NASA said the discovery brings astronomers one step closer to probing the atmospheres of distant planets the size of Earth.

The space agency said the methane-free planet, called GJ 436b, is about the size of Neptune, making it the smallest distant planet that any telescope has successfully analyzed.

"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," Stevenson said.

"This type of planet should have cooked up methane," said Associate Professor Joseph Harrington of the University of Central Florida, who led the research. "It's like dipping bread into beaten eggs, frying it and getting oatmeal in the end."

GJ 436b is 33 light-years from Earth in the constellation Leo.

The research is reported in the April 22 issue of the journal Nature.