GJ 436 b is a Neptune-sized exoplanet that orbits a M-type star. Its mass is 22.1 Earths, it takes 2.6 days to complete one orbit of its star, and is 0.0291 AU from its star. Its discovery was announced in 2004.

This unusul world is only 33 light-years away. NASA's Spitzer Space Telescope found evidence that the hot planet orbiting a star beyond our Sun lacks methane – an ingredient common to many planets in our own solar system.

Models of planetary atmospheres indicate that any world with the common mix of hydrogen, carbon and oxygen, and a temperature up to 1,000 Kelvin (1,340 degrees Fahrenheit) should have a large amount of methane and a small amount of carbon monoxide.

The planet is about 800 Kelvin (or 980 degrees Fahrenheit) – so it was expected to have methane but Spitzer's observations showed it does not.

The planet is too compact to be made mostly of hydrogen gas, like Jupiter, but may not be compact enough to be a rocky super-Earth. Researchers think it may be made mostly of an exotic form of water, an "ice" hardened by pressure rather than temperature.