In our solar system, all of the gas giant planets have abundant methane in their atmospheres. It was expected that similar planets in other solar systems would also, but one has been found which doesn’t… the planet GJ 436b, about the size of Neptune, orbiting a star 33 light-years from Earth. Based on analysis by the Spitzer Space Telescope, it appears to have almost none, which is a mystery. It should be noted that methane on these types of planets is primordial methane, a very simple molecule, left over from the planets’ original creation. On small rocky planets like Earth and Mars (and it exists on both), it can be either geological or biological in origin. In the case of Mars, the origin is still uncertain.