Gliese 436b is a mid-sized exoplanet roughly the size of Neptune in our own Solar System. It orbits its parent star, the red dwarf Gliese 436, at an extremely close-in distance of just 0.03 astronomical units, taking 2.64 days to go around once. By contrast, Mercury orbits at more than 10 times this distance from the Sun and takes 88 days. Gliese 436b was found using the transit method of discovery, whereby the planet -- as seen from the Earth -- passes first in front of and then behind its parent star. During these events, called transits, the brightness of the system is modulated, enabling astronomers to infer the presence of a planet and calculate its orbital period and mass using the laws of motion.