Gliese 436 b, also known as GJ 436 b, is an exoplanet that orbits the red dwarf star Gliese 436, located approximately 33 light-years away from Earth in the constellation Leo. It was discovered in 2004 through the radial velocity method, which detects small wobbles in a star's motion caused by the gravitational pull of its orbiting planet.

Gliese 436 b is classified as a "hot Neptune" or "warm Neptune" due to its characteristics. It is classified as a Neptune-like planet because it has a mass similar to Neptune and likely consists of a rocky core surrounded by a thick atmosphere. However, it is considered "hot" because it orbits very close to its star, with a period of only 2.6 Earth days. This close proximity causes the planet's atmosphere to be heated to extreme temperatures.

The planet's close orbit also causes it to be tidally locked, meaning the same side always faces its star. This leads to extreme temperature differences between the day and night sides of the planet. The day side experiences temperatures of around 800 degrees Celsius (1,470 degrees Fahrenheit), while the night side is significantly cooler.

Another interesting feature of Gliese 436 b is its high density, which suggests it has a substantial amount of heavy elements in its composition. This makes it an intriguing target for studying exoplanet atmospheres and their chemical compositions.

Overall, Gliese 436 b provides valuable insight into the diversity of planetary systems beyond our own and allows scientists to study the extreme conditions and properties of hot Neptune-like planets.