Triton is a SATELLITE of Neptune, the eighth planet from the Sun in our Solar System. It is the largest MOON of Neptune and was discovered in 1846 by William Lassell. Triton is the only large MOON in the Solar System with a retrograde orbit, meaning it orbits in the opposite direction to its planet's rotation. It is also the coldest known object in the Solar System, with temperatures reaching as low as -235°C.