Why is Pluto so cold?

Pluto was once counted as the ninth planet. In 2006 it was reclassified as a dwarf planet, together with Ceres, Makemake, Haumea and Eris.

Pluto is small, only about half as wide as the United States. It lies in the Kuiper Belt, a region full of icy bodies and other dwarf planets at the edge of the solar system, almost 40 times farther from the Sun than the Earth is. There it is bitterly cold, about 240 degrees Celsius below zero. Its gravity is weak: a child who weighs 30 kilograms on Earth would weigh only about 2 kilograms on Pluto.

Pluto has five moons. The largest, Charon, is about half the size of Pluto itself, which makes it the largest moon in the solar system compared with the body it goes round. Pluto's orbit is unusual too. It is an oval, tilted path with the Sun nowhere near its centre, and one trip round the Sun takes 248 Earth years. A day on Pluto lasts about 6.5 Earth days, the same time that Charon takes to circle it. Pluto even spins in the opposite direction to most of the planets.

In 2015, NASA's New Horizons spacecraft flew past Pluto and sent back pictures that showed a large, light-coloured heart shape on its surface.