Neptune emits about 2.7 times the amount of heat it absorbs from the Sun. Astronomers believe the excess heat that Neptune radiates comes from comet-like material that crashed into Neptune as the planet formed. Due to the force of gravity in the planet’s interior, the material in Neptune’s core is continually being pulled inward. As the material compacts, the molecules strike each other more frequently and with more force, releasing energy in the form of heat. Neptune’s core, which reaches temperatures of 5149°C (9300°F), is hotter than the Sun’s surface.