Because of Neptune's far distance from the Sun, it gets very little heat. The average temperature on Neptune is about -201°C (−331 °F; 72 K). Therefore, Neptune is the coldest planet in the Solar System. But in the depths of planet the temperature rises slowly. The source of this heating is unknown. Neptune is the farthest planet from the Sun, yet its internal energy is strong enough to create the fastest winds seen in the Solar System. Several possible explanations have been suggested. Firstly, radiogenic heating from the planet's core. Among the explanations is the continued radiation into space of leftover heat made by infalling matter during the planet's birth. The last explanation is gravity waves breaking above the tropopause.