Coldest measured atmospheric temperature on a planet

The coldest atmospheric temperature in the solar system is 49°K (-224°C; -371°F), detected on Uranus by Voyager 2 in January 1986. The cold spot was located in the tropopause (where pressures range from 60 to 200 millilbars) around 30 degrees south of the planet's equator.

The average surface temperature of Uranus (which, because Uranus doesn't have a solid surface, is measured from the point in the atmosphere where pressures match Earth's surface atmospheric pressure) is 76°K, four degrees Kelvin warmer than the surface temperature on Neptune, but Neptune's atmosphere is more uniform in temperature. Beyond the solar system there are undoubtedly colder planets (some exoplanets have estimated mean surface temperatures as low as 50°K), but they cannot be measured directly.

The mechanism by which these unusual cold spots appear in Uranus' atmosphere is still not clearly understood. Despite being 1.62 billion km (1 billion mi) closer to the sun than Neptune, the global average temperature on Uranus is more or less the same as Neptune's. This is because Uranus has a core temperature of just 5,010°K (4,737°C; 8,558°F), compared with at least 5,400°K for Neptune's core and 11,980°K for Saturn's. Uranus may have lost a significant amount of heat and energy in the same collision event that knocked it over onto its side.