Which is the Coldest Planet in the Solar System?

The coldest planet in the Solar System is Uranus. Measurements show that Uranus’s atmospheric temperature can drop as low as 49 K (−224.2 °C; −371.5 °F), which is colder than any other planet observed so far, including Neptune, even though Neptune is farther from the Sun.

Why Uranus is colder than expected:

Although Neptune is on average farther from the Sun, it generates significantly more internal heat than Uranus. Uranus, by contrast, radiates almost no excess heat—less than it absorbs from the Sun—and its internal heat flux is even lower than that of Earth. The reasons for Uranus’s unusually low internal heat remain unclear, but hypotheses include a possible energetic impact in its early history that dispersed its primordial heat, or a compositionally layered interior that acts as a barrier to heat transport.

Comparisons with other planets:

• Neptune: Despite its greater distance, Neptune’s internal heat keeps it warmer than Uranus; its temperatures average around −214°C.
• Saturn and Jupiter: These gas giants are considerably warmer, with Saturn’s upper atmosphere near −178°C and Jupiter’s even warmer.
• Mars: While cold, Mars is not nearly as frigid as Uranus, with minimum temperatures around −125°C.