What is coldest planet in solar system?

Uranus is the coldest planet in our solar system.

The coldest planet of our solar system is curiously Uranus, the 7th planet, and not Neptune, the 8th planet. With temperatures dropping to -218°C in Neptune’s upper atmosphere, the planet is the second coldest in our solar system.

Despite being a billion miles closer to than Neptune, Uranus has a lowest recorded temperature of -224°C.

There are two hypotheses for this discrepancy.

One is the planet's odd orientation: Uranus seems to have been knocked on its side, possibly by a giant impact long ago, which has caused the primordial heat from its core to spill out into space.

Second, Uranus has a very active atmosphere that causes it to lose heat, or some form of barrier exists in Uranus's upper layers that prevents the core's heat from reaching the surface.