Arxes

Arxes is a volcanic protoplanet, and the closest planet to the Sun. Arxes is the smallest planet in the solar system, and the only one to orbit the Sun in the opposite direction, though it is not fully understood why. This unusual retrograde orbit is believed to be the cause of the planet's ongoing volcanic activity - Arxes is incredibly volatile, spewing out magma and rock as its core is gradually torn apart by the Sun's intense heat and gravity.