The Solar System may look orderly from a distance. Venus is hotter than Mercury Mercury is the closest planet to the Sun, but Venus is the Solar System’s hottest planet. Jupiter’s moon Io is almost constantly erupting Io is only slightly larger than Earth’s Moon, yet it is the most volcanically active world in the Solar System. NASA notes that there is no known weather feature quite like it elsewhere in the Solar System. The strangest thing about the Solar System may be that these aren’t exotic worlds orbiting some distant star.