Pluto is a dwarf planet in Earth's solar system.
Due to Pluto's strange atmosphere, it had Giant Snowflakes, about the size of a hula hoop. As they don't melt in Earth's atmosphere, they're likely composed of a substance other than water. In 1958, Superman took some of these back to Earth, where he let some school kids play with them. Afterward, he left them in a time capsule by the Metropolis Museum. ("The Secret of the Space Souvenirs")
Brainiac 13 converted Pluto into a new Warworld prior to the Imperiex War as shown in the Superman: Our Worlds at War storyline. He used this Warworld to devour Imperiex. However, a powered-up Superman was able to destroy this Warworld, sending it to the moment of the universe's creation. ("Superman: Our Worlds at War")