Pluto was the ninth planet in Earth's solar system.
When the SDF-1 used a hyperspace fold to escape to The Moon, they wound up near this Pluto instead, which resulted in a long journey home for the citizens of Macross Island and the crew of the SDF-1. (Macross Saga: "Space Fold")
Behind the Scenes
Since 2006, it has been considered in the scientific community that Pluto is not, in fact, a planet, but rather is a dwarf planet of substantial size and mass. However, as most Robotech media to reference the planet were created before this time, Pluto has always been considered a planet within the Robotech universe. Robotech/Voltron, released in 2014, refers to the it only as "Pluto."
In 2015, NASA decided to name one of the chasmata discovered on Pluto the "Macross Chasma," after the original 1982 series, along with many other land-marks named after such franchises as Doctor Who, Star Trek, and Star Wars.