New Horizons sees Pluto and its moon Charon.Photo by NASA/JHUAPL/SWRI. Pluto's ego took a major blow in 2006 when it was demoted from a full-on planet to a dwarf planet. It can take back some of that pride with the news that NASA's New Horizons mission has discovered it's a bigger space object than many scientists expected. NASA announced Monday that Pluto is 1,473 miles in diameter, which the space agency says is somewhat larger than many prior estimates..