Is Pluto a planet or just a really large Kuiper Belt Object (KBO)? Those arguing that it doesn't deserve planetary status will have to reconsider because of new research from the Spitzer Space Telescope. It was previously believed that KBOs were fairly dark, with a similar reflectivity to comets. From the reflectivity, astronomers guessed that KBOs are quite large, some getting as big as 700 km (434 miles) across. But new observations from Spitzer show that they're probably more reflective than previously thought, and therefore much smaller. This means that Pluto is probably still significantly larger than other objects in the Kuiper Belt.