New measurements from NASA's New Horizons spacecraft revealed that Pluto, named for the Greco-Roman god once called Hades, is a little more swollen with ice than previously thought, making it the biggest trans-Neptunian object—more voluminous than rival dwarf planet Eris, which is nevertheless more massive. Greg Laden explains why these orbs are not considered full-fledged planets on his blog.
While Eris orbits the Sun within the 'scattered disc,' Pluto orbits in the Kuiper Belt, a collection of gravelly snowballs that Ethan Siegel says outnumber all the planets in our galaxy. The Kuiper Belt begins beyond Neptune, encircling all the planets in our solar system and extending outward for a distance equal to the gap between the Sun and Uranus. It took New Horizons more than nine years to fly to Pluto from Earth, after setting a record for highest launch speed of any man-made object, after getting a massive speed boost from the gravity of Jupiter, and without any need to slow back down. It will now take sixteen months for New Horizons to stream all the scientific data from its brief flyby of Pluto back to Earth. Meanwhile the spacecraft will attempt to visit another smaller Kuiper Belt object before it runs out of fuel and falls short of the edge of our solar system.