Pluto was reclassified as a dwarf planet on August 24, 2006. While many grieved the loss of the ninth planet, we’re now learning more than ever about this ball of ice and its faraway neighborhood at the edge of the solar system.
So, what ended Pluto’s run as a planet? The International Astronomical Union, or IAU, defines a planet by three qualities:
1. Orbits the Sun
2. Big enough for gravity to shape it into a sphere (which is called “hydrostatic equilibrium”)
3. Big enough to clear an orbit—meaning that its gravity pulls smaller objects in the neighborhood onto the planet itself, or directs them away into interplanetary space.
Number 3 is where Pluto doesn’t make the cut; it hasn’t cleared its neighborhood. It is located in the Kuiper belt, a range past the orbit of Neptune. The Kuiper belt contains many small bodies like asteroids, and Pluto is the prototype Kuiper belt object. Now there’s evidence of “Planet Nine,” a much larger, Neptune-sized planet somewhere farther than Pluto, that may act on smaller Kuiper belt objects.
Its dwarf planet status doesn’t make Pluto any less interesting to science. NASA’s New Horizons spacecraft did a flyby of Pluto in 2015, capturing images of its highly complex surface. The dwarf planet acts a lot like a regular planet from a geological perspective: unlike the familiar planets its surface is made up mostly of frozen nitrogen, and it’s covered in active ice volcanoes and constantly shifting tectonic plates. Pluto remains the brightest object in the Kuiper belt, and one of five officially recognized dwarf planets—though many more fit the criteria of dwarf planet but haven’t been officially recognized as such. At up to 4.6 billion miles away from Earth, Pluto’s neighborhood remains a fascinating area of exploration that could reveal more information about Planet Nine and will provide new insights into the origins of our solar system.