In 2000, even before Eris had appeared on the scene, the American Museum of Natural History’s Hayden Planetarium opened a new exhibit on the eight planets—Pluto was prominently excluded. The rest is history. In August of 2006, the International Astronomical Union (IAU) redefined “planet” as follows.

• The object must be in orbit around the Sun.
• The object must be large enough that it is nearly round (i.e., is in hydrostatic equilibrium).
• The object must have cleared its orbit of other objects.

The trajectory of the New Horizons probe to Pluto. (image credit: JHU/APL)

The “orbit the sun” requirement eliminates moons, such as the large moons of Jupiter.

The “nearly round” requirement eliminates small objects whose gravity is not strong enough to overcome the forces between the rocks or ices and produce a spherical surface, which has the lowest gravitational energy.

The “clear its orbit” requirement is equivalent to saying that a planet is much larger than anything else around, in which case its relatively strong gravity would indeed sweep up the neighboring objects. In fact, Ceres and a few other asteroids were originally classified as planets, before astronomers realized that they are all part of a large collection of objects with similar orbits.

The IAU also created a new category, “dwarf planet,” for those objects that meet the first two requirements above but have not cleared their neighborhood of other objects. Pluto falls into this category because of various Kuiper Belt Objects in its neighborhood. Along with Pluto, the dwarf planet category includes Eris and Ceres.

Whatever issues still remain about Pluto’s status, it is an important object in the solar system and is the subject of study, although no probe has yet reached it. In January 2006 the New Horizons spacecraft blasted off for the long voyage to Pluto (see drawing of trajectory), with the closest approach in July, 2015, at a distance of 10,300 km (6200 mi).