Pluto is a rocky, icy world. Its discovery was the result of an extensive search for Planet X, a hypothesized ninth planet based on perturbations in Uranus's orbit. Astronomer Clyde Tombaugh compared pairs of photographs of the night sky; by looking for a tiny spot that shifted position against the backdrop of stars over time, he discovered Pluto in 1930. Pluto was classified as a planet until 2006.

For decades, Pluto's mass was estimated to be similar to the mass of Earth or Mars. Revised calculations lowered that estimate over time. Then, in 1978, the discovery of Pluto's moon Charon provided the first opportunity to actually measure Pluto's mass. Studying the gravitational interaction between Pluto and Charon, the mass of Pluto was determined to be about 1/500th the mass of Earth. The revised mass meant Pluto was too small to be Planet X; however, the search for the mysterious planet was abandoned when revised measurements of Neptune's mass accounted for the discrepancies in Uranus's orbit.

Although the quest for a relatively large Planet X was over, new planetlike objects in the solar system continued to be discovered. In 1992, the discovery of many objects similar in composition to Pluto led to a debate about planet classification. These objects are members of the Kuiper belt—a region located in the outer solar system beyond the orbit of Neptune that contains trillions of small icy objects. Pluto was recognized to be a member of this group and, as other Kuiper belt objects comparable in size were discovered, astronomers questioned: If Pluto is a planet, are the other objects planets too?

In an effort to clarify the categorization of solar system objects, the International Astronomical Union (IAU) announced a formal definition of a planet in 2006. Under the new classification scheme, Pluto does not meet the criteria of a planet because there are other objects near its orbit. Pluto, along with a few other solar system bodies, is now designated a dwarf planet. It is estimated that there may be hundreds of dwarf planets in the Kuiper belt yet to be discovered. In 2008, the IAU announced another new term—plutoid—as a category for dwarf planets beyond Neptune. There is significant controversy over the IAU definitions, and debate within the scientific community continues.

Pluto is so small and distant that it has been difficult to study. With the help of the Hubble Space Telescope, additional moons have been discovered: Nix and Hydra in 2005, and two other moons (yet unnamed) in 2011 and 2012. To date, the best maps of Pluto were created by processing hundreds of Hubble images together. Pluto is only a few pixels wide on a Hubble image; using a technique called dithering, multiple images that are slightly offset can be combined to synthesize a higher-resolution image. Scientists will soon be able to study Pluto in more detail. The New Horizons spacecraft, launched in 2006, is planned to reach Pluto in 2015; it will collect unprecedented data about Pluto and other Kuiper belt objects. The spacecraft also carries some commemorative items to celebrate the historic mission, including a small amount of Clyde Tombaugh's ashes.

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