In 1978, the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time: roughly 0.2% that of Earth.

Observations of Pluto's surface by the New Horizons spacecraft revealed a variety of surface features, including mountains that reach as high as 11,000 feet (3,500 meters), comparable to the Rocky Mountains on Earth.

Pluto's mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon). As a result, Pluto has failed to clear the neighborhood around its orbit.

Pluto's surface is one of the coldest places in the solar system, at roughly minus 375 degrees Fahrenheit (minus 225 degrees Celsius).

Pluto's moons, in order of distance, are Charon, Styx, Nix, Kerberos, and Hydra.

Lowell's prediction came from deviations he initially observed in 1905 in the orbits of Uranus and Neptune. However, astronomers have since concluded that Pluto is too small to have any real impact on the orbits of the other planets, making Lowell's prediction a lucky coincidence.

The plains on Pluto's surface are composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.

With half the diameter and one eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body.

Tartarus Dorsa is named after Tartarus, the deepest, darkest pit of the underworld in Greek mythology.

Unlike many of the planets in the solar system, such as Earth, Pluto does not bulge at its center; its radius -- 737 miles (1,185 km) -- is the same at its poles and at its equator.