How big is Pluto?

Pluto is a dwarf planet in the Kuiper Belt, a zone of icy worlds that orbit the Sun beyond Neptune. It is 2,377 kilometers (1,477 miles) across, about two-thirds the width of Earth's Moon. From its discovery in 1930 until 2006, it was counted as the ninth planet.

Size

Pluto's size was hard to measure from Earth. Astronomers often size a distant body by watching it pass in front of a star, an event called an occultation, but Pluto's thin atmosphere hides its exact edge. In 2011, estimates of its diameter still ranged from 2,300 to 2,400 kilometers.

NASA's New Horizons spacecraft settled the question when it flew past Pluto on July 14, 2015. Images from its camera, taken as it approached, first gave a diameter of about 2,370 kilometers, larger than many earlier estimates. A later analysis of the images found a radius, the distance from the center to the surface, of 1,188.3 kilometers, give or take 1.6 kilometers. That makes Pluto about 2,377 kilometers across, and it is round, with no measurable flattening at the poles.

Size comparisons

Pluto is about two-thirds as wide as the Moon, which is 3,475 kilometers across, and a little less than one-fifth as wide as Earth.
Pluto's largest moon, Charon, is 1,212 kilometers across, about half as wide as Pluto. No other moon in the Solar System is so large compared with the body it orbits.
If Pluto were the size of a basketball, Charon would be a ball about 12 centimeters (5 inches) across, orbiting about 2 meters (6.5 feet) away.
Eris, another dwarf planet, is 2,326 kilometers across, about 50 kilometers narrower than Pluto. Eris has about 27 percent more mass, though.
Pluto is nearly twice as wide as Texas is from east to west.

From planet to dwarf planet

Clyde Tombaugh discovered Pluto on February 18, 1930, using a telescope at Lowell Observatory in Flagstaff, Arizona. An 11-year-old girl in Oxford, England, Venetia Burney, suggested the name, after the Roman god of the underworld. For 76 years, Pluto was the ninth planet.

Astronomers later found other icy worlds beyond Neptune, including Eris, which at first seemed larger than Pluto. In 2006 the International Astronomical Union voted on a new definition. Under it, a dwarf planet orbits the Sun and is big enough for its gravity to pull it into a nearly round shape, but it has not cleared other objects from the region around its orbit. Pluto counts as a dwarf planet because other objects may cross its path around the Sun.

A long, tilted orbit

Pluto's orbit is a stretched oval, tilted compared with the orbits of the planets. Its distance from the Sun ranges from 30 to 49.3 astronomical units, where one astronomical unit is the average distance from Earth to the Sun, about 150 million kilometers. On average, Pluto is 5.9 billion kilometers from the Sun, and one trip around takes 248 years. From 1979 to 1999, it was closer to the Sun than Neptune.

At its average distance, sunlight takes 5.5 hours to reach Pluto, and its surface can be as cold as −240 °C (−400 °F). Its tallest mountains, 2–3 kilometers high, are huge blocks of water ice. Its most prominent plains have no craters and appear to be made of frozen nitrogen.