Note: These are rounded numbers.
Diameter: 2300 km (1430 miles)
Distance from Sun: varies from 30-49 AU (1 astronomical unit = 150 million km or 93 million miles)
Orbital period (year): 248 Earth years
Rotation period (day): 6.4 Earth days
Mass: 0.00218 of Earth’s mass
Pluto was discovered during the hunt for Planet X.
The planet Neptune was discovered in 1846 because its gravity affected Uranus. But by the end of the century Uranus's orbit still didn't seem right, so some astronomers thought there was a ninth planet. One of them, Percival Lowell (1855-1916), was obsessed with finding this large unknown planet. He didn’t find Planet X before his death, but left money in his will to continue the search. In 1930, at the Lowell Observatory in Arizona, Clyde Tombaugh (1907-1997) discovered Pluto and everyone assumed this was Planet X.
Pluto wasn’t Planet X.
Soon astronomers realized that Pluto wasn’t as big as they’d thought. But they couldn’t be sure until 1978 when Pluto’s moon Charon was discovered. You can find the mass of a planet from the orbit of its moon and Pluto’s mass turned out to be less than 1% of Earth’s mass. It was way too small to have any effect on the outer planets. In fact, there never was a Planet X. Astronomers just needed a more precise mass for Neptune. They got this in 1989 from a space probe, and the orbit problem disappeared.
Pluto wasn’t named after a dog.
Venetia Burney, an 11-year-old English schoolgirl, suggested the name Pluto for the new planet. Pluto was the Roman god of the underworld, an appropriate name for a planet so far from the Sun. Disney’s cartoon dog got his name afterwards.
Pluto became a dwarf planet in 2006.
Pluto didn’t fit the International Astronomical Union’s new definition of a planet. It was classed as a dwarf planet and is now listed as 134340 Pluto in the Minor Planets Catalogue. It's the largest body in the Kuiper Belt. The Kuiper Belt is similar to the asteroid belt, but much bigger and colder. It goes from the orbit of Neptune out to about fifty times the Earth-Sun distance.
The non-planet has five moons.
Charon was named for the ferryman who took the souls of the dead over the river Styx to Pluto’s kingdom. In 2005 two small moons – later named Nix and Hydra – were discovered by the Hubble Space Telescope. Nix, the goddess of night, was Charon’s mother. Hydra was a serpent with nine heads that guarded Pluto’s realm. Two more tiny moons were later discovered. Kerberos was the monstrous three-headed dog that kept the dead from escaping. Styx was the river between Earth and Hades.
Pluto may be the largest dwarf planet – or maybe not.
Pluto and Eris are very similar in size, but we don’t know Pluto’s exact size as we can't tell where the ground stops and the atmosphere begins. We do know that Eris has a greater mass than Pluto, so if we could put them on a giant weighing scale, Eris would be heavier.
Pluto has long days and spins backwards on its axis.
Plutonian days are 6.4 Earth days long, so people who complain that there aren’t enough hours in the day wouldn’t have that problem on Pluto. The direction of its spin is opposite to the direction it orbits the Sun, so we say that it’s retrograde. (Venus also has a retrograde spin.)
Pluto is tipped on its side.
Earth is tilted on its axis by 23 degrees, so the seasons change as we orbit the sun. When your hemisphere is tipped towards the Sun, you have summer and the opposite hemisphere has winter, as shown in this diagram. But see how much Pluto is tilted. This is very similar to Uranus and we can see here that when the north pole points towards the Sun, the entire northern hemisphere would get what sunlight there is. The southern hemisphere would stay in darkness. Since Pluto’s year is 248 Earth years, this could go on a long time.
It’s COLD on Pluto.
The surface temperature on Pluto varies between from -235 to -210 degrees Celsius (-391 F to -346 Fahrenheit). Its atmosphere stays frozen except when Pluto's orbit brings it closer to the Sun than usual.
New Horizons is going to Pluto and carrying something unusual.
NASA’s New Horizons spacecraft should arrive in 2015 to study Pluto. Afterwards it will go on to explore more of the Kuiper Belt. In a tribute to Clyde Tombaugh, who died in 1997, the spacecraft carries an ounce (28 grams) of the astronomer's ashes.