Exploring the Planets

Pluto Facts

Formerly known as the ninth planet, Pluto is now classified as a dwarf planet. That means it is round, it is not a satellite, and it orbits through an area of space containing many other small bodies. It is the prototype of a class of objects orbiting the Sun beyond Neptune. Astronomers are finally beginning to expose the secrets of this distant, dark, and mysterious world.

Discoverer: Clyde Tombaugh (January 23, 1930)
Spacecraft Encounter(s): New Horizons
scheduled for July 2015
Mean distance from the Sun: 5.9 billion km
(3.7 billion mi) / 39.5 AU
Length of Year: 248 years
Rotation Period: 6.4 days
Mean Orbital Velocity: 4.7 km/sec (3 mi/sec)
Inclination of Axis: 122.46 °
Diameter: 2,302 km (1,430 mi)
Number of observed satellites: 5


Comparisons With Earth:
Diameter: 0.18 x Earth
Average Distance from the Sun: 39.5 x Earth
Mass: 0.002 x Earth
Density: 0.36 x Earth

Science from Afar
Residing in the cold, dark, distant reaches of the outer solar system, Pluto hides its mysteries well. Studying faraway worlds without close-up data from a spacecraft can be challenging. Yet from a distance, scientists have been able to gather a wealth of information about this strange world and have learned some far-out facts:

  • Its year lasts more than two Earth centuries.
  • Its day lasts as long as an Earth week.
  • Its largest moon is almost half Pluto's size, and it never rises or sets.
  • Its atmosphere comes and goes.
  • It rotates on its side.
  • Pluto is too faint to be seen with the naked eye. When viewed through a telescope, it looks like a star.
  • Stellar occultations have revealed a tenuous atmosphere on Pluto composed primarily of nitrogen and methane gas.
  • Pluto is cold: -233° C (-390° F), just 40° C (72° F) above absolute zero. At this temperature, all elements would be frozen but neon, hydrogen, and helium.
  • Pluto is smaller than Earth's Moon.

Pluto's Discovery

In the early 20th century, astronomer Percival Lowell predicted there was a planet beyond Neptune. He thought (incorrectly) that a ninth planet was needed to account for unexplained motions in the orbits of Uranus and Neptune. Clyde Tombaugh, a young astronomer at the Lowell Observatory in Arizona, took up the search for this hidden world. In 1930 he discovered the object later named Pluto.

The Blink Comparator Used to Discover Pluto
Percival Lowell purchased this Carl Zeiss blink comparator in 1911. While using it to search for a new planet in 1930, Clyde Tombaugh discovered Pluto.

The instrument allows the operator to switch the view back and forth between two photographic plates taken several days apart. The plates are exactly aligned so the distant stars remain stationary, but a planet-like object will appear to move from side to side among them.

Pluto Size Comparison

Pluto is smaller than our Moon and has a fraction of Earth's gravity. If you weigh 45 kilograms (100 pounds) on Earth, you would weigh only 3 kilograms (6.7 pounds) on Pluto.