Pluto - Known Facts

About Pluto, what scientific facts are known including trivia about the orbit, surface, and more.


Known Facts: Uranus wandered in its orbit, and Neptune's pull couldn't explain new deviations. Mathematicians theorized and astronomers searched for the ninth planet during the century before its discovery in 1930.

Lowell Observatory in Flagstaff, Ariz., hired young, self-educated astronomer Clyde Tombaugh in 1929 to renew the search that Percival Lowell's death had suspended in 1916. Tombaugh took photographic plates, some containing 1 million star images, and inspected them with a "blink comparator," which showed moving bodies against a fixed star field.

Two plates-recorded on Jan. 23 and 29 and examined on Feb. 18, 1930-revealed an extremely faint, slow-moving point within 6 deg. of the predicted position. After a careful check, the observatory announced the discovery on Mar. 13, the 75th anniversary of Percival Lowell's birth. The name chosen complemented those of the other planets. The first two letters of the name, its astronomical symbol, are also Lowell's initials.

Pluto traverses 1 1/2 deg. a year across the sky. It has moved about 71 deg. since its discovery. Its orbit (0.25) is the most eccentric, and its inclination (17 deg.) the greatest, of the planets. From 1962 until 2009 it will travel inside Neptune's orbit, reaching perihelion in 1989. The planet is so small and distant that the largest telescopes show it only as a yellowish speck. Sunlight requires 5 1/2 hours to reach its surface, which is essentially lit by starlight. It has no known moons.

Using the 200-in. Hale telescope, astronomers assigned Pluto a tentative diameter of 3,600 mi. in 1950. A partial star occultation in 1965 set an upper limit of 4,225 mi. Axial rotation, assumed from periodic fluctuations in brightness, occurs every 6 days 9 hours. In 1976 solid methane, which freezes at-373 deg. F, was detected on Pluto's surface.

No planetary flyby or probe is planned for Pluto, but if such a mission were to be scheduled, the journey could take as long as eight years.

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