discovery

Astronomical observations ofUranus and Neptune in the late 19th and early 20th centuries revealed a slight discrepancy between the actual observed orbits and what the calculated theoretical orbits of these two planets should have been. American astronomer Percival Lowell (1855–1916) believed that these discrepancies were due to the influence of an undiscovered ninth planet that he called Planet X. Lowell searched for Planet X between 1906 and 1916, but was unsuccessful in his endeavors.

Clyde Tombaugh, the discoverer of Pluto.

The task was taken up again by Clyde Tombaugh (1906–1997) in 1929. Tombaugh was an American astronomer working at the Lowell Observatory in Flagstaff, Arizona. Tombaugh’s technique was to take pictures of the same part of the night sky, but over two different dates. A comparison of the photographic plates would then reveal if any of the “stars” had changed position. Tombaugh reasoned that if one of the stellar objects had changed position in the relatively short time interval between the dates of the two plates, then it was not really a star. On February 18, 1930, Tombaugh discovered just such a moving object in a comparison of photographic plates that had originally been taken slightly earlier on January 23 and January 29, 1930. A new planet, to be named Pluto, had been discovered!

Increased accuracy in planetary observations coupled with more precise data on Neptune’s mass from the Voyager 2 spacecraft flyby in 1989 showed that the original concerns about deviations in the orbits of Neptune and Uranus were unfounded. They were simply the result of poor data and incomplete observations. There never had been a real reason to postulate the existence of a Planet X.

Pluto Loses Its Planet Status

As the decades passed and astronomers learned more about Pluto, many questioned whether Pluto could really be considered a full-fledged planet.

Pluto's status was questioned in part because it was by far the smallest of the planets. Plus, Pluto's moon (Charon, discovered in 1978) is incredibly large in comparison. Pluto's eccentric orbit also concerned astronomers; Pluto was the only planet whose orbit actually crossed that of another planet (sometimes Pluto crosses Neptune's orbit).

When bigger and better telescopes began to discover other large bodies beyond Neptune in the 1990s, and especially when another large body was discovered in 2003 that rivaled the size of Pluto, Pluto's planet status became seriously questioned.

In 2006, the International Astronomical Union (IAU) officially created a definition of what makes a planet; Pluto did not meet all the criteria. Pluto was then downgraded from a "planet" to a "dwarf planet."