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Searching for Planet X

Searching for Planet X

In many ways, Clyde Tombaugh was like his astronomer hero William Herschel, who discovered Uranus unexpectedly during a routine sky survey in 1781. Both were dedicated amateur astronomers and skilled telescope makers who devoted hours to tedious observations. Tombaugh, however, was only twenty-four years old when he discovered Pluto, whereas Herschel was in his early forties when he made his discovery. Furthermore, Tombaugh’s yearlong search for Planet X lasted much longer than that of either Herschel or Johann Galle, who discovered Neptune in 1846 on the first night he looked for it, lying less than 1 degree from its predicted position.

The search for Pluto was complicated by the fact that its orbit is highly eccentric—sometimes even passing inside the orbit of Neptune—and has a large inclination of about 17 degrees from the mean plane of the other planets. It is now known that Percival Lowell’s predictions for the position of Pluto were based on faulty calculations, and its discovery within 6 degrees of the predicted location was only a coincidence. Fortunately, Tombaugh did not limit his observations to the predicted area of the sky or to the region close to the mean orbital plane of the planets.

When James Christy discovered Pluto’s moon, Charon, in 1978, it was conclusively demonstrated that the mass of Pluto was far too small to cause observable deviations in the orbits of Uranus and Neptune; thus the two larger planets’ orbits could not be used to predict Pluto’s position. In the 1990’s, several icy objects much smaller than Pluto were discovered just beyond Pluto’s orbit in the Kuiper comet belt with periods of about 300 years, compared to Pluto’s 248-year period. In 2006, Pluto was redesignated as a dwarf planet because it did not conform to the International Astronomical Union’s revised definition of a planet.


See Also

Astronomy

Chemistry

Physics

Radio astronomy

The 1930s in America

Pluto discovery

by George J. Flynn

The Event Identification of an object orbiting farther from the Sun than Neptune

Date February 18, 1930

Place Lowell Observatory, Flagstaff, Arizona

At the time, the discovery of Pluto was considered to be the first discovery of a planet by an American. Pluto’s small size and similarity to comets sparked a long debate over the definition of a planet.

Johannes Kepler’s recognition that planets moved in elliptical orbits around the Sun coupled with Sir Isaac Newton’s demonstration that the gravitational force explained this motion allowed astronomers to calculate the orbits of planets and compare the predicted positions with observations. Two astronomers, John Adams in England and Urbain Le Verrier in France, suggested independently that irregularities in the orbits of Saturn and Uranus required an eighth planet. Their calculations of its orbit led to the discovery of Neptune in 1846, in almost the exact position they predicted. However, small irregularities remained, suggesting a ninth planet.

Percival Lowell, a diplomat from a wealthy Boston family, established an observatory in Flagstaff, Arizona. Lowell, who was interested in mathematics and astronomy, conducted several searches for this ninth planet beginning in 1905. Planets move relative to the stars, so Lowell compared photographic images of the same star field taken several nights apart. Initially, he simply placed the images next to each other for comparison. In 1911, he purchased a “blink comparator,” which alternately displayed the two images, so an object that moved appeared to jump back and forth, making his planet search more efficient.

Lowell’s death, in 1916, brought his search to a halt as legal issues over his estate were resolved. The search resumed in 1927 under the leadership of Vesto Melvin Slipher, Lowell’s former assistant who was appointed director of the Lowell Observatory. Lowell’s brother, A. Lawrence Lowell, the president of Harvard University, assisted Slipher in purchasing a 13-inch telescope for the search. However, the Lowell Observatory could not afford to hire another professional astronomer. As the project was set to resume, Slipher received a letter from Clyde William Tombaugh, a twenty-two-year-old who lived on a farm in western Kansas. Tombaugh, who could not afford a college education, was an amateur astronomer who built his own telescopes. His astronomical observations impressed Slipher, who invited Tombaugh to join the observatory staff.

On clear nights when the Moon was not visible in the sky, Tombaugh would expose a photographic plate, then rephotograph the same star field several days later. He used the blink comparator that Lowell purchased to search for moving objects. The job was tedious, requiring great concentration in order not to miss one blinking spot on the 14-by-17-inch plate, but Tombaugh was dedicated to the effort.

On February 18, 1930, Tombaugh found what he was looking for, comparing images taken on January 23, 1930, and January 29, 1930. He examined another plate, taken during poorer weather on January 21 and confirmed his discovery of an object orbiting the Sun well beyond the orbit of Neptune. The Lowell Observatory announced the discovery on March 13, 1930, Percival Lowell’s birthday and the anniversary of William Herschel’s discovery of Uranus. Initially, Pluto was believed to be the missing planet, but the orbital irregularities required Pluto to be about the mass of Earth. Since Pluto appeared as a point not resolvable in large telescopes, it was small. The discovery of Pluto’s moon, Charon, in 1978, made it possible to calculate Pluto’s mass, which was less than 0.25 percent of Earth’s mass.

The inner planets—Mercury, Venus, Earth, and Mars—are rocky objects, while the outer planets—Jupiter, Saturn, Uranus, and Neptune—are composed primarily of hydrogen and helium. Pluto, which is bright because of the high reflectivity of its icy surface, is not similar to either group of planets, appearing more like a huge comet. In 1992, astronomers David Jewitt and Jane X. Luu discovered 1992 QB1, an icy object orbiting in the “Kuiper Belt,” outside the orbit of Neptune, where many comets are thought to originate. Many more of these icy objects were soon discovered; Pluto was the most easily observable.

Impact

Determined in 1989 during the Voyager 2 spacecraft flyby, Neptune’s mass explained its orbital irregularities, eliminating the need for a ninth planet. In 2006, the International Astronomical Union grouped Pluto with other large Kuiper Belt objects as “dwarf planets.” Because Pluto is extremely difficult to observe from Earth, NASA’s New Horizons spacecraft planned to study Pluto in 2015 and observe several other Kuiper Belt objects.

Further Reading

1 

Levy, David H. Clyde Tombaugh: Discoverer of Planet Pluto. Tucson: University of Arizona Press, 1992.

2 

Minard, Anne. Pluto and Beyond: A Story of Discovery, Adversity, and Ongoing Exploration. Flagstaff, Ariz.: Northland Books, 2007.

3 

Tombaugh, Clyde W., and Patrick Moore. Out of the Darkness: The Planet Pluto. Harrisburg, Pa.: Stackpole Books, 1980.

Citation Types

Type
Format
MLA 9th
Flynn, George J. "Pluto Discovery." The 1930s in America, edited by Thomas Tandy Lewis, Salem Press, 2011. Salem Online, online.salempress.com/articleDetails.do?articleName=1930_149840801498.
APA 7th
Flynn, G. J. (2011). Pluto discovery. In T. T. Lewis (Ed.), The 1930s in America. Salem Press. online.salempress.com.
CMOS 17th
Flynn, George J. "Pluto Discovery." Edited by Thomas Tandy Lewis. The 1930s in America. Hackensack: Salem Press, 2011. Accessed December 14, 2025. online.salempress.com.