Lowell Predicts the Existence of Pluto Summary

  • Last updated on November 10, 2022

Percival Lowell instituted an observational search for the ninth planet he believed to exist in Earth’s solar system.

Summary of Event

Percival Lowell, a member of a prominent New England family, was born in Boston, Massachusetts, on March 13, 1855. Although his family was associated with textile mills, Percival pursued his formal education (at Harvard University) in mathematics with a personal interest in astronomy. Pluto (planet);search Astronomy;planets Lowell Observatory Planets;Pluto [kw]Lowell Predicts the Existence of Pluto (Aug., 1905) [kw]Pluto, Lowell Predicts the Existence of (Aug., 1905) Pluto (planet);search Astronomy;planets Lowell Observatory Planets;Pluto [g]United States;Aug., 1905: Lowell Predicts the Existence of Pluto[01340] [c]Science and technology;Aug., 1905: Lowell Predicts the Existence of Pluto[01340] [c]Astronomy;Aug., 1905: Lowell Predicts the Existence of Pluto[01340] Lowell, Percival Schiaparelli, Giovanni Virginio Slipher, Vesto Melvin Slipher, Earl Carl Lampland, Carl O. Tombaugh, Clyde William

In 1877, the Italian astronomer Giovanni Virginio Schiaparelli began an observational study of Mars Mars (planet), canals that led to his announcement that he had discerned complex and detailed surface patterns on the planet. He called these features canali, a word incorrectly translated into the English word “canals,” which proved to be a source of subsequent confusion. Lowell followed Schiaparelli’s discoveries with keen interest and imagined many great Martian civilizations.

In the early 1890’s, Schiaparelli’s eyesight had deteriorated and he could no longer make telescopic observations. Determined to continue Schiaparelli’s research, Percival founded the Lowell Observatory (opened on June 1, 1894) in Flagstaff, Arizona, and devoted it to the study of Mars and the other planets. Lowell made personal observations of the so-called Martian canals and imagined them as irrigation channels built by an intelligent and desperate civilization striving to retain its water supply from the polar caps of Mars. In 1898, Lowell founded a journal in which he published his thoughts about Mars to the scientific community, where they were met with varied receptions. His theories on Martian life were generally well received by science-fiction writers and the broad public but ridiculed by professional astronomers. Lowell became frustrated and sought ways to improve his credibility among his colleagues.

The mathematical prediction of the planet Neptune Neptune (planet) and its subsequent discovery in 1846 gave rise to the possibility that additional remote planets remained undiscovered in the solar system. Lowell thought that if he could predict the location of a ninth planet, beyond Neptune, this accomplishment would improve his reputation as an astronomer. In August, 1905, he inspired his colleagues at the Lowell Observatory to begin the first systematic photographic and visual search for “Planet X.” He expected this distant planet to be very similar to Neptune in density, size, and magnitude (brightness) and therefore easy to spot.

Lowell began his first search, from 1905 to 1907, by photographing star fields with a camera 5 inches (12.7 centimeters) in aperture (a 5-inch Brashear lens of 35-inch, or 89-centimeter, focal length). Stars;photography Photography;stars Lowell’s assistants took more than two hundred photographic plates, photographing each sky area twice to record faint stars. All of the three-hour-exposure plates were centered on the ecliptic (the plane of the solar system) and varied at intervals of 5 degrees longitude. Lowell personally examined each plate with a handheld magnifying glass. His technique was based on the assumption that a comparison of two photographs taken of the same star field would show movements of nonstellar objects such as comets, asteroids, and planets.

In 1911 and 1912, Lowell oversaw the making of a sequence of photographic plates using the observatory’s 42-inch (107-centimeter) reflecting telescope. Lowell used a Zeiss blink-microscope comparator Blink-microscope comparator[Blink microscope comparator] (a device that superimposes two photographic plates of the same region for direct comparison), which greatly expedited studies of multiple photographic plates. Unfortunately, this intensive search for Planet X also proved unsuccessful.

Assisted by Carl O. Lampland, Lowell continued his search from 1914 to 1916 with a 9-inch (23-centimeter) Brashear lens. The researchers examined almost one thousand photographic plates under the blink comparator, but with negative results.

By this time, Lowell realized that it would be difficult to discover the ninth planet observationally and attempted to compute its orbital and physical characteristics through mathematical calculations and statistical methods. He admitted that his theory would be limited and would contain some positional uncertainty, as his calculations were based on errors embedded within observations of the distant planets Uranus and Neptune.

On January 13, 1915, Lowell presented his theory on Planet X to the American Academy of Arts and Sciences. His book on the subject, Memoir on a Trans-Neptunian Planet, Memoir on a Trans-Neptunian Planet (Lowell) was published in the spring of that year. Using methods of celestial mechanics and mathematics, Lowell studied minute perturbations in the orbits of Uranus and Neptune. He attributed the irregularities in their motions to a trans-Neptunian planet—that is, a planet with an orbit intersecting that of Neptune. Unfortunately, Neptune’s orbit was not well known at the time (its revolution had been observed for only fifty years), so Lowell could not accurately calculate aberrations in its motion. Also, any deviations in the motion of Uranus were expected to be further minimized because of its great distance from Planet X; these additional errors were introduced into his calculations. In addition, Lowell studied the perturbations of comets and suggested that a distant planet could gravitationally affect a cometary orbit.

Lowell’s rigorous mathematics led him to assume that Planet X would have a mass seven times that of Earth, an inclination (with respect to the ecliptic) of 10 degrees, and an average distance of forty-three times the distance between Earth and the Sun. He also believed that the undiscovered planet would have a low density and a high albedo (the ratio of sunlight received versus sunlight reflected), as in the case of the four largest planets. In addition, Lowell expected the planet to appear faint because of its great distance from Earth and could only generally predict its location to be in or near the constellation of Gemini. Originally, he had predicted the location to be within the constellation of Libra.

Percival Lowell.

(Library of Congress)

In his attempt to locate the ninth planet, Lowell pushed himself very hard, and this overwork may have contributed to his death from a stroke on November 12, 1916, at the age of sixty-one. He left an endowment of more than one million dollars for the Lowell Observatory to continue his search. His wife challenged the will, however, and a legal battle ensued for almost a decade. Court fees depleted much of the endowment, and the observatory could not afford to maintain Lowell’s work. The search for Planet X was thus interrupted for thirteen years, but Lowell’s staff retained their enthusiasm for the search and continued it when they were able.

In 1925, A. Lawrence Lowell, Percival’s brother, provided the observatory with funds for the purchase of a 13-inch (33-centimeter) telescope with a focal length of 66 inches (168 centimeters). The new telescope began operation in 1929, at which time self-taught astronomer Clyde William Tombaugh was hired to work as an assistant at the observatory. On February 18, 1930, Tombaugh used the Zeiss blink comparator and discovered Lowell’s Planet X on photographic plates taken on January 21, 23, and 29, 1930. Astronomers Carl Lampland, Vesto Melvin Slipher, and Earl Carl Slipher took additional photographic plates on February 18, 1930, to confirm the sighting. Soon thereafter, they announced the discovery of the trans-Neptunian planet, as originally predicted by Lowell.


The press and general public were intrigued by Lowell’s search for and the eventual discovery of a ninth planet. When the discovery was announced, Lowell Observatory staff received hundreds of congratulatory telegrams and letters.

Before his death, Lowell was more recognized by astronomers in Europe than by those in the United States. Many of his American colleagues never acknowledged him as a professional astronomer; they considered him to be an anxious amateur. Unfortunately, the controversy that Lowell had begun earlier with his observations of Martian canals made him and the Lowell Observatory outcasts among much of the scientific community. This changed with the discovery of the ninth planet, however. With the exception of magnitude, the planet’s location and basic characteristics were so similar to Lowell’s prediction that astronomers did not assume its discovery came about by mere chance. In fact, Vesto Slipher stated that the discovery was simply the conclusion of a search program set forth in 1905 by Lowell and his theoretical work on the dynamical evidence of a ninth planet.

Professional astronomers contacted Lowell Observatory and requested specific coordinates of the new planet in order to compute its orbital path around the Sun. Early calculations suggested that the planet took several thousand years to orbit the Sun, but this value decreased as more observations were recorded, and today the orbit’s duration is known to be about 248 years.

The discovery of the ninth planet prompted astronomers at other institutions to begin their own searches for more distant planets. Tombaugh personally assisted in a continued search program at Lowell Observatory until the start of World War II. Since Pluto’s discovery, however, no other distant planets have been detected.

Lowell’s work in astronomy contributed to the advancement of science, not least through his establishment of the Lowell Observatory. The observatory’s equipment made possible not only the search for Pluto but also observations of Mars, Saturn and its rings, and the atmospheres of Jupiter and Uranus. Similar studies continue among astronomers worldwide.

Since its discovery, Pluto has been the subject of additional research. In the 1970’s, spectral observations suggested that Pluto had frozen methane on its surface, but it is now known that Pluto’s frozen surface consists of 98 percent nitrogen, with traces of methane and carbon monoxide. In 1978, an astronomer from the U.S. Naval Observatory examined some photographic plates taken from Flagstaff, Arizona, and found that in some images Pluto appeared slightly elongated; this observation led to the discovery of Pluto’s moon, Charon. Given a moon and a planet, astronomers are able to calculate the masses of these bodies using Newton’s laws. When Pluto was first discovered, astronomers believed its mass to be roughly equal to that of Earth; however, later measurements suggested that Pluto is actually smaller than Earth’s moon.

From the mid-1990’s onward, new discoveries made possible by improvements in technology led to debates among astronomers concerning whether particular bodies in our solar system should be classified as planets. In 2006, the members of the International Astronomical Union voted to reclassify Pluto as a “dwarf planet,” reducing to eight the official number of bodies in the solar system defined as true planets. The decision was itself the source of controversy, and the debate continued.

Outside the scientific community, Lowell’s early ideas of intelligent life on Mars and his search for a trans-Neptunian planet spawned excitement. Many newspaper reports were published on the new planet, and educators gave presentations at local observatories on observing the night sky. The Adler Planetarium in Chicago first opened its doors to the public in the midst of an atmosphere of wonder about what other planets might be hidden against the starry background sky. The discovery of Pluto undoubtedly inspired many people to look up toward the stars and to learn about astronomy. Pluto (planet);search Astronomy;planets Lowell Observatory Planets;Pluto

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Binzel, Richard. “Pluto.” Scientific American 262 (June, 1990): 50-58. Provides information on the physical characteristics of Pluto. Illustrations of Pluto and its moon, Charon, are helpful in explaining why this system is often considered to be a double planet. Includes the photograph that led to the discovery of Charon in 1978.
  • citation-type="booksimple"

    xlink:type="simple">Croswell, Ken. “Pluto: Enigma on the Edge of the Solar System.” Astronomy 14 (July, 1986): 7-22. Discusses Lowell’s strategy for the search of Planet X and Tombaugh’s actual discovery of the planet. Provides a summary of physical data for Pluto as well as information on the discovery of Pluto’s moon. Also features a concise biography of Lowell.
  • citation-type="booksimple"

    xlink:type="simple">Freedman, David H. “When Is a Planet Not a Planet? Arguments for and Against Demoting Pluto.” Atlantic Monthly, February, 1998. Reviews the debate that arose in the mid-1990’s regarding the status of Pluto as a major planet, given new discoveries.
  • citation-type="booksimple"

    xlink:type="simple">Hoyt, William G. Planet X and Pluto. Tucson: University of Arizona Press, 1980. Comprehensive work documents the transition between Lowell’s work and Tombaugh’s. A good source for the interested reader.
  • citation-type="booksimple"

    xlink:type="simple">Lowell, A. Lawrence. Biography of Percival Lowell. New York: Macmillan, 1935. Biography by Percival Lowell’s brother contains minute details of both the professional and the personal sides of the astronomer’s life. Provides an excellent chronology of Lowell’s career.
  • citation-type="booksimple"

    xlink:type="simple">Lowell, Percival. Memoir on a Trans-Neptunian Planet. Lynn, Mass.: Press of T. P. Nichols and Son, 1915. The definitive source for information on Lowell’s search for Planet X. Includes Lowell’s mathematical equations and a list of his predictions on the planet’s mass and distance from the Sun.
  • citation-type="booksimple"

    xlink:type="simple">Parker, Barry. Alien Life: The Search for Extraterrestrials and Beyond. New York: Plenum, 1998. Takes a scientific approach to the subject of the possibility of intelligent life on other planets, avoiding personal speculation. Chapter 5 includes discussion of Schiaparelli’s observations of Mars and Lowell’s subsequent theorizing about life on that planet. Includes glossary, bibliography, and index.
  • citation-type="booksimple"

    xlink:type="simple">Shapley, Harlow, ed. Source Book in Astronomy, 1900-1950. Cambridge, Mass.: Harvard University Press, 1960. Excellent source for information on many great historical moments in astronomy during the first half of the twentieth century. Chapter 14, “The Discovery of Pluto,” describes the search for the ninth planet.
  • citation-type="booksimple"

    xlink:type="simple">Tombaugh, Clyde William. “Reminiscences of the Discovery of Pluto.” Sky and Telescope 19 (March, 1960): 264-270. Tombaugh’s own thoughts about Lowell’s search for Planet X and his own years of investigation, including the final discovery of Pluto. Includes photographs of Lowell Observatory, Percival Lowell, the Slipher brothers, Carl Lampland, Tombaugh, and the Zeiss blink comparator.
  • citation-type="booksimple"

    xlink:type="simple">Tombaugh, Clyde William, and Patrick Moore. Out of Darkness: The Planet Pluto. Harrisburg, Pa.: Stackpole Books, 1980. Very readable work for the general reader provides a good review of all aspects of the search for planets in general and the personalities surrounding the discovery of Pluto.
  • citation-type="booksimple"

    xlink:type="simple">Whyte, A. J. The Planet Pluto. New York: Pergamon Press, 1980. A relatively technical work for readers who wish to dig into the details of the discovery of Pluto. Provides an objective view of the process. Includes a historical review.

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