Percival Lowell was a descendant of the Boston Lowell family. His brother A. Lawrence was the president of Harvard University, and his sister Amy was an imagist poet, critic, and publisher.

Percival graduated from the Noble and Greenough School in 1872 and Harvard University in 1876 with distinction in mathematics. At his college graduation, he gave a speech, considered very advanced for its time, on the "Nebular Hypothesis." He was later awarded honorary degrees from Amherst College and Clark University.

In the 1880s, Lowell traveled extensively in the Far East. In August 1883, he served as a foreign secretary and counselor for a special Korean diplomatic mission to the United States. He also spent significant periods of time in Japan, writing books on Japanese religion, psychology, and behavior. His texts are filled with observations and academic discussions of various aspects of Japanese life, including language, religious practices, economics, travel in Japan, and the development of personality. Books by Percival Lowell on the Orient include Chosön: The Land of the Morning Calm (1886, Boston), Noto: An Unexplored Corner of Japan (1891) and Occult Japan, or the Way of the Gods (1894); the latter from his third and final trip to the region. The most popular of Lowell's books on the Orient, The Soul of the Far East, (1888) contains an early synthesis of some of his ideas, that in essence, postulated that human progress is a function of the qualities of individuality and imagination.

He was elected a Fellow of the American Academy of Arts and Sciences in 1892. Beginning in the winter of 1893 - 94, using his wealth and influence, Lowell dedicated himself to the study of astronomy, founding the observatory which bears his name. For the last 23 years of his life astronomy, Lowell Observatory, and his and others' work at his observatory were the focal points of his life. He lived to be 61 years of age.

World War I very much saddened Lowell, a dedicated pacifist. This, along with some setbacks in his astronomical work, undermined his health and contributed to his death from a stroke on November 12, 1916.

Another of his books is The Eve of the French Revolution (1892). Lowell is buried on Mars Hill near his observatory.

Lowell became determined to study Mars and astronomy as a full time career after reading Camille Flammarion's La planète Mars. He was particularly interested in the canals of Mars, as drawn by Italian astronomer Giovanni Schiaparelli, who was director of the Milan Observatory.

In 1894 Lowell chose Flagstaff, Arizona Territory, as the home of his new observatory. At an altitude of over 2100 meters (7000 feet), with few cloudy nights, and far from city lights, Flagstaff was an excellent site for astronomical observations. This marked the first time an observatory had been deliberately located in a remote, elevated place for optimal seeing.

For the next fifteen years he studied Mars extensively, and made intricate drawings of the surface markings as he perceived them. Lowell published his views in three books: Mars (1895), Mars and Its Canals (1906), and Mars As the Abode of Life (1908). With these writings, Lowell more than anyone else popularized the long held belief that these markings showed that Mars sustained intelligent life forms.

His works include a detailed description of what he termed the 'non - natural features' of the planet's surface, including especially a full account of the 'canals,' single and double; the 'oases,' as he termed the dark spots at their intersections; and the varying visibility of both, depending partly on the Martian seasons. He theorized that an advanced but desperate culture had built the canals to tap Mars' polar ice caps, the last source of water on an inexorably drying planet.

While this idea excited the public, the astronomical community was skeptical. Many astronomers could not see these markings, and few believed that they were as extensive as Lowell claimed. As a result, Lowell and his observatory were largely ostracized. Although the consensus was that some actual features did exist which would account for these markings, in 1909 the sixty - inch Mount Wilson Observatory telescope in Southern California allowed closer observation of the structures Lowell had interpreted as canals, and revealed irregular geological features, probably the result of natural erosion.

The existence of canal like features would not be definitively disproved until Mariner 4 took the first close up pictures of Mars in 1965, and Mariner 9 orbited and mapped the planet in 1972. Today, the surface markings taken to be canals are regarded as an optical illusion.

Although Lowell was better known for his observations of Mars, he also drew maps of the planet Venus. Lowell observed spoke like features and a central dark spot, yet it is now known that Venus' atmosphere is opaque. In an article published in Sky and Telescope in July 2003, it was suggested that in fact Lowell was observing an image of the blood vessels in his own eye.

Lowell's greatest contribution to planetary studies came during the last decade of his life, which he devoted to the search for Planet X, a hypothetical planet beyond Neptune. Lowell believed that the planets Uranus and Neptune were displaced from their predicted positions by the gravity of the unseen Planet X. Lowell started a search program in 1906 using a camera 5 inches (13 cm) in aperture. The small field of view of the 42 inch (110 cm) reflecting telescope rendered the instrument impractical for searching. From 1914 to 1916, a 9 inch (23 cm) telescope on loan from Sproul Observatory was used to search for Planet X. Although Lowell did not discover Pluto, Lowell Observatory (690) did photograph Pluto in March and April 1915.

In 1930 Clyde Tombaugh, working at the Lowell Observatory, discovered Pluto near the location expected for Planet X. Partly in recognition of Lowell's efforts, a stylized P-L monogram () — (the first two letters of the new planet's name and also Lowell's initials), was chosen as Pluto's astronomical symbol. However, it would subsequently emerge that the Planet X theory was mistaken.

Pluto's mass could not be determined until 1978, when a satellite came close enough for observation. This confirmed what had been increasingly suspected: Pluto's gravitational influence on Uranus and Neptune is negligible, certainly not nearly enough to account for the discrepancies in their orbits. In 2006, Pluto was reclassified as a dwarf planet by the International Astronomical Union.

In addition, it is now known that the discrepancies between the predicted and observed positions of Uranus and Neptune were not caused by the gravity of an unknown planet. Rather, they were due to an erroneous value for the mass of Neptune. Voyager 2's 1989 encounter with Neptune yielded a more precise value of its mass, and the discrepancies disappear when using this value.

Although Lowell's theories of the Martian canals, of surface features on Venus, and of Planet X are now discredited, his practice of building observatories at the position where they would best function has been adopted as a principle. He also established the program and setting which made the discovery of Pluto by Clyde Tombaugh possible. Craters on the Moon and on Mars have been named after him. Lowell has been described by other planetary scientists as "the most influential popularizer of planetary science in America before Carl Sagan."

While eventually disproved, Lowell's vision of the Martian canals as an artifact of an ancient civilization making a desperate last effort to survive, had an enormous influence on the development of Science Fiction - starting with H.G. Wells' enormously influential "The War of the Worlds" which made the logical further inference that creatures from a dying planet might seek to invade Earth.

The image of the dying Mars and its ancient culture was retained, in numerous versions and variations, in most SF works depicting Mars in the first half of the Twentieth Century. Also when proven to be factually mistaken, the vision of Mars derived from his theories remains enshrined in works which remain in print and widely read as classics of Science Fiction.

Tombaugh was born in Streator, Illinois. After his family moved to Burdett, Kansas, Tombaugh's plans for attending college were frustrated when a hailstorm ruined their crops. Starting in 1926, he built several telescopes with lenses and mirrors he ground himself. He sent drawings of Jupiter and Mars, as well as his telescopes to the Lowell Observatory. These resulted in a job offer. Tombaugh was employed at the Lowell Observatory from 1929 to 1945.

Following his discovery of Pluto, Tombaugh earned bachelor's and master's degrees in astronomy from the University of Kansas in 1936 and 1938. During World War II he taught naval personnel navigation at Northern Arizona University. He worked at White Sands Missile Range in the early 1950s, and taught astronomy at New Mexico State University from 1955 until his retirement in 1973. He died in Las Cruces, New Mexico, in 1997, aged 90. He was survived by his widow, Patricia and their children: daughter Annette and son Alden. Tombaugh was the great - uncle of Los Angeles Dodgers pitcher Clayton Kershaw. Tombaugh was an active Unitarian - Universalist.

The asteroid 1604 Tombaugh, discovered in 1931, is named after him. He discovered hundreds of asteroids, beginning with 2839 Annette in 1929, mostly as a by-product of his search for Pluto and his searches for other celestial objects. Tombaugh named some of them after his wife, children and grandchildren. The Royal Astronomical Society awarded him the Jackson - Gwilt Medal in 1931.

Approximately one ounce of his ashes is being carried on the New Horizons spacecraft, launched in 2006 for a flyby of Pluto in 2015. The container includes the inscription: "Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's 'third zone'. Adelle and Muron's boy, Patricia's husband, Annette and Alden's father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906 – 1997)".

While a young researcher working for the Lowell Observatory in Flagstaff, Arizona, Tombaugh was given the job to perform a systematic search for a trans - Neptunian planet (also called Planet X), which had been predicted by Percival Lowell and William Pickering.

Tombaugh used the observatory's 13 inch astrograph to take photographs of the same section of sky several nights apart. He then used a blink comparator to compare the different images. When he shifted between the two images, a moving object, such as a planet, would appear to jump from one position to another, while the more distant objects such as stars would appear stationary. Tombaugh noticed such a moving object in his search, near the place predicted by Lowell, and subsequent observations showed it to have an orbit beyond that of Neptune. This ruled out classification as an asteroid, and they decided this was the ninth planet that Lowell had predicted. The discovery was made on Tuesday, February 18, 1930, using images taken the previous month. The name "Pluto" was reportedly suggested by Venetia Burney, then an 11 year old English school girl, who died in April 2009, having lived to see the reclassification of Pluto as a dwarf planet. It won out over numerous other suggestions partly because it was named after the Roman god of the underworld because he was able to render himself invisible and because Percival Lowell's initials PL formed the first 2 letters. The name Pluto was officially adopted on May 1, 1930.

Following the discovery, starting in the 1990s, of other Kuiper belt objects, Pluto began to be seen not as a planet orbiting alone at 40 AU, but as the largest of a group of icy bodies in that region of space. After it was shown that at least one such body was larger than Pluto, on August 24, 2006 the International Astronomical Union (IAU) reclassified Pluto, grouping it with two similarly sized "dwarf planets" rather than with the eight "classical planets".

Tombaugh's widow Patricia stated after the IAU's decision that while Clyde may have been disappointed with the change since he had resisted attempts to remove Pluto's planetary status in his lifetime, he would have accepted the decision now if he were alive. She noted that he "was a scientist. He would understand they had a real problem when they start finding several of these things flying around the place."

Tombaugh continued searching for some years after the discovery of Pluto, and the lack of further discoveries left him satisfied that no other object of a comparable apparent magnitude existed near the ecliptic. No more Trans - Neptunian objects were discovered until (15760) 1992 QB₁, in 1992.

However, more recently the relatively bright object Makemake has been discovered. It has a relatively high orbital inclination, but at the time of Tombaugh's discovery of Pluto, Makemake was only a few degrees from the ecliptic near the border of Taurus and Auriga at an apparent magnitude of 16. This position was also very near the galactic equator, making it almost impossible to find such an object within the dense concentration of background stars of the Milky Way.

Tombaugh discovered nearly 800 asteroids during his search for Pluto and years of follow up searches looking for another candidate for the postulated Planet X. Tombaugh is also credited with the discovery of comet C/1931 AN, though its orbit is currently unknown. He also discovered hundreds of variable stars, as well as star clusters, galaxy clusters, and a galaxy supercluster.

Tombaugh was probably the most eminent astronomer to have reported seeing unidentified flying objects and to support the extraterrestrial hypothesis. On August 20, 1949, Tombaugh saw several UFOs near Las Cruces, New Mexico. He described them as six to eight rectangular lights, stating: "I doubt that the phenomenon was any terrestrial reflection, because... nothing of the kind has ever appeared before or since... I was so unprepared for such a strange sight that I was really petrified with astonishment." Another sighting by Tombaugh a year or two later while at a White Sands observatory was of an object of −6 magnitude, four times brighter than Venus at its brightest, going from the zenith to the southern horizon in about 3 seconds. The object executed the same maneuvers as in Tombaugh's first sighting.

Tombaugh later reported having seen three of the mysterious green fireballs, which suddenly appeared over New Mexico in late 1948 and continued at least through the early 1950s. Despite this, the final report of Project Twinkle claimed that he "... never observed an unexplainable aerial object despite his continuous and extensive observations of the sky."

In 1956 Tombaugh had the following to say about his various sightings: "I have seen three objects in the last seven years which defied any explanation of known phenomenon, such as Venus, atmospheric optic, meteors or planes. I am a professional, highly skilled, professional astronomer. In addition I have seen three green fireballs which were unusual in behavior from normal green fireballs... I think that several reputable scientists are being unscientific in refusing to entertain the possibility of extraterrestrial origin and nature."

Shortly after this in January 1957, in an Associated Press article in the Alamogordo Daily News titled "Celestial Visitors May Be Invading Earth's Atmosphere," Tombaugh was again quoted on his sightings and opinion about them. "Although our own solar system is believed to support no other life than on Earth, other stars in the galaxy may have hundreds of thousands of habitable worlds. Races on these worlds may have been able to utilize the tremendous amounts of power required to bridge the space between the stars..." Tombaugh stated that he had observed celestial phenomena which he could not explain, but has seen none personally since 1951 or 1952. "These things, which do appear to be directed, are unlike any other phenomena I ever observed. Their apparent lack of obedience to the ordinary laws of celestial motion gives credence."

In 1949, Tombaugh had also told the Naval missile director at White Sands Missile Range, Commander Robert McLaughlin, that he had seen a bright flash on Mars in August 1941, which he now attributed to an atomic blast. Tombaugh also noted that the first atomic bomb tested in New Mexico would have lit up the dark side of the Earth like a neon sign and that Mars was coincidentally quite close at the time, the implication apparently being that the atomic test would have been visible from Mars.

In June 1952, Dr. J. Allen Hynek, an astronomer acting as a scientific consultant to the Air Force's Project Blue Book UFO study, secretly conducted a survey of fellow astronomers on UFO sightings and attitudes while attending an astronomy convention. Tombaugh and four other astronomers, including Dr. Lincoln LaPaz of the University of New Mexico, told Hynek about their sightings. Tombaugh also told Hynek that his telescopes were at the Air Force's disposal for taking photos of UFOs, if he was properly alerted.

Tombaugh's offer may have led to his involvement in a search for near Earth satellites, first announced in late 1953 and sponsored by the Army Office of Ordnance Research. Another public statement was made on the search in March 1954, emphasizing the rationale that such an orbiting object would serve as a natural space station. However, according to Donald Keyhoe, later director of the National Investigations Committee on Aerial Phenomena (NICAP), the real reason for the sudden search was because two near Earth orbiting objects had been picked up on new long range radar in the summer of 1953, according to his Pentagon source.

By May 1954, Keyhoe was making public statements that his sources told him the search had indeed been successful, and either one or two objects had been found. However, the story did not break until August 23, 1954, when Aviation Week magazine stated that two satellites had been found only 400 and 600 miles out. They were termed "natural satellites" and implied that they had been recently captured, despite this being a virtual impossibility. The next day, the story was in many major newspapers. Dr. LaPaz was implicated in the discovery in addition to Tombaugh. LaPaz had earlier conducted secret investigations on behalf of the Air Force on the green fireballs and other unidentified aerial phenomena over New Mexico. LaPaz vehemently denied his involvement in the search, although the New York Times reported on August 29 that a source close to the project said that the story was true and LaPaz was indeed involved, in fact had been the one to spot and identify the objects as natural rather than artificial satellites. The same source denied the search had anything to do with flying saucers.

However, both LaPaz and Tombaugh were to issue public denials that anything had been found. The October 1955 issue of Popular Mechanics magazine reported: "Professor Tombaugh is closemouthed about his results. He won't say whether or not any small natural satellites have been discovered. He does say, however, that newspaper reports of 18 months ago announcing the discovery of natural satellites at 400 and 600 miles out are not correct. He adds that there is no connection between the search program and the reports of so-called flying saucers."

At a meteor conference in Los Angeles in 1957, Tombaugh reiterated that his four year search for "natural satellites" had been unsuccessful. In 1959, Tombaugh was to issue a final report stating that nothing had been found in his search. His personal 16 inchtelescope was reassembled and dedicated on September 17, 2009 at Rancho Hidalgo, New Mexico, near Animas, New Mexico, adjacent to Astronomy 's new observatory.