Discovery of Pluto (1930): Clyde Tombaugh Finds Planet X

On 18 February 1930 a young assistant at Lowell Observatory in Flagstaff, Arizona, named Clyde Tombaugh spotted a tiny moving dot on a pair of photographic plates. It was a new world beyond Neptune, soon named Pluto. The discovery of Pluto ended a quarter-century search for a mysterious “Planet X” and added a ninth planet to the textbooks, a status it kept until astronomers reclassified it as a dwarf planet in 2006.

Background: the search for Planet X

After William Herschel discovered Uranus in 1781, astronomers noticed that its orbit did not quite match predictions. Calculations of these irregularities led to the discovery of Neptune in 1846, one of the great triumphs of mathematical astronomy. By the late 19th century some astronomers believed that small leftover discrepancies pointed to yet another planet farther out.

The most determined of them was Percival Lowell, a wealthy Boston businessman who founded Lowell Observatory in Flagstaff in 1894, initially to study Mars. From 1906 Lowell organised searches for what he called “Planet X”, predicting its likely position from the orbits of Uranus and Neptune. He found nothing before his death in 1916, and a legal dispute with his widow over his estate held up further searches for years.

Clyde Tombaugh joins the hunt

In the late 1920s the observatory’s director, Vesto Slipher, revived the search with a new 13-inch photographic telescope. He needed someone patient enough to do the work. Clyde Tombaugh, a 22-year-old farm boy from Kansas with no university degree, had sent the observatory careful drawings of Jupiter and Mars made with telescopes he built himself. In January 1929 Slipher hired him.

Tombaugh’s method was painstaking. He photographed the same patch of sky on nights several days apart, then examined the two plates in a blink comparator, a device that rapidly switched the view between them. Stars stayed fixed, but anything moving against them, such as an asteroid or a distant planet, would appear to jump back and forth. Each plate could contain hundreds of thousands of stars, especially in regions near the Milky Way.

What happened in February 1930

Tombaugh took plates of an area in the constellation Gemini on 23 and 29 January 1930. On the afternoon of 18 February, while blinking them, he noticed a faint object, of about 15th magnitude, that had shifted by a small amount between the two dates. Its slow motion matched what was expected of a body far beyond Neptune. He checked it against another plate and then informed Slipher.

The observatory staff spent several weeks confirming the object and measuring its motion before making any announcement. The discovery was made public on 13 March 1930, a carefully chosen date: it was the anniversary of Lowell’s birth and of Herschel’s discovery of Uranus. Newspapers around the world celebrated the finding of the “ninth planet”.

Naming Pluto

Lowell Observatory received more than 1,000 suggestions for a name. In Oxford, England, 11-year-old Venetia Burney, who was interested in classical myths, suggested Pluto, the Roman god of the underworld, who could make himself invisible. Her grandfather Falconer Madan, a retired librarian at the Bodleian Library, passed the idea to the astronomer Herbert Hall Turner, who cabled it to the observatory.

Staff members voted unanimously for Pluto, and the name was announced on 1 May 1930. It had the added appeal that its first two letters were Percival Lowell’s initials, and the planet’s symbol combined the letters P and L. Burney received a reward of five pounds from her grandfather. The name soon spread: Walt Disney’s cartoon dog Pluto appeared in 1930 and took the name in 1931, and the element plutonium, first produced in 1940–41, was later named after the planet.

Key people

• Clyde Tombaugh (1906–1997): the discoverer, who later earned degrees, continued searching the sky for asteroids and comets and taught astronomy in New Mexico.
• Percival Lowell (1855–1916): the founder of Lowell Observatory, whose predictions and fortune launched the search.
• Vesto Slipher (1875–1969): observatory director who hired Tombaugh and announced the discovery.
• Venetia Burney (1918–2009): the schoolgirl who proposed the name.

A smaller world than expected

Pluto turned out to be far smaller than Lowell’s Planet X. Its brightness suggested at first a body perhaps as large as Earth, but estimates fell steadily over the following decades. When James Christy of the US Naval Observatory discovered Pluto’s large moon Charon in 1978, astronomers could at last calculate Pluto’s mass: only about 0.2 per cent of Earth’s. That was far too little to disturb Uranus or Neptune.

Pluto’s orbit also proved unusual. It is tilted about 17 degrees to the plane in which the major planets orbit and is so elongated that between 1979 and 1999 Pluto was closer to the Sun than Neptune. The two can never collide, because Pluto completes two orbits for every three made by Neptune, a resonance that keeps them far apart.

In the early 1990s analyses using the more accurate mass of Neptune measured by the Voyager 2 spacecraft showed that the supposed discrepancies in the outer planets’ orbits did not exist. Pluto’s discovery near Lowell’s predicted position was therefore largely a coincidence, made possible by Tombaugh’s thorough survey.

Reclassification as a dwarf planet

In 1992 astronomers David Jewitt and Jane Luu found the first object, apart from Pluto and Charon, in a belt of icy bodies beyond Neptune now known as the Kuiper Belt. Hundreds more followed. In 2005 a team led by Mike Brown announced Eris, an object comparable in size to Pluto and more massive.

This forced astronomers to decide what a planet was. On 24 August 2006, at its General Assembly in Prague, the International Astronomical Union adopted a definition requiring a planet to have cleared the neighbourhood around its orbit. Pluto did not qualify and was reclassified as a dwarf planet, later given the minor-planet number 134340. The decision remains controversial with some scientists and much of the public. Critics, including members of the New Horizons mission team, argue that the orbital-clearing rule is hard to apply and that a planet should be defined by its own physical properties rather than its surroundings.

Legacy

Pluto’s story continued with NASA’s New Horizons spacecraft, launched in January 2006, which flew past Pluto on 14 July 2015 and revealed a surprisingly active world with nitrogen-ice plains, mountains of water ice and a thin atmosphere. The spacecraft carried a small portion of Clyde Tombaugh’s ashes. Whatever its classification, Pluto opened the way to understanding the outer Solar System as a vast region of icy worlds.

Timeline

• 1894: Percival Lowell founds Lowell Observatory in Flagstaff, Arizona.
• 1906: Lowell begins searching for Planet X.
• 1929, January: Clyde Tombaugh joins Lowell Observatory.
• 1930, 23 and 29 January: Tombaugh photographs the region containing Pluto.
• 1930, 18 February: Tombaugh discovers Pluto using a blink comparator.
• 1930, 13 March: The discovery is announced.
• 1930, 1 May: The name Pluto is officially announced.
• 1978: James Christy discovers Pluto’s moon Charon.
• 2006, 24 August: The IAU reclassifies Pluto as a dwarf planet.
• 2015, 14 July: New Horizons flies past Pluto.

Why it matters

The discovery of Pluto was a triumph of patient observation by a self-taught astronomer, and for 76 years it defined the outer edge of the known planetary system. Its later demotion showed how science revises its categories as new evidence appears, and Pluto remains the gateway to the Kuiper Belt and the frozen outskirts of the Solar System.