Pluto: Discovery, Characteristics, and Dwarf Planet Status

Introduction: Pluto as a Puzzling Planet

Pluto — nicknamed the "puzzling planet" or "runaway planet" — was discovered by Clyde Tombaugh, a young American astronomer, at the Lowell Observatory in Arizona in 1930. Pluto is the largest known dwarf planet and the ninth largest object directly orbiting the sun. It is the coldest, smallest, and most distant planet from the sun. Similar to other Kuiper Belt objects, Pluto's landscape consists mainly of rock and ice. Astronomers have long been puzzled by the origin of Pluto, with earlier work suggesting that it is a residual planetesimal.

Theories on Pluto's Origin

There are varied hypotheses about Pluto's origin. One possibility is that Pluto was knocked out of its original position by Neptune's largest moon, Triton, implying that Pluto never possessed fully planetary status. This concept was later rejected, however, as subsequent research established that there is no meaningful interaction between Neptune's and Pluto's orbits. Another strand of hypothesis argues that the formation of a distant rocky core in the Sun's protoplanetary disc resulted in the emergence of Pluto.

J. Hunter Waite Jr. and Christopher Glein proposed a cosmochemical model arguing that Pluto is essentially a giant comet containing gas and ice. The discovery of Pluto's place in astronomical history also traces back to the earlier discovery of Uranus by Sir William Herschel in the late eighteenth century — a finding that revealed a world beyond Saturn, which had for centuries been recognized as the outermost planet. Urbain Le Verrier's subsequent identification of Neptune, based on perturbations observed in Uranus's orbit, further fueled the drive for exploration that ultimately led to the discovery of Pluto.

Theorizing that puzzling perturbations in the orbits of Neptune and Uranus were caused by the gravitational influence of an unknown planetary body, Percival Lowell — founder of the famous Flagstaff Observatory in Arizona — proposed the existence of a ninth planet, which he called Planet X.

Discovery of Pluto and Its Naming

Lowell's search for Planet X began in 1903, but he died in 1916 without finding it, despite 13 years of dedication to mathematical theory and planet searching. Fortunately, Lowell left behind substantial work on the mathematical theory, meaning the remaining task was purely observational. His observatory team continued the search until Tombaugh recorded an exceptional moving object on photographic search plates on January 21st, 23rd, and 29th, and again in February 1930.

Using the new astronomical technique of photographic plates combined with a blink microscope, Tombaugh adopted a straightforward approach: observing the same area of sky two to three days apart and looking for a star that appeared to move. NASA's Solar System Exploration resource documents this milestone in detail. It was subsequently discovered that Pluto has five satellites: Charon (1978), Nix (2006), Hydra (2006), Kerberos (2013), and Styx (2013), with the largest — Charon — being approximately half the size of Pluto.

The discovery of Pluto attracted massive global interest, resulting in more than 1,000 name suggestions. Proposed names included Lowell, Artemis, Indana, Atlas, Vulcan, Osiris, Zymal, Cronus, Apollos, Perseus, Erebus, Zeus, Idana, Bacchus, and Minerva. Ultimately, the suggestion of an eleven-year-old schoolgirl from Oxford, Venetia Burney — who proposed naming the planet after the Roman god of the underworld — was adopted, and Planet X became Pluto.

Physical and Orbital Characteristics

Pluto is regarded as one of the strangest yet most fascinating objects in the solar system. Its year lasts 248 Earth years, meaning Pluto has not yet completed even one full orbit since its discovery. A single day on Pluto lasts 153 hours, equivalent to approximately 6.39 Earth days. The planet's diameter is roughly 45% that of Earth.

Pluto follows a tilted, elliptical orbit around the sun — tilted 57 degrees from the plane of the solar system — which is unusual compared with the more circular orbits of the major planets. Although Pluto is on average 39 astronomical units from the sun, its oval-shaped orbit can place it as far as 49.3 astronomical units away, and periodically brings it closer to the sun than Neptune. Between 1979 and 1999, Pluto was near perihelion and was therefore closer to the sun than Neptune during that period.

It is intriguing that, although Pluto's and Neptune's orbits appear to cross, they do not actually intersect — meaning the two bodies never collide, maintaining their orbital stability despite their apparent closeness. Unlike the gas giant planets beyond Mars, Pluto is rocky and small, with a mass of 1.31 × 10²² kilograms (0.22% of Earth's mass) and a volume of approximately 0.5 billion cubic miles (0.66% of Earth's volume). Its density of 1.86 g/cm³ is about 33.7% that of Earth.

Bibliography

Ley, Willy. 1956. The Demotion of Pluto. New York: Galaxy Publishing Corp.

NASA Science. 2019. Solar System Exploration. April 25th. Accessed April 27, 2019. https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth/.

Stern, S., F. Bagenal, K. Ennico, and George Gladstone. 2015. "The Pluto System: Initial Results from Its Exploration by New Horizons." Science vol. 350, issue 6258.

Stern, A., and J. Mitton. 2005. Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System. Wiley-VCH, 1997.

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