Pluto

Pluto, officially designated 134340 Pluto, is a dwarf planet located in the Kuiper Belt, a vast region of icy bodies beyond Neptune. Long regarded as the ninth planet of the Solar System following its discovery in 1930, it was reclassified as a dwarf planet in 2006 when the International Astronomical Union adopted a more restrictive definition of what constitutes a planet. Pluto remains one of the most studied trans-Neptunian objects and continues to hold scientific, historical and cultural significance. Composed mainly of rock and various ices, it is much smaller than the inner planets and possesses a geological and atmospheric complexity that has drawn continued interest, particularly after observations made by the New Horizons spacecraft in 2015.

Physical Characteristics

Pluto is the largest known trans-Neptunian object by volume, though slightly less massive than Eris. Its radius is roughly 1,188 kilometres, making it about 0.1868 times the radius of Earth. In terms of mass it represents around 0.0035 Earth masses, or about one-sixth of the Moon’s mass, while its volume is about one-third that of the Moon. Its density reflects a mixture of icy and rocky components, consistent with other large bodies in the outer Solar System.
The dwarf planet has a rotation period of approximately 6 days, 9 hours and 17 minutes, a consequence of tidal locking with its largest moon, Charon. Pluto’s axial tilt is extreme, causing dramatic seasonal variations over its long orbit. Light from the Sun takes about 5.5 hours to reach Pluto.
Pluto’s surface displays widespread geological diversity. New Horizons revealed nitrogen ice plains, water-ice mountains and regions exhibiting potential cryovolcanic activity. The heart-shaped Tombaugh Regio, dominated by nitrogen ice, exemplifies the climatic and geological processes shaping the dwarf planet.

Orbit and Resonance

Pluto follows a moderately eccentric and inclined orbit, travelling between approximately 30 and 49 astronomical units from the Sun. This orbital eccentricity periodically places it closer to the Sun than Neptune. Despite this proximity, the two bodies cannot collide because they are in a 3:2 mean-motion resonance, with Pluto completing two orbits for every three of Neptune’s. This stable dynamical relationship ensures long-term protection from close encounters.
A Plutonian year lasts 247.94 Earth years; thus, Pluto will complete its first full orbit since its discovery in the year 2178. The orbital architecture of the Kuiper Belt, including resonant and scattered objects, has placed Pluto at the centre of studies of Solar System evolution.

Moons of Pluto

Pluto has five known natural satellites:

• Charon, the largest, with a diameter more than half that of Pluto. The Pluto–Charon system is often described as a binary dwarf planet because their barycentre lies between them rather than inside either body.
• Styx, Nix, Kerberos and Hydra, smaller irregular moons discovered through improved telescope observations. These moons follow nearly circular orbits within the system’s equatorial plane.

The tidally locked relationship between Pluto and Charon means each body always shows the same face to the other, a feature that has important implications for their mutual evolution and thermal histories.

Discovery

The discovery of Pluto followed decades of speculation about deviations in Uranus’s orbit, which led some astronomers to propose the existence of an additional planet beyond Neptune. In 1906 Percival Lowell initiated a search for this Planet X, supported by detailed mathematical calculations performed at Lowell Observatory.
Although Lowell unknowingly captured faint images of Pluto in 1915, the object was not recognised at the time. Following Lowell’s death and an extended legal dispute over his estate, the observational search resumed in the late 1920s. Clyde W. Tombaugh, a young astronomer appointed to conduct the investigation, used a blink comparator to compare photographic plates taken on different nights. On 18 February 1930 he identified a moving object on plates from January of that year. After further verification, the discovery was announced on 13 March 1930. The first precovery images date back as far as 1909.
The discovery cemented Pluto’s status as the first known member of what would later be identified as the Kuiper Belt.

Naming and Symbol

The name Pluto was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, who had a keen interest in classical mythology. Her suggestion was conveyed to astronomers at Lowell Observatory and accepted unanimously. Mythologically, Pluto is the Roman god of the underworld, equivalent to the Greek Hades, a fitting association for a distant and faint world. The initials PL also acknowledge Percival Lowell, strengthening the observatory’s support for the name.
Pluto’s original astronomical symbol, a monogram formed from the letters P and L, is still used occasionally but has fallen out of favour as planetary symbols are increasingly discouraged in formal scientific contexts. Several alternative symbols exist in astrological traditions, including those depicting a bident or stylised orbital paths. The name also permeated popular culture, inspiring the naming of Disney’s character Pluto in 1930.

Re-evaluation of Planetary Status

By the late twentieth century, Pluto’s small mass and size raised doubts about its classification as a planet. The discovery of similarly sized or larger objects in the trans-Neptunian region, particularly the scattered-disc object Eris in 2005, prompted reconsideration. In 2006 the International Astronomical Union adopted a definition of “planet” requiring an object to clear its orbit of debris. Pluto did not satisfy this requirement and was therefore reclassified as a dwarf planet.
Despite this formal designation, ongoing debates among planetary scientists reflect differing views on whether clearing the orbit should be a criterion for planethood. Several researchers continue to refer to Pluto and other dwarf planets as planets in a functional or geological sense.

Exploration by New Horizons

NASA’s New Horizons spacecraft performed the first flyby of Pluto and its moons on 14 July 2015. It collected high-resolution images, atmospheric data and measurements of surface composition. The mission transformed scientific understanding of the dwarf planet, revealing unexpected complexity, including:

• Vast nitrogen-ice plains and convective glacial activity.
• Mountain ranges composed of water ice.
• Layered hazes in the thin nitrogen-based atmosphere.
• Evidence for past or ongoing cryovolcanism.
• Complex interactions within the Pluto–Charon system.

Data returned by New Horizons continue to be analysed, offering fresh insight into the geology and climatic variations of distant icy worlds.

Pluto in Culture and Science

Pluto’s discovery and subsequent reclassification have made it a cultural icon and a subject of intense public interest. Its changing status reflects broader developments in astronomy, including the understanding of planetary formation and the recognition of the Kuiper Belt as a major component of the Solar System.