Pluto
Pluto is a dwarf planet in the Kuiper Belt of the Solar System. It is smaller than Earth's Moon but has varied terrain, glaciers and a tenuous atmosphere. Pluto was regarded as the ninth planet for 76 years after its discovery; in 2006, the International Astronomical Union classified it as a dwarf planet.[2][4]
Size and physical characteristics
Pluto has a diameter of approximately 2,376 km, about two thirds that of the Moon and less than one fifth that of Earth. Its mass is 1.30 × 1022 kg, roughly 460 times smaller than Earth's. Its mean density of about 1.85 g/cm³ reflects a mixture of rocky material and ice.[1][5]
Surface gravity is approximately 16 times weaker than on Earth. An object that presses on a support with a force of 100 newtons on Earth would exert a force of about 6 newtons on Pluto.[5]
Orbit and rotation
One orbit around the Sun takes approximately 248 Earth years. Pluto's orbit is notably elongated and inclined by about 17° to the ecliptic. Its distance from the Sun varies from roughly 4.4 billion km to 7.3 billion km.[1]
Pluto is occasionally nearer the Sun than Neptune; the most recent such interval lasted from 1979 to 1999. This does not mean that their paths intersect dangerously or that the two bodies approach each other closely.[6]
Their orbital motion is locked in a 3:2 resonance: Neptune completes three revolutions around the Sun while Pluto completes two. The repeating geometry and three-dimensional arrangement of the orbits prevent close encounters.[7]
Pluto rotates once every 6.4 Earth days. Its rotation is synchronised with Charon's orbit, so Pluto and its large moon permanently show the same faces to one another.[8]
Surface and ice mountains
Pluto contains plains, mountain ranges, valleys and impact craters. One of its most prominent features is the bright, heart-shaped Tombaugh Regio, named after the discoverer of Pluto.[9]
Water ice is an important structural material in the mountains. At Pluto's temperatures it is strong enough to support high relief. Along the edge of Sputnik Planitia, New Horizons photographed mountains about 3.5 km high beside smooth icy plains.[10]
The surface varies greatly in colour. Bright ice occurs alongside reddish and dark terrain produced by complex organic material that forms when radiation processes simpler compounds.[11]
Sputnik Planitia and glaciers
The western half of Pluto's bright heart is Sputnik Planitia, a basin covered mainly by nitrogen ice. Its surface is divided into large cells. Models link this pattern to convection, with warmer ice rising slowly and cooled material sinking.[12]
The plain contains few impact craters, indicating a geologically young surface that is continually renewed. Nitrogen ice can flow slowly and form glaciers. Isolated hills of water ice occur within it and may be carried by the surrounding softer ice.[12][13]
Dark streaks and diffuse patches near the northern margin of the plain may record temporary releases of liquid nitrogen from beneath a glacier. This interpretation was proposed from New Horizons images taken in 2015; the liquid itself was not observed directly.[14]
Temperature and atmosphere
Pluto's surface temperature is approximately −240 to −226 °C, depending on location and conditions. At such temperatures, nitrogen, methane and carbon monoxide can exist as surface ices.[2]
Pluto has a tenuous nitrogen atmosphere containing methane and carbon monoxide. Surface pressure is of the order of one pascal, roughly one hundred thousand times lower than Earth's sea-level pressure. The atmosphere and surface ice exchange material: ice sublimates as it warms, while gas can condense onto the surface as conditions cool.[1][2]
Backlit images revealed an extended layered haze. Its particles form through photochemical reactions involving nitrogen and methane. Their small size scatters sunlight and gives the haze a blue appearance.[11]
Internal structure
Pluto consists of rocky material and ice. Models allow for a rocky core surrounded by an ice shell. Some landforms and tectonic features are consistent with the possible survival of a subsurface ocean, although no present-day layer of liquid water has been confirmed directly.[12]
Moons and the Pluto–Charon system
Pluto has five known moons: Charon, Nix, Hydra, Kerberos and Styx. Charon was discovered in 1978; the other four are much smaller and were found later with the Hubble Space Telescope.[3][15]
Charon is about 1,212 km in diameter, roughly half the diameter of Pluto. The system's common centre of mass lies outside Pluto's surface: both bodies orbit a point in the space between them. One complete cycle takes approximately 6.4 days.[1][15]
Because Charon is so large relative to Pluto, the pair is often described as a double system. Pluto nevertheless retains its official classification as a dwarf planet.[8][4]
Discovery and classification
Clyde Tombaugh discovered Pluto on 18 February 1930 by comparing photographs of the star field taken at Lowell Observatory. The distant object shifted against the background stars between images made on different nights.[16][1]
On 24 August 2006, the International Astronomical Union adopted a definition of a planet and established the category of dwarf planet. Pluto is massive enough to be nearly round and orbits the Sun, but it does not gravitationally dominate its orbital region, which contains other trans-Neptunian objects. Under the adopted definition, it is therefore a dwarf planet.[4]
New Horizons exploration
New Horizons became the first spacecraft to explore Pluto at close range on 14 July 2015. It had launched on 19 January 2006 and travelled for more than nine years before reaching the system. The spacecraft performed a flyby rather than entering orbit around Pluto.[17]
The mission returned detailed images of the surface and moons and measured the composition of the ices and atmosphere. The data revealed that this small, distant world has diverse terrain and active processes that renew its surface. Analysis continues to produce new findings many years after the flyby.[12][14]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 Pluto Fact Sheet // NASA, National Space Science Data Center.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Pluto Facts // NASA Science.
- ↑ 3.0 3.1 Pluto Moons // NASA Science.
- ↑ 4.0 4.1 4.2 Pluto and the Solar System // International Astronomical Union.
- ↑ 5.0 5.1 Planetary Fact Sheet — Metric // NASA, National Space Science Data Center.
- ↑ Charon Discovery Image // NASA Science.
- ↑ In Depth: Kuiper Belt // NASA Solar System Exploration.
- ↑ 8.0 8.1 Charon // NASA Science.
- ↑ Pluto Features Given First Official Names // NASA. 2017.
- ↑ Pluto's Majestic Mountains, Frozen Plains and Foggy Hazes // NASA Science.
- ↑ 11.0 11.1 New Horizons Finds Blue Skies and Water Ice on Pluto // NASA. 8 October 2015.
- ↑ 12.0 12.1 12.2 12.3 Five Years after New Horizons' Historic Flyby, Here Are 10 Cool Things We Learned About Pluto // NASA. 2020.
- ↑ Pluto's Mysterious, Floating Hills // NASA Science.
- ↑ 14.0 14.1 NASA's New Horizons Finds Evidence of Recent Liquid on Pluto's Surface // NASA Science. 5 August 2026.
- ↑ 15.0 15.1 45 Years Ago: Astronomers Discover Pluto's Moon Charon // NASA. 2023.
- ↑ Charon Discovery Image // NASA Science.
- ↑ New Horizons // NASA Science.