Pluto imaged by New Horizons from a range of 35,445  kilometers.

Figure 1. Image of Pluto captured by New Horizons on 14 July 2015, from a range of 35,445 kilometers (22,025 miles). A dominant feature is the bright white "heart", the large left lobe of which has been named Sputnik Planitia – a . This million-square-kilometer nitrogen glacier. Sputnik Planitia literally forced the dwarf planet to reorient itself so that the basin now faces almost squarely opposite Pluto’s moon Charon. 

Pluto's orbit

Figure 2. Pluto's orbit in comparison with those of the eight major planets of the Solar System.

Comparison of the orbits of Pluto and Neptune

Figure 3. Pluto has an exceptional orbit (A) that is both relatively inclined and markedly eccentric. Here (1) it is compared with the orbitof Neptune (2). At perihelion. Pluto may come within Neptune's path but the inclination of 17° (B) means that there is never a possibility of collision.

Pluto interior, hypothetical. Credit: Lunar and Planetary Institute

Figure 4. This enhanced color mosaic combines some of the sharpest views of Pluto that the New Horizons spacecraft obtained during its 14 July flyby. The pictures are part of a sequence taken near New Horizons' closest approach to Pluto, with resolutions of about 77–85 meters (250–280 feet) per pixel, revealing features smaller than half a city block.

Model of the internal structure of Pluto

Figure 5. Model of the internal structure of Pluto.

Pluto is a dwarf planet of which is also classified as a trans-Neptunian object, Kuiper belt object, and plutoid. It is smaller than the eight 'classical' planets of the Solar System and usually farther than Neptune is from the Sun. Pluto is only two-thirds as big as our own Moon and has only one-fifth of the Moon's mass.


Its size had led to astronomers to question for some time whether Pluto should be called a planet at all. The debate about Pluto's status intensified in 2005 when it was shown that Eris (formerly known as 2003 UB313), an object further out in the Kuiper Belt, might be larger than Pluto (although a more recent estimate suggests that Eris is about the same size or slightly smaller than Pluto). On 24 August 2006, at a meeting of the International Astronomical Union (IAU) in Prague, astronomers voted to downgrade Pluto's status. It is no longer considered to belong in the same category as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Instead it has been placed in a new category of "dwarf planets." It has also been given the minor planet designation of 134340 Pluto. However, this decision to demote Pluto is opposed by several hundred astronomers. In June 2008, the IAU defined a new subcategory of dwarf planets to be known as plutoids. A plutoid is any dwarf planet that orbits beyond Neptune. Only four known objects belong to this category: Pluto, Eris, Haumea, and Makemake.



Pluto was discovered by the American astronomer Clyde Tombaugh at the Lowell Observatory in 1930 and is named after the Roman god of the underworld. The name was first suggested by an English schoolgirl, Venetia Phair, and was quickly accepted by the astronomers at Lowell, who also noted that its first two letters are also the initials of Percival Lowell, the founder of Lowell Observatory.


To begin with, it was thought that Pluto might be the long-sought-for Planet X, a world beyond the orbit of Neptune that was believed to be disturbing the movements of Neptune and Uranus. But astronomers quickly realized that the mass of Pluto was much too low to have any noticeable effect on the orbits of its inner neighbors. Tombaugh had been searching in a part of the sky where his calculations indicated that Planet X might lie. However, it's now known that there are no unexplained perturbations in the orbits of Neptune and Uranus. Tombaugh's discovery of Pluto, by use of a blink comparator, turned out to be just a lucky accident.


Pluto's orbit

Pluto has a far more eccentric – stretched-out – orbit than any of the eight classical planets of the Solar System (Figures 1 and 2). Most of the time, Pluto is much farther from the Sun than is Neptune. However, for about 10 years on either side of perihelion its elongated orbit carries it closer to the Sun than Neptune. The last time this happened was at the end of the twentieth century, from 21 January 1979, to 11 February 1999 (perihelion was on 5 September 1989).


Pluto's orbit is also unusual in being highly inclined and in having a 3:2 resonance with that of Neptune, so that Pluto completes three orbits for every two of Neptune. There is no danger, though, that these two worlds will ever collide. As Pluto approaches perihelion, it also reaches its maximum distance from the ecliptic due to its 17° orbital tilt, so that it is always far above or below the plane of Neptune's orbit. In fact, Pluto and Neptune never come closer than 18 astronomical units (AU) apart.



Unlike most of the eight classical planets, but similar to Uranus, Pluto spins on its side – that is, with its poles almost in its orbital plane. This extreme axial tilt, together with the high orbital tilt, makes for pronounced seasonal climatic changes – though, of course, it is always very cold!



Pluto has very thin atmosphere with a surface pressure 100,000 times less than that on Earth. It is composed of 98% nitrogen and small amounts of methane and carbon monoxide – much like the atmosphere of Neptune's moon Triton.


Pluto's atmosphere gradually freezes and collects on the surface as the planet moves away from the Sun. Yet, interestingly, observations have shown that between 1989, when Pluto was at perihelion, and 2002, the atmospheric pressure increased threefold. The explanation probably has to do with the fact that materials take time to warm up and cool off, which is why the hottest part of the day on Earth, for example, is usually around 2 or 3 pm rather than local noon, when sunlight is the most intense.



The plains on Pluto's surface are composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. The mountainous regions, on the other hand, are made of water ice. Pluto's surface exhibits large differences in both brightness and color. Indeed Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus. The color varies from charcoal black to dark orange and white. Notable features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), Cthulhu Macula, or the "Whale" (a large dark area on the trailing hemisphere), and the "Brass Knuckles" (a series of equatorial dark areas on the leading hemisphere). Sputnik Planitia, the western lobe of the "Heart", is a 1,000 kilometer-wide basin of frozen nitrogen and carbon monoxide ices, divided into polygonal cells, which are interpreted as convection cells that carry floating blocks of water ice crust and sublimation pits toward their margins; there are obvious signs of glacial flows both into and out of the basin. No craters are visible on Pluto's surface in the images sent back by New Horizons, indicating that its surface is very young, with an age, according to recent studies of between 140,000 and 270,000 years.


Internal structure

Pluto is thought to have a differentiated internal structure, with rocky material having settled into a dense core surrounded by a mantle of water ice (Figure 5). This differentiation would have been caused by heat from the decay of radioactive elements which would have eventually heated the ices enough for the rock to separate from them. It is possible that such heating continues today, creating a subsurface ocean of liquid water 100 to 180 kilometers thick at the core-mantle boundary. Researchers at Brown University have simulated the impact thought to have formed Sputnik Planita, and showed that it might have been the result of liquid water welling up from below after the collision, implying the existence of a subsurface ocean at least 100 km deep. 


Moons of Pluto

Pluto has five known satellites. The largest of these, Charon, is so big in comparison with Pluto – more than half its diameter – that the Pluto-Charon system is often referred to as a double planet. The average separation distance is 19,640 kilometers (roughly eight Pluto diameters).


Uniquely, among planets in the Solar System, Pluto has a rotation period exactly equal to that of one of its moons (Charon). Although it is common for a satellite to travel in a synchronous orbit with its planet, Pluto is the only planet to rotate synchronously with the orbit of its satellite. Thus tidally locked, Pluto and Charon keep the same faces toward each other as they travel through space. From 1985 to 1990, Earth was aligned with the orbit of Charon around Pluto enabling daily observations of the mutual eclipses of the pair. The data collected showed that Pluto has a highly reflective south polar cap, a dimmer north polar cap, and both bright and dark features in the equatorial region. Its geometric albedo ranges from 0.49 to 0.66, compared with Charon's (darker) 0.36 to 0.39. This difference, together with Pluto's higher density (between 1.8 and 2.1 grams per centimeterscompared with Charon's 1.2 to 1.3 grams per cubic centimeter), suggests that the two bodies formed separately rather than as the result of a single collision, though this is by no means certain. Pluto's higher density indicates it is made of 50% to 75% rock mixed with ices, whereas Charon seems to be mostly ice with very little rock.


Two small outer satellites were discovered in 2005, a third in 2011, and a fourth in 2012. See Pluto, moons.


Pluto's origin

Several odd facts about Pluto, especially its highly inclined and eccentric orbit and its small size, led to early speculation that it might be a moon of Neptune that escaped. The modern view, however, is that Pluto is a displaced Kuiper Belt object.



The only spacecraft to have flown past Pluto is New Horizons, which encountered the Pluto-Charon system in 2015.


Pluto statistics
mean distance from Sun 5,910 million km (3,673 million mi, 39.4 AU)
diameter 2,370 km (1,473 mi)
diameter (Earth = 1) 0.187
mass (Earth = 1) 0.002
density 1.85 g/cm3
axial period 6.39 days
axial inclination 122.5°
orbital period 247.7 years
orbital inclination 17.2°
orbital eccentricity 0.248
number of moons 5
atmospheric composition Mostly N2, with some CO and CH4
mean temperature -230°C (-382°F)
escape velocity 1.21 km/s (4356 km/h, 2726 mph)
albedo 0.49–0.66


Pluto size comparison