Pluto

Pluto

**Pluto** is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered. It is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, a dwarf planet in the scattered disc. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance
 with Neptune prevents them from colliding. Light from the Sun takes
about 5.5 hours to reach Pluto at its average distance (39.5 AU).

Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.

## Physical Characteristics

Pluto's surface is composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide. Pluto's surface is quite varied, with large differences in both brightness and color. Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus. The color varies between charcoal black, dark orange and white. Pluto's color is more similar to that of Io with slightly more orange, significantly less red than Mars.

Notable geographical features include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), Cthulhu Regio, 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 Planum,
 the western lobe of the "Heart", is a 1000-km-wide plain 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 towards their margins; there are obvious signs of glacial flows both into and out of the plain.

## Satellites

Pluto has five known natural satellites: Charon, first identified in 1978 by astronomer James Christy; Nix and Hydra, both discovered in 2005; Kerberos, discovered in 2011; and Styx, discovered in 2012. The Plutonian system is highly compact: the five known satellites orbit within the inner 3% of the region where prograde orbits would be stable. Closest to Pluto is Charon, which is large enough to be in hydrostatic equilibrium and to cause the barycenter of the Pluto–Charon system to be outside Pluto. Beyond Charon there are four much smaller circumbinary moons, Styx, Nix, Kerberos, and Hydra.

The Pluto–Charon system is one of the few in the Solar System whose barycenter lies outside the primary body; 617 Patroclus is a smaller example, and the Sun–Jupiter system is the only larger one. The similar sizes of Charon and Pluto has prompted some astronomers to call it a double dwarf planet. The system is also unusual among planetary systems in that each is tidally locked
 to the other, which means that Pluto and Charon always have the same
hemisphere facing each other. From any position on either body, the
other is always at the same position in the sky, or always obscured.