Pluto is the outermost of the known planets, in an atypical orbit that is both eccentric (e = 0.25) and significantly inclined (17 degrees) to the plane defined by the orbits of the inner planets. At perihelion, it is actually closer to the Sun than the planet Neptune. It is the only planet that cannot be placed in either the terrestrial or gas‐giant classification. Overall, in size (with a diameter of 2,290 km, Pluto is the smallest of the known nine planets), mass (0.2 percent that of Earth, or about one‐sixth that of the Moon), and density (about 2 g/cm 3, indicating a composition of roughly half ice and half silicate materials), it is more like a moon of one of the outer planets or the largest and closest of a belt of comet‐like objects orbiting beyond Neptune.
Pluto has its own relatively large and close moon, Charon. Pluto and its moon therefore form another binary planetary system with a period of 6.4 days and in an orbit that is tilted 106° to their orbit about the Sun. Both rotate synchronously with their mutual orbit; hence they keep the same faces toward each other. Spectroscopy shows that methane is present as surface ice (the planet's temperature is 45 K) and as a thin atmosphere (with nitrogen N 2, the surface pressure is only 10 −6 that of Earth's atmosphere) that possibly surrounds both the planet and its moon. The planet's surface reflectivity is not uniform, but this changes over time. As the planet moves outward from the Sun, its temperature falls and the atmosphere freezes out, falling as snow on the surface of the planet and increasing its reflectivity. When it moves inward, closer to the Sun, solar heating causes the surface ice to return to its gaseous state in the atmosphere.
The unusual physical properties of Pluto are matched by its unique orbital relationship to Neptune. Pluto orbits the Sun twice during three Neptune orbits. This results in an unusual circumstance: Whenever Pluto is closest to Neptune's orbit, Neptune is always on the other side of the Sun. In fact, if Pluto were not in this orbital resonance with Neptune, it is likely that long ago it might have been gravitationally expelled from the solar system.
All these properties suggest that Pluto is not a true planet in the sense that the other planets are — namely, that they are objects that are the result of gravitational assemblage of prior small objects, or planetesimals. Rather, Pluto (and its moon Charon) are left over pieces of outer solar system planetesimals, some of which were accreted into the major planets, others of which became moons, and others that were lost to the solar system in gravitational interactions with other planets. (See Table 1 for Pluto's physical and orbital data.)
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