Pluto

I

Introduction

Pluto, planetary body counted as the ninth planet in the solar system after its discovery in 1930. In 2006 the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet. The new IAU definition of a planet that changed Pluto’s status is not accepted by some scientists, who continue to recognize Pluto as the ninth planet. Pluto revolves about the Sun once in 247.9 Earth years at an average distance of 5,880 million km (3,650 million mi). Pluto’s orbit is so eccentric that at certain points along its path Pluto is slightly closer to the Sun than is Neptune. Pluto is about 2,360 km (1,475 mi) in diameter, about two-thirds the size of Earth’s moon. Its composition links Pluto with Kuiper Belt Objects (KBOs), icy bodies found beyond Neptune in the outer solar system. The first space mission to explore Pluto, the National Aeronautics and Space Administration’s (NASA’s) New Horizons spacecraft, was launched in January 2006 and is scheduled to fly by Pluto in 2015. Pluto was named after the god of the underworld in Roman mythology.

II

Observation from Earth

All the information astronomers have on Pluto comes from observation through large telescopes on Earth or space telescopes in orbit. Pluto was discovered as the result of a telescopic search inaugurated in 1905 by American astronomer Percival Lowell, who postulated the existence of a distant planet beyond Neptune as the cause of slight irregularities in the orbits of Uranus and Neptune. Continued after Lowell’s death by members of the Lowell Observatory staff, the search appeared to end successfully in 1930, when American astronomer Clyde William Tombaugh found Pluto near where Lowell predicted another planet. Pluto became the ninth and most distant known planet in the solar system. However, the new planet posed a puzzle—Pluto appeared to be too small to affect the orbits of Uranus and Neptune. Astronomers later detected errors in Lowell’s calculations and determined that the irregularities Lowell noted in the orbits do not exist. Clyde Tombaugh’s discovery of Pluto now is seen as a lucky accident that rewarded hard work.

A

Pluto’s Moons

For many years very little was known about Pluto, but in 1978 astronomers discovered a relatively large moon orbiting Pluto at a distance of only about 19,600 km (about 12,180 mi) and named it Charon. The orbits of Pluto and Charon caused them to pass repeatedly in front of one another as seen from Earth between 1985 and 1990, enabling astronomers to determine their sizes accurately. Charon is about 1,200 km (750 mi) in diameter, making Pluto and Charon the parent-satellite pair closest in size to one another in the solar system. Some scientists have called Pluto and Charon a double planet.

Pluto and Charon are tidally locked, meaning that they always keep the same face toward each other as they rotate. The lengths of Charon’s “day” and “month” (one orbit) are thus the same as Pluto’s “day” (6.4 Earth days). Charon’s orbit is also retrograde (clockwise viewed from Pluto’s north pole). Although Charon is thought of as a satellite of Pluto, both in fact orbit around a common center of mass (called a barycenter) that is situated in space between the two bodies.

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Astronomers using the Hubble Space Telescope (HST) detected two more small moons orbiting Pluto beyond Charon in late 2005. Follow-up observations with the HST in February 2006 confirmed the existence of the moons, which orbit Pluto in the same plane and same direction as Charon at distances of about 49,000 km (about 30,400 mi) and about 65,000 km (about 40,380 mi). The two moons later were officially named Nix and Hydra, in outward order. Additional observations with the HST in March 2006 showed that the two moons are the same color as Charon. The three moons have a neutral color, like Earth’s moon, and contrast with Pluto, which has a pinkish hue.

The findings that all three moons orbit in the same plane and have the same color lend support to the theory that Pluto’s system formed from the collision of two large bodies about 4.6 billion years ago. Pluto survived the collision, according to this theory, and the material that afterward was thrown into orbit around Pluto eventually formed Charon and the two newly discovered moons.

B

Relation to Neptune’s Orbit

Every 248 years Pluto’s elliptical orbit brings it within the orbit of Neptune. Pluto last traded places with Neptune in 1979 and crossed back outside Neptune’s orbit in 1999. No possibility of collision exists, however, because Pluto’s orbit is inclined more than 17.2° to the plane of the ecliptic (the plane in which Earth and most of the planets orbit the Sun) and is oriented such that it never actually crosses Neptune’s path. Neptune completes three orbits around the Sun in the time it takes Pluto to make two orbits. Astronomers have discovered a number of other objects that have the same 3/2 orbital relationship with Neptune but are smaller than Pluto. Dubbed plutinos, these bodies include Orcus, Ixion, Rhadamanthus, and Huya.

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