Pluto Fast Facts about a Dwarf Planet in Orbit

Pluto is smaller than seven of the solar system's moons: Earth's Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton. It is two-thirds the size of Earth's Moon, but 12,000 times farther away.The tiny sizes of and great distances to Pluto and Charon make taking good images almost impossible. One of the best Hubble Space Telescope images available of the two is shown in the upper color insert on page C-6. Charon is so difficult to distinguish that it was not discovered until 1978.

Pluto's mass is not accurately known because of the difficulty of discriminating between the masses of Pluto and Charon: The two orbit around their common center of mass and act more as binary planets than as a planet and its moon. The total mass of the Pluto-Charon system is estimated at 0.0024 Earth masses, but its division between Pluto and Charon is not well constrained, and therefore the densities of the two bodies are equally poorly constrained. Cathy Olkin and her colleagues at the Lowell Observatory have made the best and most recent estimate. Based on fine measurements of the motion of Pluto and Charon taken using the Hubble Space Telescope over a number of days in 2003, the team estimates that Charon contains about 11 percent of the total mass of the system and Pluto contains the remaining 89 percent, with errors of about 1 percent. All the data given on the two bodies here, including masses, volumes, and radii, must be considered estimates, with possible errors ranging from 1 percent to as much as 10 percent.

Each planet, and some other bodies in the solar system (the Sun and certain minor planets), has been given its own symbol as a shorthand in scientific writing. The symbol for Pluto is shown on page 111.

Fundamental Information about Pluto The following table makes clear how small and poorly known Pluto is. Though it is considered a dwarf planet, far more is known about various moons than is known about Pluto. Pluto is smaller than all of Jupiter's Galilean satellites (Io, Europa, Ganymede, and Callisto) as well as Saturn's Titan, Uranus's Titania and Oberon, and Neptune's Triton. Pluto is hardly larger than four or five other Kuiper belt and possible Oort cloud objects already found.

Fundamental Facts about Pluto equatorial radius 715 miles (1,151 km), or 0.18 times Earth's radius (though esti mates for Pluto's radius vary by about 3 percent)

equatorial radius 715 miles (1,151 km), or 0.18 times Earth's radius (though esti mates for Pluto's radius vary by about 3 percent)

ellipticity

~0, but cannot be determined with present data more accu-

rately than within 10 percent error

volume

1.53 x 109 cubic miles (6.39 x 1010 km3), or 0.0059 times

Earth's volume

mass

2.84 x 1022 pounds (1.29 x 1022 kg), or 0.0022 times Earth's mass

average density

111 pounds per cubic foot (1,750 kg/m3) according to a NASA

report, but other estimates range from 102 to 131 lb/ft3 (1,600

to 2,060 kg/m3)

acceleration of gravity on

1.9 feet per second squared (0.58 m/sec2), or 0.06 times

the surface at the equator

Earth's gravity

magnetic field strength

unknown, but unlikely to have a magnetic field

at the surface

rings

zero

moons

three

Pluto's orbit is unusually eccentric, moving from 29.7 AU from the Sun at perihelion to 49.85 AU at aphelion. Pluto was most recently at perihelion during 1989. During about two decades of each orbit, Pluto's eccentric path carries it inside the orbit of Neptune, which has a perihelion distance of 29.8 AU. Pluto was most recently closer to the Sun than Neptune between 1979 and 1999, a period that naturally

Symbol for PlutĀ»

Many solar system objects have simple symbols; this is the symbol for Pluto.

encompasses Pluto's perihelion. Pluto's and Neptune's orbits are constrained in a 3:2 resonance, meaning that for every three times Neptune orbits the Sun, Pluto orbits twice. This resonance creates stable and long-lasting orbits.

PLUTO'S ORBIT

rotation on its axis ("day")

6.39 Earth days

rotation direction

retrograde (clockwise when viewed from above the North

Pole); the opposite direction to Earth's rotation

sidereal period ("year")

247.9 Earth years

orbital velocity (average)

2.95 miles per second (4.74 km/sec)

sunlight travel time (average)

five hours, 27 minutes, and 59 seconds to reach Pluto

average distance from the Sun

3,670,050,000 miles (5,906,380,000 km), or 39.482 AU

perihelion

2,756,902,000 miles (4,436,820,000 km), or 29.658 AU from

the Sun

aphelion

4,583,190,000 miles (7,375,930,000 km), or 49.305 AU from the

Sun

orbital eccentricity

0.2488, the highest in the solar system

orbital inclination to the ecliptic

17.14 degrees

obliquity (inclination of equator

122.5 degrees, the highest of any planet in the solar system,

to orbit)

but similar to Uranus's obliquity

Pluto's orbit is also highly inclined to the ecliptic plane (by 17.1 degrees). Its inclination carries Pluto from eight AU above the ecliptic plane to 13 AU below it.When Pluto is closest to the Sun and inside Neptune's orbit, it is also about eight AU above the ecliptic. Between Pluto's resonance with Neptune and its movement out of the ecliptic plane it is never closer than 17 AU to Neptune; their closest approach happens when Pluto is near aphelion. In fact, Pluto makes closer (12 AU) and more frequent approaches to Uranus. Characteristics of Pluto's orbit are listed in the table on page 110.

Pluto's orbital axis lies almost in its orbital plane, similar to but even more extreme than Uranus's obliquity. Pluto has the highest obliquity of any solar system body and as a result experiences large variations in solar heating over the course of its highly eccentric orbit. Pluto's high eccentricity, inclination, and obliquity all mark it as significantly different from the other, larger planets. From these characteristics alone Pluto might have been suspected to be a different type of solar system body, even before the Kuiper belt was discovered.

All the modern observations of Pluto have occurred when Pluto was at or very near its perihelion. Understanding of the planet is therefore based on measurements taken when the planet is at its warmest, and, in particular, atmospheric and surface characteristics may be different when the planet is at aphelion.

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