Pluto and Charon Help

By — McGraw-Hill Professional
Updated on Sep 16, 2011

Introduction to Pluto

Pluto , named after the god of the underworld, follows an eccentric orbit that ranges from slightly inside that of Neptune (29.7 AU) to about 50 AU at aphelion. Pluto and its moon, Charon , named after the ferry boatman who took dead souls to Pluto for judgment, receive 1/900 as much sunlight per unit area as Earth when the two are at perihelion. At aphelion, the system receives only 1/2500 as much sunlight per unit area as Earth.

Figure 7-10 illustrates the orbits of Pluto-Charon, Neptune, and Earth to scale. You might wonder if either Pluto or Charon will ever crash into Neptune. The answer is no because of a phenomenon called orbital resonance . The Pluto-Charon system makes exactly two solar orbits for every three orbits of Neptune; as a result, the two systems can never get any closer than 17 AU to each other. Unless some other celestial object intervenes and gravitationally upsets the orbit of Neptune or the orbit of Pluto-Charon, a cosmic collision will never take place.

The Outer Planets Pluto And Charon

Figure 7-10. The orbits of Earth, Neptune, and Pluto, to scale. The orbits of Pluto and Neptune overlap, but orbital resonances ensure that the two planets will never collide.

The Year And The Day

The equatorial plane of Pluto intersects its orbital plane at an angle of 58 degrees, but the rotation of Pluto is retrograde. There are pronounced seasonal changes in the path of the Sun across the sky at any particular location; the Sun always appears to rise in the west and set in the east.

Pluto takes about 6 days and 9½ Earth hours to rotate once on its axis. Charon follows a prograde orbit (in the same direction as Pluto rotates) over Pluto’s equator and completes one orbit every Plutonian day, so Charon always stays over the same spot on Pluto. An observer on Pluto would see Charon hanging almost perfectly still in the sky. In addition, Charon, like most planetary moons, keeps the same side toward Pluto constantly.

The Pluto-Charon system takes 248 Earth years to make one complete journey around the Sun. Its orbit is a pronounced ellipse. Thus the variations in this system’s distance from the Sun, as well as its extreme axial tilt, affect the seasons. The maximum-to-minimum ratio of solar irradiation is about 2.8:1.


Pluto is approximately 2400 kilometers (1500 miles) in diameter; this is smaller than the Earth’s moon. Charon is about half the diameter of Pluto. The centers of the two objects are about 20,000 kilometers (12,500 miles) apart. If the Pluto-Charon system could be brought close to Earth for size comparison, the result would look like Fig. 7-11.

The Outer Planets Pluto And Charon Composition

Figure 7-11. The Pluto-Charon system, to scale as it would appear next to Earth.

Both Pluto and Charon have about twice the density of water. This implies that they consist of a combination of ices and rocky materials. They may in fact be huge “dirty snowballs,” consisting of primordial matter that never accreted into an object large enough to properly be called a planet. Controversy has arisen here; an outspoken group of astronomers has expressed their belief that Pluto would not be called a planet if it were discovered today.

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