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Pluto and Charon Help (page 2)

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

Atmosphere

Pluto has a thin atmosphere consisting largely of nitrogen. However, astronomers think that this atmosphere, which is on the order of one-millionth the density of Earth’s atmosphere at the surface, exists only when Pluto is near perihelion. When the system moves farther from the Sun, the atmosphere is believed to freeze onto the surface. Because Pluto’s gravitation is weak, the atmosphere extends to considerable distances from the planet, enveloping Charon. The atmosphere might be blown into a teardrop shape by the solar wind, in much the same way as a comet’s tail is blown away from the Sun.

Although no probe has yet flown near Pluto, images of the planet have been obtained through the Hubble Space Telescope. The surface is pinkish red; this is thought to be caused by the presence of methane ice. There are bright and dark regions, with the south polar region being especially reflective. High-resolution images of Pluto and Charon will be obtained when and if a close flyby is made. Some astronomers believe that when this happens, Pluto and Charon might be reclassified as a double comet.

What Makes A Planet?

Astronomers have been searching for a large planet beyond Neptune ever since Neptune itself was discovered. Pluto is not massive enough to account for observed aberrations in the orbits of Uranus and Neptune. Perhaps such a “Planet X” does not exist, and the so-called perturbations in the orbits of Uranus and Neptune are caused by some unseen (or unseeable) object or effect. Maybe “Planet X” is a large, massive object with albedo (reflectivity) so low that we cannot see it even with the largest Earth-based telescopes.

Astronomers are almost certain that there are thousands or millions of asteroids and dormant comets in solar orbits beyond the orbit of Neptune. This Kuiper Belt is a disk-shaped swarm of primordial rocks and “dirty snowballs”; the Oort Cloud is a larger, spherical congregation of such objects that encloses the Solar System like a bubble. Every once in a while, an object from one of these swarms undergoes a gravitational interaction or collision with another object and is hurled into the main part of the Solar System. If the object passes near Neptune or Uranus, the gravitation of the large planet can send it diving toward the Sun. A few decades later, we on Earth discover a new asteroid or comet.

We might say that in order to be a planet, a celestial object must be spherical, must orbit the Sun (and not some other planet), and must be larger than a certain diameter (say, 500 kilometers) or have more than a certain amount of gravitation (say, 5 percent that of the Earth). However, no official standard yet exists. Depending on the set of criteria adopted, assuming scientists ever agree on one, Pluto-Charon may be “demoted” to the status of a double comet or else hundreds, maybe thousands, of objects now considered primordial matter will be reclassified as planets.

Practice problems of this concept can be found at: The Outer Planets Practice Problems

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