Pluto
After 76 years of glory, the small ball of rock and ice known as Pluto was relegated
to the solar system backwaters in 2006 when astronomers dropped it from the
list of planets. Instead, it's simply the most famous member of the Kuiper
Belt, a broad doughnut-shaped ring of objects that extends outward from just
inside the orbit of Neptune, the most distant planet.
Because it is so far from the Sun, astronomers had a hard time measuring Pluto's
size. Astronomers finally got it right in the 1980s, after James Christy discovered
a companion object. By watching Pluto and the companion, named Charon, eclipse
each other, they measured Pluto's diameter at about 1,400 miles -- about one-third
less than the diameter of Earth's Moon.
Pluto is basically a ball of frozen
nitrogen, methane, and carbon dioxide wrapped around a small core of rock.
On average, it's farther from the Sun than any other planet, so its surface
is bitterly cold.
When it passes closest to the Sun, as it did in 1989, some
of its surface ice vaporizes, giving Pluto a thin, cold atmosphere of nitrogen
and methane. As the planet retreats from the Sun, though, the atmosphere should
freeze and fall to the surface as fresh ice.
Pluto (at lower left) and Charon look like featureless orbs even
to Hubble Space Telescope. Their orbital motion is shown in the sequence
of inset images.
The atmosphere vaporizes and refreezes
because Pluto follows a highly elliptical orbit, so its distance from the Sun
varies by almost two billion miles. That odd orbit led to a lively discussion
about Pluto's origin and even its classification. Its orbital path lies within
the Kuiper Belt, a region that encircles the realm of the planets. The belt
may contain millions of balls of rock and ice that are similar to Pluto, including
many that are almost as big -- and a few that may be even bigger.
Astronomers
have detected frozen water on the surface of Charon, which is different from
the composition of Pluto. That suggests that Charon formed from the debris
from a collision between Pluto and another large body, which may be the same
process that gave birth to Earth's moon.
Pluto's Moons
1. Charon
2. Hydra
3. Nix
Keywords
New Horizons Mission to Pluto
Pluto
Pluto's Moons
This document was last modified: September 13, 2008
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