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Pluto: Planet or Not?
by Devon Hamilton PhD, Senior Scientist,
Joe Wilson, Science Programmer and
Suzanne Taylor, Science Programmer
April, 2004
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In 1930, American astronomer Clyde Tombaugh made an amazing discovery:
a tiny planet smaller than our moon orbiting the sun almost 5 billion
kilometres away. It was named Pluto for the Roman God of the underworld,
and has since been considered the ninth and final planet of our Solar
System.
But over the past few years, scientists have been discovering more and
more objects like Pluto in the distant reaches of our Solar System. Some
occupy very similar orbits, and are known as "Plutinos". In
October of 2002, the biggest of these until that point was found: an
object over half Pluto's diameter that astronomers have since dubbed "Quaoar" (pronounced
kwa-whar) after a Native American god. Just last fall another, even larger,
object was found further away from Pluto, named "Sedna".
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For many scientists, these latest findings have confirmed
what they suspected all along: Pluto might not be a planet at all, but
the largest object orbiting in what astronomers call the "Kuiper
Belt", and that it's only a matter of time before we discover a Kuiper
Belt object bigger than Pluto.
Traditionally, a planet is thought to be a body orbiting a star, with
enough mass to gravitationally pull itself into a spherical shape, but
not enough mass to make its own energy and shine. This sounds straightforward
enough, doesn't it? But there are other things to consider.
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The Kuiper belt is where we get most of our comets. In
composition, Pluto is more comet-like than any of the other planets.
It consists largely of various ices (water, carbon dioxide and methane)
with a few rocks thrown into the mix. In size it is smaller than many
of the moons of our solar system, but it does have a moon of its own.
However, some asteroids are known to have satellites (or "moons") of
their own as well, and they're far too small to be spherical like a planet.
Pluto is also the runt of the litter of planets in our solar system.
It is by far the smallest, and the oddest. The inner four planets of
our solar system - including Earth - are all rocky, small worlds; the
next four planets are all swollen gas giants. All eight of these planets
follow elliptical orbits that are almost circular, and they all orbit
in roughly the same plane about the Sun. On the other hand, Pluto has
a comparatively steeply inclined and elongated orbit that actually brings
it inside Neptune's orbit.
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The orbit of Pluto is unlike any of the other planets,
including the Kuiper-belt object Quaoar. Sedna’s orbit is much,
much further away than Pluto’s however, and is also very elliptical.
Credit: NASA/Caltech
and A. Feild (STScI)
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The International Astronomical Union considered reclassifying
Pluto in 1999, and were swamped with angry e-mails. Should we revise
our system of classification based on new evidence like the discovery of
Quaoar? Or should we leave Pluto alone, and realize that it is, after all,
just a name?
You be the judge!
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Pluto: Planet or Not?
Review the evidence below and vote.
Why Pluto should be considered a planet:
- It has enough gravity to pull itself into a sphere.
- It has its own moon.
- It’s larger than Quaoar and Sedna and the other objects.
- It has an atmosphere.
- It’s been called a planet since it was discovered in 1930.
Why Pluto shouldn’t be considered a planet:
- It has a much more eccentric and tilted orbit than any of the other
planets.
- It’s a lot like the comets and other Kuiper Belt objects found
at the edge of our solar system.
- It’s smaller than four of the planetary moons in our solar
system.
- It has very little in common with Neptune and the other outer planets.
- It is most similar to Quaoar and Sedna, the latest Kuiper Belt objects
to be discovered.
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