|Is Pluto a terrestrial planet or is it a gas giant?
|Question Date: 2004-01-11|
Pluto is NEITHER. Terrestrial planets
are made up
(mostly) of metal (iron) and rocks
Jovian planets are giant gas balls
not unlike the SUN although they have a small
rocky central core. Pluto is a rock ice
planet---more like Europa, a satellite of
In fact, Pluto is probably the largest of the
so-called KUIPER BELT objects . These are
mainly icy objects (like comets, in fact) that
reside beyond the orbit of Neptune and extend out
to a few hundred astronomical units---1 AU =
distance from sun to Earth. So you see Pluto
is very different as far as the other planets
go...it is more like a satellite of a gas giant
(icy plus some rock) not unlike the comets in
Depending on your definition, Pluto may or may not
be a planet. I think that the best way to think of
it is as an overgrown comet.
There is a class
of objects orbiting the sun at about eight light
hours' distance called the Kuiper belt. These
are icy bodies that typically are some hundreds of
kilometers in diameter (large asteroid-sized), but
vary. Pluto, which is just over a thousand
kilometers in diameter, is probably just the
largest of these Kuiper objects. One of the moons
of Neptune, Neried, is probably another Kuiper
object that has been captured by Neptune's gravity
(Neptune, of course, is a gas giant).
that mean that Pluto is a planet? Well, the
word "planet" was made up by humans to
larger objects in our solar system that are not
moons of something bigger yet, so Pluto is only a
planet if we say it is. Pluto is smaller than
the Earth's moon.
That's a good question. We don't really know what
the exact composition of Pluto is but we can make
an educated guess. Scientists have calculated that
the density of Pluto is about 2 grams per cubic
centimeter. Based on this, they estimate that the
Pluto is probably a mixture of 70% rock and 30%
water ice. Rock is very dense, and ice is much
less dense, so if you mix them together, you get a
planet that has a medium density. Pluto is so far
from the sun that it is very cold, which explains
Neither. The terrestrial planets are Mercury,
Venus, Earth, and Mars. The Gas Giants are
Jupiter, Saturn, Uranus, and Neptune.Pluto is
the largest known Kuiper Belt object. Most
astronomers consider it a planet, although there
was a proposal a few years ago to reclassify it to
a minor planet. This was generally rejected
although mostly for historical reasons; we all
learned that Pluto was a planet in grade school
and would be reluctant to lose a planet.
Pluto were discovered today, few would call it a
planet. As a Kuiper Belt object it would be a big
dirty snow ball. That is, it is mostly ice and
dust, just like a comet, in fact many comets start
out as Kuiper Belt objects.
solar views for
more about the Kuiper Belt.
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