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Is Pluto a terrestrial planet or is it a gas giant?
Question Date: 2004-01-11
Answer 1:

Pluto is NEITHER. Terrestrial planets are made up (mostly) of metal (iron) and rocks (silicates).

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 Jupiter.

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 composition!

Answer 2:

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).

Does that mean that Pluto is a planet? Well, the word "planet" was made up by humans to describe the 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.

Answer 3:

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 the ice.

Answer 4:

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.

If 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.

See solar views for more about the Kuiper Belt.

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