|MadSci Network: Astronomy|
Is Pluto a moon?
Nope. Pluto is quite definitely a planet by current definitions (more on this later on). It is 1,500 miles in diameter (Neptune's moon, Triton, is 1,680 miles). It is in orbit around the Sun (though it is a wild orbit), it has an atmosphere (when the planet gets close enough to the Sun for its ices to vaporize), and a single moon (Charon, 750 miles in diameter). The Pluto-Charon pair rotate around each other in 6.4 days and always present the same hemispheric face to one another (kind of like seeing the same face of the Moon, but always over the same place on Earth).
Pluto's status as a planet has been questioned now for almost the last ten years. Perhaps it was because of some of its characteristics:
Pluto's highly elliptical (oval-shaped) orbit is tilted about seventeen degrees out of the disc that the other planets' orbits describe around the Sun. Pluto's orbit also crosses Neptune's (don't worry about a collison, there's still about 240 million miles between the two at their orbital crossing), and there is a 20-year interval in which Pluto is actually closer to the Sun than Neptune (incidentally, it's on its way out to the far point of its 248-year orbit. It crossed Neptune's orbit back in February of 1999). This extremely strange orbit was more than enough for some people to think that Pluto had wandered in from outside and been captured by the Sun's gravity.
Pluto's status as a planet was first questioned in the early 1990s after astronomers began discovering large objects out past the orbit of Neptune. The first one (called 1992QB1) was a 120-mile-wide (estimated) object that was definitely in the orbital disc. As of March 1999, more than 130 Kuiper Belt Objects (KBOs) had been catalogued.
Right, what the heck is this Kuiper Belt? It's a ring of icy objects that was left over after the formation of the solar system. Theory goes that as Neptune moved away from the Sun billions of years ago, it scattered most objects in its path out to the Oort Cloud of comets that surround our solar system at very large distances. Theory also goes that Triton (one of Neptune's moons) was captured as a moon during this time. Incidentally, I have also read that the composition, size and density of Triton and Pluto are quite similar (except that Triton is larger than Pluto).
Okay, so where are we? Pluto is a planet. It may have been pulled into its orbit by Neptune when the solar system was still being formed. Triton more than likely was pulled into Neptune's orbit during this time. (And there are some who regard Triton as a once-planet before Neptune captured it.)
So what is a planet, then?
This is the critical question here. Unfortunately, there is no real definition yet, because no one really knows how a planet forms.
Some astronomers suggested that a planet be at least 1000 kilometers (602 miles) in diameter, which Pluto would pass, but not the asteroids circling between Mars and Jupiter (four of these, Vesta, Ceres, Juno, and Pallas, were once considered planets in the mid-1800s). This was discarded by planetary astronomers for being too arbitrary.
Some astronomers are trying to define an upper bound to a planet's mass as 12 times the mass of Jupiter. The reason for this is that an object with between 13 and 80 times the mass of Jupiter cannot fuse hydrogen, but could spark deuterium fusion in their cores (deuterium is a hydrogen atom with a neutron). A number of objects of this nature have been discovered in the last ten years. They are called "brown dwarves" and can be found orbiting a star or alone in space. More than 80 times the mass of Jupiter (which is about 1/12 the mass of the Sun), and you have the lightest class of true stars - red dwarves.
The traditional view of the distinction between brown dwarves and planets was that if a stellar companion with 10 Jupiter masses formed the way a star does (from a cloud of collapsing interstellar gas), it's a brown dwarf. If it formed by gradually accumulating gas and dust inside a circumstellar disk, it's a planet. Again, this goes into the bin because you have no idea how the object really formed.
There are "extrasolar planets", which are objects found to be orbiting nearby stars. (I believe there are more than 30 of these catalogued so far). There are "protoplanets", which are found in infant star systems; "rogue planets", which were tossed out of their solar systems; "planetesimals", which are planet fragments that have reformed into real planets, and "planetary bodies", like Neptune's moon Triton, that may have once been planets before being captured by a larger body. But the true definition of a planet seems not to have any real definition yet.
Myself, I like the theory that says that a planet is an object big enough for gravity to dominate its material strengths so that it tends to flow into a spherical shape. Only objects like this that orbit a star would qualify for planetary status. Unfortunately, this means that the gas planets would not get classified as "planetary bodies", since they are not in a solid state as the inner planets are.
I hope I haven't confused you too much about Pluto's status, but here's something that could be a comfort: If Pluto gets demoted as a planet, then Mercury would have to be demoted as well. Pluto has an atmosphere, Mercury does not. Pluto has an orbiting moon. Mercury is a dead, very hot (when facing the Sun) chuck of rock orbiting by itself.
Articles that may help:
http://www.lowell.edu/ users/buie/pluto/planet.html Yes, Pluto really is a planet - Article
http://hybner.pp.se/solar/eng/plu to.htm Everything you could possibly want to know about Pluto (These pages are EXCELLENT!)
Good luck to you!
Be seeing you,
Web Software Engineer, Edmark Corporation http://www.edmark.com/
Try the links in the MadSci Library for more information on Astronomy.