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Twelve New Moons Found for Saturn
sebFlyte writes "Auntie is reporting that astronomers have found 12 new moons orbiting Saturn. Most of these are thought to be captured bodies, and they bring the total number of Saturn's moons up to 46, which is 17 short of Jupiter's total of 63. The new moons don't seem to have been named yet."
There are set guidelines, but not everybody uses the same ones.
One of the common things I've seen used (still not universal, though) is using orbit to define what something is, and not size, not composition, not even satellites (I've seen this from both those who consider Pluto a planet and a comet, as well, even though it definitely excludes Pluto from planethood).
For moons, it's mainly a matter of wether or not it's the dominant thing in its orbit. Ring material isn't considered moons, because no single object dominates any particular orbit, but there are also small objects which orbit in gaps in Saturn's rings that are otherwise empty. They're the primary thing in their orbit, so they're considered moons.
In the case of planets, orbital domination is used, but also orbital shape and spacing is used. All the major planets (Mercury out to Neptune) are effectively alone in their orbits. Small objects cross their orbits, others orbit them, but the planets comprise the vast majority of material in their orbit. Earth is the only exception, since a considerable portion of the mass in our orbit is also tied up in our moon, which is what is what brings the "double planet" opinion - when taken together, the rest of the material along their orbit can be statistically discounted, since it's only an invisible fraction of the Earth and Moon's combined mass. Jupiter is the only planet that actually shares an orbit with objects that don't orbit it (as opposed to simply having its orbit intersected by them), but even the trojan asteroids' movements are controlled by Jupiter. Everything else in that orbital area has been cleared out long ago.
In addition, all the planets orbit inside the approximate plane of the solar system, and have fairly circular orbits, and their average distances from the sun follow a pattern.
Ceres doesn't get planetary status in this system. It fits into the pattern of orbinearlyts, it's in the plane, and it has a circular orbit, but it's not the dominant mass in its orbit due to Jupiter's influence. It hasn't cleared the other asteroids the way other planets in the solar system did in their own orbits. Ceres may be a considerable fraction of the asteroid belt's total mass, but not to such an extent that the rest of the belt can be discounted for mass purposes.
In this definition, Pluto doesn't meet any of the requirements. It doesn't fit the orbital pattern, it has a classical trans-Neptunian cometary orbit and not a planetary one (far from circular, tilted dramatically out of the solar disk), and it's not unique in its orbit due to other large comets with simmilar orbis and even comparable size, like Sedna.
Getting caught up on size and composition clouds the issue, really. If a comet the size of earth were to come in from deep space, loop just inside the orbit of mercury, then zip back out into space not to return for thousands of years, I don't think there'd be any argument. Despite it's size, it has the orbit of a comet, so it's a comet. Pluto's orbit shares many things in common with trans-neptunian comets - it's eccentric, tilted out of the plane of the planets' orbits. It's size doesn't really matter, since many asteroids and comets in the outer solar system are much larger than those that pass through the inner solar system. They're beyond the grip of Jupiter's gravity, so many of the largest objects have survived where ones in the inner solar system were eventually sucked up by the major planets, and comets that pass through the inner solar system loose mass on every orbit, comets that don't approach the sun keep all of their material, instead increasing in size from collisions with other objects.