I was watching CNN tonight and there was something about several new moons being discovered around Jupiter, bringing the total number to 47. I have tried searching the the SDMB archives and elsewhere on the internet trying to get a definition of how big an orbiting object has to be in order to be called a “moon”. From everything I gather, a moon could be as small as a pebble, as long as someone in authority defines it as a moon. If any little rink-a-dink piece of orbiting rock is technically a moon, isn’t it likely Jupiter has thousands? Millions? And what about Earth?
BTW, my Webster’s New Universal Unabridged Dictionary 2nd Edition defines moon as “any planetary satellite” which doesn’t really help. And if there is no size limitation for calling an object a moon…shouldn’t there be?
Astronomy is rife with similar size demarcation problems. When is a planet a planet and when is a planet a star? Some astronomers say Pluto isn’t really a planet, but rather a large Kuiper Belt object. Some astronomers say the Earth-Moon systrem is really a double-planet. These lines have always been somewhat grey and fuzzy, and I think the debate will continue for some time.
Thanks, Squink. I enjoyed that Jupiter link. It seems that the answer is indeed “Yes” —moon counts are completely subjective.
And to think thousands of elementary school kids are still forced to memorize the current number of official moons orbiting this or that planet as if it means something. Seems silly. Also seems silly for CNN to cover this “news”.
Not to skirt the question, but I’ve never understood why there need be such precise size demarcations.
As long as everything is labeled in a consistent way, ie, so everybody knows what particular body “Pluto” is, why worry about whether its a small moon or large asteroid?
Trigonal: Precisely. The only friction we could have is if we discover, say, a body in the Oort Cloud (that mass of rocks orbiting out beyond Neptune, which may or may not include Pluto and Charon) exactly half the size of Pluto. Is it the tenth planet to be actually named? Is it just another hunk of rock to be assigned a serial code? Different astronomical bodies could differ, causing just the inconsistencies you fear.
And I do agree that having schoolkids remember the minor moons of any body is silly. Should they know Io, Europa, and such? Sure. At least until the next test. But small moons are more or less meaningless unless someone decides to major in it or somehow take a more active interest in the subject.
Just mention that there’s a lot of crap floating around Uranus and that we aren’t sure how much of it meets our rather arbitrary standards for classification into the moon category.
The other night I heard on coast to Coast (which I’ll admit may not be a credible source) that Jupiter, all in all, is only slightly larger than the planet Earth, and almost everything we’re looking at is storms/clouds/dust/etc…
So if what we call a “planet” is really mostly a bunch of gas, I assume then the definition of a moon could really be skewed.
Is it possible that, in actual solid matter, some of Jupiters moons are actually larger than the solid matter of the planet itself?
Just standardize the system using gravity as the basis, setting 1 G as the standard. Have it so that anything that has at least .1 G be classified as a “planet”. Anything between that and, say, .03 G or .01 G (or whatever) be classified as a “moon”. Anything less than that be classified as “debris”.
Of course, I just came up with random numbers… feel free to adjust them appropriately…
FWIW, I think the biggest reason why Pluto isn’t really a planet it lots of people’s minds is that it has a highly eccentric orbit, and actually crosses the orbit of Uranus and is closer to the sun at some points - but it’s still sort of an arbitrary classification I guess.
I think the only strict requirement for calling something a ‘moon’ is that it orbits both around the sun with its planet, and around the planet itself in some fashion. But somehow I just don’t see myself getting excited when I look up in the sky and say:
"Ah! Look at that! That miniscule patch of black sky where you can’t see anything - that’s RQM-773X9Q - the pebble moon! There’s going to be some wackos out tonight?
Pluto will never be a moon, as it doesnt orbit anything(except, technically, the Pluto/Charon common barycentre
but it could be reclassified as a planetoid or even a Kuiper belt object-
A moon has to be orbiting a planet- small moons could be referred to as moonlets if necessary.
There aren’t any (known) cases of moons having stable satellites of their own, so we could have a three part classification system-
planets round stars, moons round planets-
and by size
planets>planetoids>asteroids>meteoroids
moons>moonlets>meteoroids/ ring particles
Surely there’s no need to learn the names of the moonlets.
About Jupiter: Its believe the core of the planet is the size of Earth. The rest of its size does indeed come not from solid matter, but a very dense atmosphere.
And while the size of Jupiter’s solid core may be close to that of Earth’s, it’s mass is much greater than that of Earth. When it comes to celestial bodies, mass is much more impartant than diameter.
But small moons are more or less meaningless unless someone decides to major in it or somehow take a more active interest in the subject.