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New Moon of Jupiter Discovered
xihr writes "Astronomers have discovered a new moon of Jupiter, bringing its known retinue of satellites to a whopping 40. The new moon, designated S/2002 J1, is only 3 km wide, and has a highly inclined and eccentric orbit.
Astronomy.com has the story."
-- Language is a virus from outer space.
Anything orbiting a planet is a satelite, but not every satelite is considered a moon.
Paul Lenhart writes words!
Like all of the newly discovered moons, this little bit of rock is just another captured asteroid - its retrograde (backwards) orbit is a dead giveaway. Simulations show that most of the captured moons will eventually wander back to the asteroid belt - so this is the solar system equivalent of a one night stand.
BTW, the rings of Jupiter are close to the planet - this new moon is not. It's so far away you couldn't see it with the naked eye if you were so unwise as to stand on the surface of Europa or Io and look for it.
Simple (more or less) - if it orbits another body (not a star), and is not man-made, it's a moon. Size is not really a consideration - if you can detect it, it's worth cataloging.
I believe that Dactyl, the moon of the asteroid Ida, is only about 100 meters across.
The exception occurs in planetary rings, where a moon has to be bigger than the ordinary rubble of the ring to be considered a moon.
Actually, even better:
If it freely orbits a star, is IN THAT STAR'S PLANE OF THE ECLIPTIC, it's a planet (note: this knocks Pluto off the list). You still need some arbitrary limits here, though: I'd say as long as the normalized dot product of the orbit's normal vector with the plane of the ecliptic is within four sigma, you can call it a planet (note - don't take the absolute value - if it's going the wrong way, it's not a planet). Interestingly enough, this places most of the asteroids in 'planet' status.
That pretty much limits planets to objects which were clearly formed from the accretion disk of the star, as opposed to 'leftover junk' (which something like Pluto most assuredly is). Of course, this also means that a planet can get knocked out of its original orbit, and lose its 'planet' status, but this is also an acceptable side-effect if you want definitional consistency.
For 'moon', any body which orbits a planet within two sigma of ITS plane of ecliptic should be considered a 'moon'. Here we need a good definition of 'orbit' - if, at any time, an object's orbit brings it *away* from the center of mass of its solar system, and towards its planetary primary, it's in orbit around that planetary primary, and not its star. This means, incidentally, that the Earth's Moon is not a moon - it's another planet that happens to co-orbit the sun within the same boundry space as the Earth, and the two planets perturb each other's orbits.
Any body which does not fall under this criteria is a 'satellite' of its primary, but not a 'planet' or 'moon'.
While this definition leads to a few counter-intuitive situations (or at least, counter-traditional), that's inevitable when formalizing terminology. People use words too loosely to expect all possible cases to fit the 'traditional' nomelcatures, so when formalizing, you sometimes have to accept a few deviances (for example, if we were to normalize biological taxonomy, dogs, wolves, coyotes, and jackals would all be the same species - since they're genetically capable of interreproducing. It's only weight of tradition that keeps them seperate, a very unscientific concept.)
-Hentai [in vita non pacem est]
Here is a link to an intersting chart with orbital data for all moons in the solar system, including S2002 J1.
Planetary Satellite Mean Orbital Parameters
Jupiter doesn't have rings, eh? NASA might disagree with you there...