J002E3

[Ivan Seeking]

The motion of J002E3, showing how the object was captured into its current chaotic orbit around the Earth. The Sun is to the left in these animations. Animations created by Paul Chodas and Ron Baalke.

http://neo.jpl.nasa.gov/j002e3.html


So is this still being called a moon?

http://news.bbc.co.uk/2/hi/science/nature/2251386.stm

[selfAdjoint]

NASA seems to be content with "object". Some astronomers object to calling every satellite in the solar system a "moon". They want to reserve Moon as a proper name for THE Moon.

[Ivan Seeking]

The meaning of the word "planet" seems to be a little subjective as well. I once read that the biggest difference between a planet and a moon is that a moon has no atmosphere. Don't a number of "moons" have atmospheres; and doesn’t at least one planet, Pluto, lack any atmosphere?

[Phobos]

Astronomers still consider the Earth to have 1 moon, even though there are a couple curious nearby objects like J002E3. Note that last paragraph in the second link..."Earth's second one is called Cruithne. It was discovered in 1986 and it takes a convoluted horseshoe path around our planet as it is tossed about by the Earth's and the Moon's gravity. "

Originally posted by Ivan Seeking
The meaning of the word "planet" seems to be a little subjective as well. I once read that the biggest difference between a planet and a moon is that a moon has no atmosphere. Don't a number of "moons" have atmospheres; and doesn’t at least one planet, Pluto, lack any atmosphere?

Lots of gray areas in astronomical definitions like this.

You are correct that that "no atmosphere" is a bad definition of a moon, but maybe presented in a certain context it can get you close enough (e.g., if speaking in general to little kids or if you needed to take a guess on a game show [:)])

Saturn's moon Titan has quite an interesting atmosphere (which NASA will be sending a probe into soon...yippee!!). The planet Mercury has no real atmosphere except for the temporary particles the sun blasts off it. Pluto's* thin atmosphere disappears during its "winter" (it freezes and snows down to the surface). Asteroids don't have atmospheres but they are not moons (ok, a few are, like my namesake). Comets have gases surrouding their icy/rocky cores, but they are not planets.

In general, planets are the large, non-star objects orbiting a star. Moons are the things orbiting the planets. I would guess that Cruithne and J002E3 are not called moons because they're too odd (non standard orbits around the planet).

* keeping that particular debate of planet-hood (planet-dom?) aside

[Ivan Seeking]

Is this a situation where the definitions have been forced to evolve due to having better information about these various types of bodies?

[Nereid]

Originally posted by Ivan Seeking
Is this a situation where the definitions have been forced to evolve due to having better information about these various types of bodies? That's part of the reason; others include nostalgia (a.k.a. history - look at the fuss about whether Pluto is a planet or not), stickiness (also a.k.a. history - 'moon' vs 'satellite'), and the lack of a linguistic dictator (even though the IAU does a pretty decent job of standarisation, not every editor of every relevant peer-reviewed scientific journal is always consistent).