View Full Version : what is the difference between a moon and a planet?

07-14-03, 05:16 PM
this is a subject i know nothing about. someone please enlighten me. why is a moon not a planet?

07-14-03, 05:36 PM
A planet is a planet becuase it orbits a star.
A moon is a moon becuase it orbits a planet.

That simple. I doubt theres any depthy explanation as to why

string theorist
07-14-03, 05:42 PM
zxanth's very correct. and it doesn't get any simpler...:)

07-15-03, 12:14 AM
so they're really all big rocks? i don't think moons get enough respect.

does it strike anyone else as odd that there are these giant perfectly spherical rocks flying around in perfect orbit? i never thought of it that way before but it is mindboggling.

07-15-03, 12:39 AM
i always wonder when i jump up why i tend to come down. bummer!

James R
07-15-03, 12:41 AM
They aren't perfectly spherical, and they don't fly around in perfect circular orbits. And they aren't all rocky, either.

07-15-03, 12:48 AM
Not all moons are rocky either. Europa, one of Jupiter's moons,
is completely covered in water ice, with crisscrossing tracks or
cracks in the ice. Many scientist believe there may be a vast
liquid water ocean under the ice, in which case there is the
possibility of life in the ocean.

07-15-03, 02:08 AM
and the Moon Titan has huge seas of natural gas!

I bet Enron would LOVE to get thier hands on that Moon..

07-16-03, 01:21 AM
oh poo in know that, i'm just saying. the universe is still an amazing thing to lay people like myself.

07-16-03, 02:38 PM
It was a debate for a while to name Pluto a moon because it has many characteristics of one but ultimatly they decided to keep it a planet because has a moon. Tritan, Pluto's moon, is the coldest known place in our solar system.

string theorist
07-16-03, 03:27 PM
I thought titana was the coldest...

Fraggle Rocker
07-16-03, 04:53 PM
So then, what is the difference between a planet and an asteroid? They both orbit stars.

I'll grant the distinction between a planet and a comet. Cometary orbits are so eccentric that they're qualitatively different. But asteroids orbit in a more-or-less planetary manner. They're just small.

07-16-03, 10:35 PM
Several large moons are bigger than the smallest 2 planets. (3 planets if you include that new one)

07-16-03, 11:33 PM
Here's a really good site about this.

A Good Definition of the Word "Planet": Mission Impossible? (

07-16-03, 11:53 PM
and a natural body that orbits a moon is a moonlit?

Pluto and Charon orbit each other and some have assume them as a dual planet system with Pluto being the bigger of the to.

07-17-03, 01:07 AM
it would be called a Satelite.

Actualy, anything that orbits Anything else is a satelite.. the earth is a satelite of the sun, the sun is a satelite of the galactic core.. (the big black hole in the middle of the milkyway)

Anyone else think otherwise?

07-17-03, 08:41 AM
There is a case for the distinction in kind between moons and planets-
planets orbit stars, moons orbit planet- there is no known case of a moon having a natural satellite of its own; the distinction in this case is orbital based rather than size based.

In fact all bodies in space are on a more or less sliding scale, mass wise-
meteoroids blur into meteors blur into asteroids blur into planetoids blur into planets blur into brown dwarfs blur into red dwarfs blur into orange, yellow dwarfs, subgiants, giants, hypergiants;
each size grouping has many different compositions, but these compositions are determined by the history of the object and can be considered separately.
SF worldbuilding at

Fraggle Rocker
07-24-03, 09:53 PM
Nobody ever spoke to my question about what defines the difference between a planet and an asteroid.

07-24-03, 11:25 PM
If a planet orbits a star and a moon orbits a planet...
(No, I am not going to ask what orbits a moon)
Why do people say that Pluto is not a planet, but a moon?

Is there an actual scientific debate on this, or are these just people that are listening to crackpots?

James R
07-24-03, 11:36 PM
Fraggle Rocker:

A planet is bigger than an asteroid. That's the only difference, I think.


<i>Why do people say that Pluto is not a planet, but a moon?</i>

Who says that?

07-24-03, 11:49 PM
Originally posted by James R

<i>Why do people say that Pluto is not a planet, but a moon?</i>

Who says that?

This was the first page that popped up in my search engine (there were quite a few more).

Pluto (like Ceres) was found as the result of a deliberate search. It was the same sort of search that had turned up Neptune. This relied on the observation that an outer planet had wobbles in its orbit which seemed to be caused by an undiscovered body further out. Careful calculation of Uranus' wobbles had led directly to the location of Neptune. Now Neptune also seemed to be wobbling, and a location for Pluto was calculated. It was very approximate, and Tombaugh (under Lowell's direction) had to search a big area before he found it. We now know that Pluto is far too small to cause any wobbles in Neptune (they weren't real), and Pluto's discovery is much more analogous to that of Ceres than Neptune. Like Ceres, Pluto is surprisingly small compared to the other planets, and further turned out to be much smaller than first thought. In fact, it is barely larger than Ceres, and smaller than many of the major moons. Its orbit is also somewhat comet-like (as is its composition), since its path is neither circular nor in the ecliptic. Nonetheless, for decades it was accepted as the ninth planet.

The complaints began when, as with Ceres earlier, other objects began to turn up in very "Pluto-like" orbits. These are all part of the Kuiper Belt -- the remains of the outer disk that originally formed the Solar System. Its presence had been predicted, but because the Kuiper Belt Objects (KBOs) are small and much further away than the asteroids, there was a gap of six decades rather than two years between the discovery of the largest example and smaller ones. Now, however, we know over 100 of these KBOs, and the next larger one is more than half Pluto's size. We may well find a larger KBO in the near future. A sizeable contingent of astronomers feels that if we are not going to call Ceres a planet, we really shouldn't call Pluto one either. The only real difference between them is that Pluto had a much longer grace period before its orbital companions were found. Alternatively, we should restore planethood to Ceres. Everyone agrees that we shouldn't call all objects orbiting the Sun "planets", so we have to have a reasonable way to exclude objects that are "too small" (or too low in mass). As with Godzilla, size matters, but where should we draw the line? Is "roundness" is a good criterion?
Source (

James R
07-24-03, 11:56 PM

There is debate over whether Pluto should be called a planet or not, but I don't think anybody is arguing it should be called a moon. If anything, it is an asteroid (or a Kuiper-belt object).

07-25-03, 12:05 AM
Originally posted by James R

There is debate over whether Pluto should be called a planet or not, but I don't think anybody is arguing it should be called a moon. If anything, it is an asteroid (or a Kuiper-belt object).

Got it.

I got a half-assed quote from someone that I hadn't researched yet.


07-26-03, 11:48 AM
Pluto is probably best regarded as a Kuiper Belt Object; in terms of chemical composition and size, it is very similar to the objects residing in this belt. A moon is usually thought of to be a body in orbit around a much more massive object, which is not a star (satellites of stars of course, are called planets by default). Asteroids, comets and KBO's are not really moons since like planets, they have independent orbits around the Sun but are generally thought of as being too 'small' to be a planet (which was why Pluto seemed so enigmatic until observational evidence for the Kuiper Belt's actual existence came in).