They’re called “rogue planets”.

Also, I’m not sure why you want a distinction between a moon and a planet. “Moon” is a dynamical term, it tells you that one object is gravitationally bound to another. It doesn’t tell you anything about the mass, size, density or composition. Known moons range in size from fractions of a kilometer to over 2600 km, and the term would also apply to a body like Neptune in orbit around a more massive planet. Most people would call that a double-planet system.

I like the “dominate its orbit” approach, but that’s just my opinion – I can see the value of the original IAU definition, even if Ceres & six or eight TNOs end up being planets (again).

This brings up a topic I haven’t heard addressed yet – what would you call a spheroid object somewhere between the mass of Ceres and 10 Jupiters floating out in deep space, not orbiting anything other than perhaps the Galactic SMBH?

Clearing it’s orbit, dominating it’s orbit, that all seems kinda vague, you know?

My definition is simple and that’s it’s appeal. Yes, it would result in most moons and certain asteriods being declared planets. I say that’s an acceptable outcome.

The one problem I see is that any liquid or gaseous body, no matter how small, could be round and therefore declared a planet. Maybe this isn’t so simple after all!

Right. My problem with the IAU definition is that it is poorly drafted. Scientific definitions should be scientifically rigorous. I don’t particularly care if scientists classify Pluto as a planet, a dwarf planet or a pluton. Pluto is obviously a completely different object than Jupiter and any scientific classification should acknowledge that, just as we differentiate terrestrial planets from gas giants.

Obviously they also should have said “orbits a star” rather than “the sun”.

They said “sun” deliberately. Hundreds of planets had already been discovered by scientists in 2006, and the people in the room voting on the definition knew that.

Here was the original draft proposal:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

Notice that in this version, planets can be found around any star in the universe, but Pluto remains a planet. They added “has cleared the neighbourhood around its orbit” specifically to exclude Pluto and the other recently discovered small worlds. But that would have relegated many of the recently discovered Jupiter-sized worlds around other stars to “dwarf planet” status because they were in fairly exotic orbits.

Jupiter would have to be moved REALLY far out to not dominate its orbit.

Take a look at the dimensions of the Oort cloud. Can an object of any size in the Oort cloud be a planet under the official definition? Why not?

Earth’s orbit is affected by Jupiter, but is dominated by Earth – see quasi-satellites such as 3753 Cruithne.

Jupiter would have to be moved REALLY far out to not dominate its orbit. Neptune, six times further out than jupiter and only 1/20th its mass, completely dominates its orbit, complete with trojans in its Lagrange points, and objects such as Pluto in orbital resonance.

And yes – if the Earth were in orbit around Jupiter, it would be a moon, not a planet. That’s why “orbits a star” and “dominates its orbit” should be in the definition.

(1) A “planet” [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(http://www.iau.org/public_press/news/detail/iau0603/)

Notice that “dominating the orbit” is ChH’s version, not the official one. Notice also that the official definition pretty much ensures that there will never be more than 8 planets in the universe. Those 1235 potential planets Chris mentions above? None of them are planets according to the official definition. What if we discover an Earth-sized object in the Kuiper belt? Also not a planet, since it will almost certainly not have had time to “clear the neighborhood” around it’s orbit. Even a body the size of Jupiter would be classified a “dwarf planet” under the current definition if it’s orbit were far enough from the Sun.

In fact it’s not obvious that Earth fits the current definition. Earth’s current orbit has been shaped by it’s interaction with Jupiter. Every other object near Earth’s orbit would have been shaped by Jupiter as well. Any object whose orbit was too eccentric would have been tossed out of the system, by Jupiter. So which planet “cleared the neighborhood”?

So what is the scientific value of the current definition? It was almost certainly written to explicitly exclude Pluto. In fact it seems to have no other purpose at all.

Words mean what we want them to mean, and by golly we can include Pluto when we say “planet” if we want to. It’s not as if doing so would lead to any confusion.