Funny how those cute pictures of the object pop up – when all we really can see right now with the best telescopes is a dot. Oh well…

In defining a planet, it is important to make a distinction that will be durable, apply to all stellar systems, and be non-anthropocentric.  

Definitions that say an object "must not be part of a swarm" are silly, as we would never know when planets might be demoted by the discovery of nearby kin.

Statements that an object cannot be a planet because it is part of the Kuiper belt or Oort cloud are similarly logically challenged.  A second flaw to such arguments is that they require "true" planets to be similar to earth or the earlier-discovered planets in arbitrary and ill-defined ways.

Calling a planet "any object, orbiting a star, which is at least as large as Pluto" arbitrarily sets Pluto as a standard.  What is so universal about its size?  If it had been 100 miles less in diameter, we would still have called it a planet, and THIS would be the standard of the aforementioned statement.

A planet cannot be defined by its orbital plane – we must allow things to be planets which orbit outside of the plane of the ecliptic.  Why?  Because we should not be able to demote an object from planetary status purely because its orbit changes.

What about "bigness?"  We have to distinguish between planets and stars.  This is easy.  A planet becomes a "star" as soon as it is big enough to cause some sort of fusion reaction by sole virtue of its size.  Whether it has already completed the fusion of whatever material it is capable of fusing is irrelevant.  We cannot allow stars to "become" planets based on their age alone.

The best definition is, therefore, that an object is a planet if it is at least large enough to form a sphere based on its own gravity, and if it is too small to ignite a fusion reaction of deuterium, again based on its own gravity.  Yes, this makes Ceres a planet too, as well as Quaoar, Varuna, and Sedna, and dozens to hundreds of other objects – so be it!  The more the merrier!

- Promethium

or do we arbitrarily eliminate moons from the definition? :-) After all, the Moon orbits the sun just as the Earth does, though it just happens to look like it goes around Earth from our perspective.

I’ve also wondered what exactly counts as spherical – after all none of these is exactly a sphere, due to rotation and mountains and valleys. Is there a mathematical definition that makes sense here – largest mountain peak/valley depth greater than some fraction of nominal object radius? But then you’d need a bit of a detailed survey to determine status.

And is the sphere for this criterion including any oceans, or only solid surface? What about objects without a visible solid surface?

Still, the spherical criterion does seem the most objective I’ve heard so far…

That Spitzer isn’t as good as Hubble. The public is getting brainwashed by NASA with a steady stream of colorized Spitzer photos that sure are cute – but not Hubble quality. The public won’t be able to distinguish the difference – and fight for saving the Hubble as the priceless unique instrument it truly is – unless they are told.

Obviously, a moon orbits a planet. You are just muddying the waters here. Also, you seem to think there is an issue of sphericity. You apparently don’t know that objects with a diameter of about 700 KM or more are gravitationally pulled into a spherical shape. How round is round is not even an issue, this is one of the easiest questions to answer.

Ceres is a sphere, but all the other asteroids are potato-shaped or irregular.

I think Sedna should be a planet. However, for those who don’t like that idea, I propose a new word: PLANETOID. A planetoid is a planet (by definition) that some think is too small to be called a planet. That could include anything smaller than Pluto, for those who don’t like a logical definition, an arbitrary one is what you deserve.

a zero-gravity chunk of water self-gravitates into a sphere at much less than 700 km… The criterion in question here obviously depends on the characteristics of the materials that make up the planetary body, and its temperature. Is that what we want?

You seemed to be trying for an objective criterion, and I was just indicating that even this apparently objective measure is (slightly) arguable.

For example, Eros turned out to consist of two roughly spherical pieces loosely joined. Given that Eros is only 33 km in longest diameter and 13 km in smallest, as a physicist I know I can represent that as a sphere with some rather large (60% amplitude) harmonics. Obviously one would just that to NOT be a spherical object.

But you can get other asteroid numbers from this fact sheet, which indicates even Ceres has major and minor diameter numbers that differ by about 3% (932 km vs. 960 km). Pallas varies from a sphere by over 15% (570 x 525 x 482 km). Where’s our boundary – 10%? Or is even 3% variation from a sphere too much?

And what’s this exclusive attitude when it comes to moons, anyway?

This Galileo photo gives some justification to calling the Moon a planet in its own right. Pluto’s Charon is considerably closer to its primary in size (though both Pluto and Charon are smaller than our Moon).

More thoughts on this issue from Cornell:

Pluto and Charon (1/10 of Pluto’s mass) are definitely a double planet and most people would consider Earth and the Moon (1/81 of Earth’s mass) a double planet.

It is possible that once some Earth-sized extrasolar planets with moons are discovered, the term “double planet” will be defined more strictly, but so far it is completely unofficial.

However, what I actually meant when I said the Moon orbits the Sun, not (in the obvious sense) the Earth, is embodied in this diagram of the Moon’s motion. The Moon never exhibits “retrograde” motion – and in fact, as pointed out there, the Sun’s pull on the Moon is actually twice that of Earth.

So does the Moon really orbit the Earth? Or do they just co-orbit the Sun? Depends on your perspective :-)

Thanks apsmith! I had heard about the Moon never actually being pulled away from the Sun by the Earth, but had never seen such a good discussion of it. I’m living on a double planet now! (er, living on half of a double planet?)