@LouisIngenthron Haumea yes, Titan no according to current defitions, yes historically. Historically moons were called secondary planets.

@freemo Are smaller comets and asteroids planets?

@LouisIngenthron It would have to have a nearly round shape, orbit the sun. smaller objects arent usually nearly round, so usually not.

@freemo Haumea is pretty far from round too; it's an ellipsoid.

Okay, two more:

Is the sun a planet?

Is a hypothetical body the size and mass of earth, but not orbiting any star (i.e. a "rogue" body) a planet?

@LouisIngenthron I dont make the rules. Haumea is considered a planet by scientists so they must feel it is sufficiently round.

the sun doesnt orbit itself, so its not a planet.

The proper name for a "planet" without a star is a "isolated planetary-mass object" .. it is not a planet no.

@freemo Okay, I thought I was done, but that raises a new question: Is Alpha Centauri C, aka "Proxima Centauri", a planet?

@LouisIngenthron No, it doesnt orbit itself, also its a star (as sufficient mass to have fusion.

@freemo No, but the point of the question was that it *does* orbit another star (two actually).

@LouisIngenthron ORbiting a star isnt enough, there are specific criteria.

@freemo Right, but by those criteria, Pluto is a dwarf planet, which is distinct from a main-class planet.

But okay, so it sounds like your definition is:
1) Big enough for geostatic equilibrium
2) Small enough for no fusion
3) Orbiting a star

Correct?

@LouisIngenthron I never said what type of planet pluto is. Pluto is a planet. There are several types: Major, minor, dwarf, etc.. those are categories of planets

My definition? What my definition. I use the official definition of a planet. It must be in orbit around its star, not a star, large enough to be nearly-spherical. These are just the standard definition for a planet.

A "Major Planet" is a class of planets that have the added criteria of clearing their path around the star with no similar sized or larger objects within itspath. But not being a major planet is not the same as saying something isnt a planet.

@freemo Okay, that's where I take issue. The unmodified term "planet" has a very strong connotation, both in scientific and common use, of referring only to major planets, not to the larger category (to the point where Wikipedia doesn't even have a separate article on "major planet"; it just redirects you to "planet"... it does, however, have a separate article on "dwarf planets")

Personally, I think we should classify them only by size:
1) Asteroid - not rounded
2) Barren Planet- round, but not massive enough to hold atmo (pluto, mercury, the moon)
3) Terrestrial planet- has atmo, but clear surface division between gas & liquid if any (venus, earth, mars, titan)
4) Gas giant planet- massive enough that atmo smoothly transitions from gas-to-liquid with altitude
5) Dwarf stars
6) Mainline stars
7) Giant stars,
8) Neutron stars,
9) Black holes

@LouisIngenthron People do tend to assume, improperly, that planet means "major planet". But that seems silly to me.

I was fine with the old definition really.

Everything is a planet, you just have primary and secondary planets depending if its a primary or not.

@freemo Well, that depends on which "old definition" you mean. Originally, it included the sun and moon. Earth didn't even qualify for the definition for a long time, and for a while, it included at least three asteroids.

@LouisIngenthron yup, i mean the one that includes the moon as a planet, a secondary planet.