Depending on what you consider “local population” you could end up with only one planet in the solar system.

Jupiter has a mass over three times that of Saturn the next largest object in the solar system, and over 317 times that of Earth, the largest of the “terrestial planets.”

I think we will find that the general public will not accept a definition that does not include Pluto. Clearly there is no total agreement among astronomers - only if 95% or so agreed that Pluto did not fit the definition of a planet would they be able to convince the public. As divided as they are, they may as well accept that Pluto will be considered a planet and accept a definition that includes it.

But once the number of planets gets too high, perhaps a little beyond the teens, the media will no doubt count them as follows:

* 9 “regular” planets (including Ceres) and

* a dozen or more “plutons,” including Pluto.

Let me throw in my two bits at coming up with a definition of a planet. This definition is partly a compilation of ideas already expressed by others on the internet. I have taken these ideas and applied Occam’s-Razor, together with some of my own ideas, to come up with what I believe a more simple and concise definition.

• Definition of a planet
Rule 1: A planet must not be so massive that it can support any fusion reactions, even briefly.
Rule 2: A planet must not contain degenerate matter, such as a core of solid neutronium that might be left over from the death of a star.
Rule 3: A planet must have sufficient mass to have a shape determined by gravity and not by the strength of its materials. This means it is spheroid, i.e., nearly spherical.
Note: These rules allow certain objects in our Solar System such as moons, Ceres and some Kuiper Belt objects to also be classified as planets. How the object formed, or whether or not it is in orbit around a star is not part of this definition of a planet. However, it is generally accepted that most planets do form by accretion in the disc of gas, dust and rocks left behind by a proto star and therefore do orbit stars or Brown Dwarfs. Is seems quite reasonable that some planets might be ejected from their birth stellar systems and become wandering planets in interstellar space. Such objects would not by such action somehow magically metamorphose and cease to be planets.

• Roles of planets (with special meanings applied to certain terms) It would seem useful to distinguish between what an object IS and what ROLE it plays in a system.
o Major Planet — a planet that is in orbit around a star.
o Minor Planet — i.e., a natural satellite, or moon — a planet which is in orbit around another planet. This definition allows moons in turn to, themselves, have moons.
o Wandering Planet — A wandering planet would be one which is not in orbit around any other object, except perhaps, though not necessarily, around a Galaxy.
o These roles could also be applied to planetesimals. (see below)

• Definition of a sub-planet, i.e., a planetesimal (again exapting a word for this definition)
o Planetesimals would be any objects in space too small to fit the definition of a planet, e.g., asteroids, comets and other space debris larger than gas and dust. These may exist in orbits around a galaxy, a star, a brown dwarf, a planet, or other planetoid.

• Sub-classes of planets
Just as we have sub-classes of stars based on certain criteria such as size and luminosity, so also can we have sub-classes of planets. Planets might be sub-classed as: Giant, Dwarf, Gas, Rocky, Icy and so on. Sub-classifying planets should not make them non-planets.

Wade Starks

The definition is simple and has no arbitrary numbers. In fact, it has no numbers at all. This is good because it means the definition is based on basic principles.

I like original proposal more than the counter-proposal: “A planet is a celestial body that (a) is by far the largest object in its local population, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) does not produce energy by any nuclear fusion mechanism.”

The population-based counter-proposal is not as simple as the size-based original proposal. It will require various somewhat arbitrary numbers to define “by far the largest” and “local population”. The counter-proposal is not easy to apply to other planetary systems. We are are likely to know the mass (and therefore the roundness) of extra-solar planets, but we are unlikely to know much about the “local populations” of extra-solar planets. Another problem is that the counter-proposal would not, without revision, categorize an extra-solar double-Jupiter as a planet.

The fact that the original proposal opens the doors to a lot of dwarf planets is not a problem. Just because there are a lot more planets, that does not mean we will require kids to memorize all their names. They will simply need to learn the eight main planets (the four rocky mid-size planets and the four gas giant planets) and know that there are also a lot of dwarf planets that do not need to be memorized.

The counter-proposal is a reasonable proposal. I have no problem with demoting Pluto and saying there are only eight planets, if that is what the majority of astronomers want to do. But I think the original proposal is better: it is simpler, has fewer arbitrary numbers, and is easier to apply to other solar systems.

Plutoid is a term for the circular file; it, too, is Sol-centric.

I’m not too keen on the idea that Pluto’s moon can become a planet. A Planet’s primary orbit should be around the central star. In a case of near-identical sized objects orbiting each other, that could be termed a planetary pair; a key concept here is orbiting each other so each has a sine-wave orbital path– i.e. Athena major and Athena minor. If it orbits a planet without significant impact on the planet’s eliptical orbital, IT’S A MOON. Of course, then you have to define “significant impact”, which could be twice the planet’s diameter.

Here’s a simple view:

Planets should be a product of the star’s system formation (with other caveats).
1) Primary planets should be within a narrow angle of the sun’s plane of rotation, i.e. 15 degrees.
2) Secondary planets orbit in a plane tilted larger than the primary/principle ( these objects would appear to have been captured subsequent to the system formation or as a result of a massive collision between a primary object and an extra-system mass).
3) Those objects with sufficeint mass to form a ball, but fall outside the definition of a planet should be called planetoids– NOT PLANETS, NOT DWARF PLANETS. The term dwarf planet to me sounds like “almost pregnant”– to wishy-washy

I’m sure there are situations in the Galaxy that would play havoc with this simplistic approach. One system’s planet would be another’s plantiod, but that’s to-well into our future to debate now. Just make a definition that’s (largely) applicable, regardless of star system referenced.

Just a simpleton’s opinion…

With that said I think the new planet classification system is great. The rules seem simple enough to me (as a layman) and it astounds me when people say it may be confusing for the general public. I think some people are making this new system more confusing than it is. The rules are straightforward and anyone with a passing love for space/science will be able to understand them easily. The public who doesn’t ‘get it’ really has no interest to begin with and shouldn’t be factored into the equation.

Even though there is no formal definition of a planet I feel that if you were to take a random sampling of people off the street, sit them down in a room and show them illustrations of the following (non-labeled) celestial objects on flashcards, then ask them to state yes or no if what was on the flashcard was a planet or not, it would probably (9 out of 10 times) go like this:

Vesta – “No”
Sedna – “Yes”
Jupiter – “Yes”
The Sun – “No”
Eros – “No”
Halley’s Comet – “No”
Europa (with Jupiter in background) – “No”
Earth – “Yes”
Ida – “No”
The Voyager Spacecraft – “No”
Pluto – “Yes”
The Moon (so long as it was a familiar photograph) – “No”
Europa (with NO Jupiter in background) – “Yes”
A perfectly round cantaloupe floating in space – “Yes”

The point to all this is the definition of a planet is imbedded into our psyche. A giant sphere floating in space (unless it is in flames or understood as a moon) is a planet. Add to this the fact that nature determines if it becomes a sphere or not just makes this whole debate a no-brainer to me.

In addition I feel some people think that upon christening these new (minor) planets the major-planets will somehow be demystified. I mean come on people, how can you look at Jupiter or Saturn and say if you throw in Ceres it will make the former two loose their magic? Quite the contrary I think it will further illustrate the magnificence of them. When it comes down to it all what does it matter to the scientist? Ceres is Ceres, Jupiter is Jupiter and everything is what it is, what point is there in labeling it all?

Well I will tell you what the point is… excitement for us normal people. I see my two Nephews (ages 9 and 12) now excited over the prospect of 3 new planets. It has sparked INTREST in them and they want to know more about the solar system and have been going online looking at ALL the planets as a result of this. Even myself have been online the past couple days reading all about Ceres, Charon and ‘Xena.’ I have to admit before all of this I knew very little about Ceres… Now I am fascinated by it.

This system, though helpful for the professional scientific community (which in reality, will view things the same no matter however the vote goes), should really also be considered for us ‘normal’ people of the world. I truly feel that if passed this new definition (as well as announcements of future ‘new’ planets) will spark a new founded public interest in our own solar system. Instead of a static system where 8 (or 9) planets are the ‘end-all-be all’, we will have dynamic fluid solar system where new things are still possible. That is exciting… and anything that makes kids and adults excited and interested in science cannot be a bad thing.