However, I personally think that this isn’t a real ‘definition’. We now say that the first criterion for naming something a planet, is that it must orbit a star. Otherwise, it is either a star itself or a rogue planet (‘planetar’). I think most people would agree with this criterion.

It would, however, demote Pluto – it isn’t just orbiting the Sun, it’s orbiting a barycenter (its and Charon’s) that is not within Pluto itself. So I’d consider Pluto-Charon a double planet if I look at this criterion only.

The second criterion is that it must have reached ‘Hydrostatic Equilibrium’ – its shape must resemble a nice roundish ball. Most objects that are considered planets, like Earth, Ceres, Pluto or Charon, have reached this balance. This makes ‘big rocks’ like comets, small asteroids, etc. ‘other objects’ – they aren’t round.

The third would be ‘clearing the neighbourhood’. I disagree with this one, per your reasoning – even Jupiter would have a hard time doing so at 200 AU.

I suggest we change the third criterion to this: The object must orbit the Sun near the Invariable Plane or the plane of the Sun’s equator. All current planets could therefore be considered planets, but Pluto or Eris couldn’t, as their inclinations are huge.
However, Ceres’s is only ~10°. Would it be a planet? I don’t know. It’s round, it orbits the Sun, but it’s tiny – its surface is only just greater than the island of Madagascar’s.

A further criterion, to differ dwarf stars (like Sirius B) from planets, is that they can only produce so much light by themselves – our current planets don’t produce light, they only reflect it. Dwarf stars do produce light, making them stars and not planets.

I suggest:
Planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Dwarf planets – Ceres, Eris, Haumea, Makemake, Sedna, Orcus and all the others that we have yet to discover
Double dwarf planets – Pluto-Charon
Minor planets – asteroids, comets, trojans, etc.

ps: I think we also need a definition of ‘moon’ (Jupiter has over 60 moons atm – 60!), so that only big chunks like Ganymede, the Moon, Titan, etc. are moons, but Phobos, Deimos, Nix, etc. are dwarf moons. But that’s beside the point.

You’re referring to the invariable plane. All planetary orbital planes wobble around the invariable plane, and the inclination of the Earth’s orbit has a 100,000-year cycle relative to the invariable plane, which varies from 0.1° to 3°. This cycle closely matches the 100,000-year pattern of ice ages.

It has been hypothesized that a disk of dust and other debris is in the invariable plane, and that this has influence on the Earth’s climate. The Earth currently traverses this plane around January 9^th and July 9^th, when there is an increase in radar-detected meteors and meteor-related noctilucent clouds.*

^*Source: Wikipedia — Milankovitch cycles.

Why? Shouldn’t it be the plane that defines the solar system’s net angular momentum? That would be (approximately) the orbit of Jupiter- with a dash of Saturn thrown in, and Mercury still fail.

KC: The LHB happened half a billion years after accretion finished. My point is that the assumption that planetary orbits must be stable is probably not valid.

@ Greg: Mercury does have a tail. It isn’t very bright, but there are plenty of pictures of it floating around the web. Pluto is an icy planet just like Triton, Ganymede, Enceladus, etc. It is big, round, and has a geologic history. Comets meet none of those criteria.

In the Kingdom of the Blind, the man with one good eye is King; however, where depth perception is required, he is handicapped.

If we make the #1 criterion for planethood be that an object orbits in the same plane as its entire planetary system, how do we classify exoplanet systems with planets orbiting in several different planes? Are none of them planets? Do we pick one and use its orbital plane, call that a planet, and the others not planets? By this criterion, we should demote Mercury, which has an orbit inclined to the plane of most of the solar system’s planets by about seven degrees.

Au contraire, mon ami. The planetary plane is determined by the Sun’s equator, not the ecliptic (which you’re thinking of), and the orbital inclination to the Sun’s equator of the 8 major planets are: Mercury — 3.38°; Venus — 3.86°; Earth — 7.25°; Mars — 5.65°; Jupiter — 6.09°; Saturn — 5.51°; Uranus — 6.48°; Neptune — 6.43°. All well within 8°.

Whereas the orbital inclination of Pluto to the Sun’s equator is 11.88° (O.K., not much, but a benchmark has to be set somewhere), and its orbital eccentricity makes it an oddball, as I have already stated above.

As for your hypothesis of extra-solar planets in eccentric orbits, I don’t think it is a coincidence that all the major 8 planets of the Solar System orbit more or less within the planetary plane — Protoplanetary disc.

If you found a perfect cube exactly one meter on each side composed of pure titanium halfway between the sun and Alpha Centauri, would you call it a cube of titanium or something else? It isn’t orbiting a star. Why does your answer change when we shift from titanium to silicon/oxygen/etc. and increase the size?

Ask any class of kindergarten kids. If we find a purple cow on Gliese 581c it’s still a purple cow.

Confusion in a different direction. If our Moon “orbits Earth” because of the Hill Sphere argument – as you point out in your first post, then why doesn’t Charon “orbit Pluto” for the same reason – especially if Pluto’s Hill Sphere is so much bigger?

Does the barycenter argument trump the Hill sphere argument?

-Alice

When I talk to school kids about the solar system (on rare occasions), I like to emphasize that the solar system looks like this:

* four terrestrial planets
* four jovian planets
* asteroids
* Kuiper belt objects (of which Pluto is one of the largest but not the largest now).

and also

* Comets and the Oort cloud
* Dust

@IVAN3Man: All the types of music you list still fall under the broad category of music. They illustrate the tremendous variety of styles that music encompasses. No one is suggesting calling the objects that don’t dominate their orbits “classical planets.” That term can be reserved for what Stern and Levison label the “uber-planets.” You said it best in the statement: “everything else can be given subcategories.” So just like all the styles you listed fall under the broad umbrella of music, all objects in hydrostatic equilibrium fall under the umbrella of planets. The “unter-planets” can be distinguished from the 8 larger ones without taking away their status as one subtype of planet.

@ Greg: Like stars, planets may come in many varieties, but they do share a fundamental characteristic, namely being in hydrostatic equilibrium; being differentiated internally with a core, mantle, and crust; and having meteorological and geological processes. The issue is not their being in hydrostatic equilibrium in and of itself but the fact that when an object gets large enough to attain this state, it starts developing the same geological processes and differentiation as planets (processes that inert asteroids do not have). Haumea may not be round, but it is large enough to have this differentiation. And any planet placed close enough to its parent star would grow a comet-like tail due to atmospheric outgassing. You are incorrect in saying this would not apply to any of the other planets. Place Earth in Mercury’s orbit, and it too would start looking and acting like a comet. Therefore, this argument is nothing more than a “red herring.” Also, Pluto is significantly larger than any known comets.

Pluto, Haumea, Makemake, and Eris can be considered to have dual status as both Kuiper Belt Objects and as planets (specifically as “unter-planets” or what we call dwarf planets).

If we make the #1 criterion for planethood be that an object orbits in the same plane as its entire planetary system, how do we classify exoplanet systems with planets orbiting in several different planes? Are none of them planets? Do we pick one and use its orbital plane, call that a planet, and the others not planets? By this criterion, we should demote Mercury, which has an orbit inclined to the plane of most of the solar system’s planets by about seven degrees.

Why do we need a precise definition of a planet? “A rose by any other name smells as sweet…” or words to that effect.

Shouldn’t we spend time LOOKING for new stuff instead of arguing about this? Don’t we have other more important things that would benefit from studied debate?

In this paper, they wrote about how the “sweeping of the orbit,” or dynamical dominance can be calculated. It clearly excluded Pluto, Ceres and their types from the list of planets… ok, ok, so he called them überplanets and unterplanets- all planets of some type, I suppose… but I think if you look at the math behind the funny words (here I mean “sweeping up its orbit” not “überplanet” and “unterplanet”) it makes sense.

So what if Neptune hasn’t swept up Pluto? The little guy is in a stable, resonant orbit thanks, at least in part, to Neptune’s superior gravity. Same with the trojan asteroids in Jupiter’s orbit; they’re there, but they’re being dominated by Jupiter’s gravity and have been swept into a couple neat little, Lagrangian piles.

To satisfy Lab Lemming – note mu applies only to mature systems that have stopped planetary accretion (a birthday cake looks a lot different when you start than when you finish, obviously there’s going to be a bunch of steps along the way before reaching the final product!)

For Example, using the IAU definition, our solar system had 8 planets 4 billion years ago. But shortly after that, resonance between Jupiter and Saturn disrupted the orbits of the outer solar system. So 100 million years later, at 3.9 million years, we only had at most two planets- Mercury, and Venus. Earth, Mars, Jupiter, Saturn, Uranus, and Neptune didn’t cease to exist, but they stopped adhering to the definition of ‘planet’. This is because gas giants tried to clear each other out of their orbits, and in the process they pushed the asteroid belt and/or the inner kupier belt inward, into Mars and Earth’s neighborhood. The big impact basins on the moon are a result of the Earth-Moon system “reclearing” their orbit at this time.

There also are many categories of music genres: Classical, Folk, Jazz, Country & Western, Rhythm & Blues, Rock & Roll, Pop, Heavy Metal, Punk Rock, Reggae, Electronic, Acid, Techno, Rap, Skiffle (kitchen sink music), etc.

As to the question, “Is Pluto a planet?” Is Rap ‘music’ Classical?

The term classic means something that has stood the test of time and has the “highest rank or excellence”; therefore, the distinction of “planet” should be reserved for the classical planets: The Rocky Planets — Mercury, Venus, Earth, Mars; The Gas Giants — Jupiter, Saturn, Uranus, Neptune.

Everything else can be be given sub-categories. That is it. Period!

While there are many objects that fit in the category of “star,” they all share fundamental characteristics – thermonuclear fusion in the core, a main sequence of some sort, existence mostly as a ball of superheated plasma. That there are many different sizes of stars with different chemical components doesn’t mean that the word “star” doesn’t have a pretty specific meaning.

Planets, on the other hand, seem to come in many different flavors. There are your rocky terrestrials, your gas giants – but the fact that they are all orbiting stars – and the fact that they are all on the same orbital plane – gives me a much more concise feeling about their definition.

If you took Pluto and put it in a much more wildly eccentric orbit that made it come significantly closer to the sun, you’d call it a comet, because it would be one. That wouldn’t be the case for any of the other objects we consider planets.

The fact that Pluto is NOT a planet makes it more interesting, not less. The idea of trying to nail down what planets are – and how they relate to the solar system at large – is a good thing.

If multiple rocky bodies are in orbit around each other, all of them that meet the above definition are planets, and all of them are also considered moons of the most massive planet.

If you meet such an object in the orbital plane of a star, outside that orbital plane, around a gas giant, or floating free in space, it wouldn’t matter. It’d be a planet.

As we find other solar systems, our definition will really need to be radically altered. Why not get rid of all the traditional ideas and just pick a template we can reliably use anywhere?

Presumably, objects in the solar system got their individual M/m values due to the processes by which they formed and ended up in their current orbits. If so, then the gap between Pluto, etc. and the planets is an indication of their different origins. Isn’t that why we’re trying to come up with a definition of “planet” in the first place: To create a physically meaningful way to categorize things that orbit a star?