How many planets would there be if Pluto were still a planet?

And it is like that for the vast majority of “things” in life, at least in the English language. Is there a “formal scientific definition for shoe” that would be rigorous enough to exclude arbitrary crap that one might strap onto their foot that wouldn’t socially be considered a “real shoe” and that would actually be used by scientists in a meaningful sense to make fine distinctions, or is there a “legal definition of satin” that courts would recognize? (E.g. “The court rules that the aforementioned witness statement contains a factually false statement because it said that Sally was wearing a satin dress from James Brothers Dressmakers, Inc., but according to State v. Hernan, the word “satin” and “broadcloth” are mutually exclusive and the expert testimony established that James Brothers only manufactures broadcloth attire.”

There’s glory for you.

You’re getting a couple of things mixed up. The “perturbations” of Neptune’s orbit weren’t real at all, and were just plain observational error. The perturbations of Mercury’s orbit, meanwhile, were at times explained via a hypothetical closer planet called Vulcan, but were actually due to general relativity. GR effects are most significant when you’re very close to a mass, so Mercury is the only planet where they have any real significance.

On the “cleared its orbit” definition, you get a real problem. Jupiter and its Trojans can probably be considered acceptable, since they’re all pretty small, but there is a major Solar System object that does share its orbit with another planet-sized object. Do we really want a definition of “planet” that excludes Earth?

Personally, I think that we should just drop use of the word “planet” entirely, and just refer to rockballs, gasballs, and iceballs. Mercury, Venus, Earth, Luna, Mars, and Ceres are rockballs; Jupiter, Saturn, Uranus, and Neptune are gasballs; and Pluto, Eris, Quaoar, Sedna, and the major moons of the gasballs, plus a whole bunch of other objects most of which haven’t been discovered yet, are iceballs. There’s really no justification for considering Mercury and Jupiter to be the same category of objects, and very little for considering Ceres and Pluto to be the same, nor is there much justification for considering Earth, Luna, and Ceres to all be different categories.

Classic Mark Twain Humor:

Mark Twain: “How many legs does a dog have, if you call the tail a leg also?”
Second Banana: “Five?”
Mark Twain: “No, four. Just because you call a tail a leg does not make it a leg.”


As others have said, it’s all a matter of how you want to define things. It’s easy to create a definition that includes the nine traditional planets and no others: a planet is any object which orbits the sun, has a mass larger than 10[sup]22[/sup] kilograms, and whose orbit is contained within 50 AU’s of the sun. These parameters would exclude Ganymede, Titan, Triton, Titania, Eris, Ceres, Pallas, Vesta, Sedna, Haumea, Makemake, Quaoar, Chariklo, Damocles, Charon, the Moon, and all of the other questionable cases.

The definition of clearing the neighbourhood says “it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence.” It does not exclude the Earth…

We had a logically inconsistant list, not a definition that could be used to categorise newly discovered objects. There is no sense in which “the solar system has 9 planets” can be said to be correct.

Which is exactly what we got - planet and dwarf planet.

This approach is not useful when we start looking at other solar systems.

I think you have misunderstood what “clear its orbit” means. The moon and the Trojans are included in that definition as captured bodies. The moon is gravitationally bound to Earth and thus has been “cleared,” as it no longer orbits the sun independently. I don’t think there is any planet that has completely removed all other detritus from its orbit.

There have been two method proposed for measuring “clearing its orbit,” that I am aware of, and both are listed in this Wiki page. The first is Soter’s µ which measures what proportion of the mass in the orbit is taken up by the planet. Everything where µ>100 is a planet. Everything below isn’t. The second is Stern & Levison’s Λ which measures its potential for clearing an orbit. There everything where Λ>1 is a planet.

Using those it is pretty clear that Earth counts (µ=1,700,000 Λ=153,000) and Pluto doesn’t (µ=0.077 Λ=0.00295). For µ you get two broad and clearly distinct populations. First are the 8 planets with about a 2 order of magnitude range. Second are the dwarf planets with about a 1 order of magnitude range. And there is a about a 5 order of magnitude range between them. And for Λ you get the same 5 order of magnitude gulf between planets and dwarf planets.

Did you mean to say lithospheres, aerospheres, and crystallospheres?

Nicely done. I couldn’t think of a definition that would exclude Eris. but the entire orbit within 50 AUs does it quite cleanly.

I’ve never understood the logic to the answer of “four” to this question. Look at a dog’s front and back legs. They are very different. But we call them all legs because they do share similarities despite also having obvious differences. In the same vein, a tail has similarities and differences to the legs. It wouldn’t be outrageous to create a term that included the legs and tail. Appendage is close, but that would also include the head. Suppose instead of a new term we just started to replace an existing term. So now “legs” is used for all appendages below the neck and a dog would indeed have 5 legs.

I guess the point of the quote, though, is to indicate that you can’t just define words the way you want? That despite what you want to call things, it doesn’t really change what is really there? Is that the idea?


Here you go. Or you could have simply deduced that there was something that the IAU was redefining.

The list wasn’t logically inconsistent. It didn’t have any real scientific utility though. Since the definition wasn’t useful to categorize newly discovered objects, the solution would have been to stop using the term planet to cover the diverse set of object that orbit the sun and come up with a new term. And there is the sense that everybody knows what the nine planets are, and there was the sense that there was something that the IAU was redefining, which was a definition that had nine planets. Pluto could simply have been designated as a dwarf planet, a type of planet. And Jupiter as a gas planet, another type of planet, along with other designations that described greater commonality between the objects. It served no purpose to redefine terms and create more confusion if the object was to clear up confusion to start with.

A leg is an appendage used for walking.

If you include Pluto on the list of planets there is no logical way you can exclude Eris, which is 25% more massive.

Luna is an object, it orbits the Sun, it has a mass larger than 10[sup]22[/sup] kg, and it’s always within 50 AU of the Sun.

You’re probably assuming that the “orbits the Sun” bit disqualifies it, but now you have to define “orbiting the Sun”. The gravitational force of Sun on Luna is greater than the gravitational force of Earth on Luna, a situation which does not occur for any of the satellites of any other planet.

Could you single out the part that you regard as a definition? I don’t see any definition in the paragraph you linked to. In fact, since the paragraph indicates that the status of Pluto was unclear, it implies there wasn’t any definitive definition of a planet at the time.

A list is not a definition.

First of all, a list is a definition for a set of arbitraty items. That is how we arrived at the definition because we had insufficient evidence to determine what did or did not qualify as a planet, a term that was defined in antiquity, and then redefined based on a non-specific consensus definition.

I believe you are not looking for a formal definition, but a scientific definition, that was accepted and agreed upon by a consensus of scientists. I won’t claim such a thing existed, nor that it matters. The solution to eliminating ambiguity is to create new terms for more specific definitions. There’s nothing wrong with coming up with a new definition, but this is a discussion about labels, not physical properties, and there was no compelling need to reuse the old label which was so well known. You’re an ornithologist aren’t you? Why weren’t the traditional names of birds used in the taxonomy of birds? To be more specific, new labels are a better solution than using more general ones that are well known.

In other words, you were bullshitting before when you claimed there was a “formal definition of a planet” before the current one, and when you offered the link to Wikipedia as containing one. :dubious:

It’s really not worth discussing this with you if all you are doing is making stuff up and redefining terms to suit your argument.

No. Did you look at the page? It contains a chart showing the definition of the planets from 1930–2006. That’s a formal definition whether you like it or not. I’m not redefining terms, you fail to define them in the first place, and the IAU is responsible for the changed definition that this thread is about.

Here’s anotherformal definitionthat demonstrates there were nine planets before 2006. How many dictionary, encyclopedia, and textbooks definitions do I have to show you before you accept that you are wrong?

If there is a formal definition on that page, please quote it directly.

That of course is not in any way a formal scientific definition of a planet, merely a list of them.

A scientific definition would allow new objects to be classified when they are found. No such formal definition existed before 2006.

Playing linguistic games does not help your point.