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

The question’s in the thread title, and the answer’s not nine. As I understand it, a large part of the reason why we stopped thinking of Pluto as a planet was that we started discovering a lot of stuff in the outer solar system that would also meet the criteria for planethood at the time. How many are there? I see lists of trans-Neptunian objects and dwarf planet candidates on Wikipedia, but I don’t have any idea how many of these would have been considered planets. What’s the deal?

Probably 10, although we can’t say for certain. The problem is that there was no set of criteria for planethood before the IAU passed their resolution. Or, to put it another way, at that time, a planet was whatever people meant when they said “planet”. Likely the consensus would be that anything as large or larger than Pluto would be a planet. That would mean Eris (current data indicates it’s the same size as Pluto, albeit somewhat more massive) would definitely be in, but other bodies would probably not be. But I’m sure there’d be people arguing that a few other bodies are also planets and there’d definitely be people arguing that Pluto and Eris are not planets.

The answer depends on who you accept as the authority on the subject.

Obviously there is a scientific utility to better categorizing the objects that orbit a star. But there are historical reasons for considering Pluto to be a planet, while excluding other celestial objects. The current definition from the IAU of eight planets was arrived at by a vote, and it is a an arbitrary assigment of a definition. The IAU did not determine the definition of a planet to begin with, and have assumed, without any scientific basis, the authority to redefine it. They could have just as easily arrived at a new term which applied only to the innermost eight planets without throwing Pluto under the bus. The answer is still nine, or not, depending on which definition you choose to accept. But up until this redefinition, there was no consensus that anything other than the traditional nine bodies were planets.

Define planet. Exactly where you set the minimum requirements will greatly change the answer. Much of the problem is there really wasn’t a set definition before Pluto was demoted other than, we know one when we see one.

At one point in time Vesta was considered a planet. After further measurement we realized that it was really quite small, and there were a lot of similar kinds of objects like it. So we started calling them asteroids, and Vesta was demoted. Much the same thing is now happening with Pluto. We are starting to find a lot of Pluto like bodies. Some are significantly smaller, but still have a similar composition and are found in the same rough part of the solar system.

So presuming we have to have a definition that keeps Pluto, but hopefully excludes most of the 200+ discovered plutinos, we would probably have a definition based on size. I would say Eris would definitely have to qualify. It appears to be almost exactly the size of Pluto. And Haumea, Makemak, Sedna, Quaoar, Orcus, Varuna, and 2007 OR10 would also likely be planets in any definition that includes Pluto. Although Orcus would be pretty small. A 1000km diameter is a likely alternative point to draw a line. So my guess is we would have around 16 planets. And probably a few dozen more waiting to be discovered.

This was another of those sort of bass-ackward ways of defining things – You start with the list of objects that you wanted to include as a planet, based on “everyone’s a-priori ideas”, and then try to come up with a consistent definition that includes just the objects you want and only those.

We had a similar thing in the marine mammal preservation crowd – First there was the Marine Mammal Protection Act, which gave blanket protection to them all without giving a species-by-species list, then everyone got to arguing whether that included Polar Bears. Everyone who had an opinion tried to propose a specific definition for what a “marine mammal” is that would either (according to each individual’s point of view) include or exclude Polar Bears. That’s the same sort of thing that’s happened here with the planets, I think.

That’s sort of inherent in the history of the term. “Planet” comes from the Greek for “wanderer”, as in “wandering star”. To the ancient astronomers, planets were the objects in the sky that looked like stars, but moved around in the sky, as opposed to the stars which were fixed. It took about 2000 years before the Renaissance astronomers came to realize that the planets were objects not unlike the Earth itself, and a couple hundred years after that to discover that there were more objects like the planets that weren’t visibile to the naked eye.

So, in the classical sense, the term “planet” describes only those moving objects that were visible to the naked eye 2500 years ago - Mercury, Venus, Mars, Jupiter, and Saturn. The modern definition is a retroactive one designed to describe what those objects are, as opposed to what the ancients believed them to be.

The answer is nine. There were no criteria for planethood at the time other than “traditionally considered a planet”. How many there would be if we create or accept a definition that includes Pluto, but not anything “lesser”, is a more difficult question.

It’s a problem of taxonomy. There is no precise line between planet, dwarf planet and minor planet. To answer the OP’s question, you first have to decide exactly what a planet is. If you choose “gravitationally self-rounded” as a sensible cut off point, which is one of the dwarf planet criteria and much less arbitary than “Pluto sized or larger”, the answer could be several hundred, with the vast majority beyond the orbit of Neptune. A precise answer can’t be given. The outer solar system hasn’t been comprehensively surveyed, due to the dimness of the objects there and their low apparent motion. The point at which an icy body relaxes into hydrostatic equilibrium isn’t known, but is estimated to be between 200km and 400km diameter.

The real wild-card is Sedna, with it’s highly elliptical orbit. It was only detected because it currently happens to be relatively near the sun, it would not be possible to detect for most of it’s roughly 11,000 year orbit. It’s not currently known if there are a lot of Sednas out there, or if it’s a freak object.

The IAU’s definitions of planet and dwarf planet are pretty sensible. It makes sense to distinguish the eight major planets from a large number of relatively small ice-balls. A key part of the distinction is whether the object is large enough to clear it’s orbit of other bodies (please don’t be ultra-literal about this, it’s possible for small bodies to orbit stably in one of the lagrangian points of a planet, but not larger objects). This ties nicely in with the history of the solar system. The eight planets went through an additional stage of formation compared to the dwarf planets, they grew large enough to gravitationally disrupt nearby bodies and either eat or eject them. It’s by no means a perfect definition, but it is a useful one.

If tradition were the only criterion, the answer would be 7 and Pluto would never have been added to the list of planets. Nor would Uranus and Neptune. Or the Earth. The Sun and Moon would still be planets, though.

The number of planets has changed over the centuries, so there’s no reason why it couldn’t change again with the discovery of new objects. The count has been as low as 6 and as high as 18 or so. Now it’s 8 and is not likely to change again. Not impossible, though.

Huh? I answered that in the second post of the thread.

Yes of course. Traditions can’t possibly be from the 20th century.

Pardon me for finding “but other bodies would probably not be.” to be less than definite.

Remember, we only discovered Neptune because something was perturbing the orbit of Uranus. We only discovered Pluto because something was perturbing the orbit of Neptune. Except Pluto turned out not to be massive enough to be the cause, it was, IIRC (and I may not) inaccuracy of measurement caused by following Newton’s laws, and not Einstein’s.

So, the outer planets were only searched for and found based on their effects on the orbit of closer ones. Pluto and others are too small and/or distant to fit that particular definition of “planet”. 'Tho we thought Pluto did fit that definition for a time.

There was some discussion years ago when Pluto was demoted giving other definitions, like a planet should sweep it’s orbit free of debris, or some such. Feh, Jupiter doesn’t even sweep it’s orbit free of everything. [EDIT] Alka Seltzer: already said this.

But as it was said, we don’t have a rigid definition of planet.

Using the wondering stars definition, perhaps to update it it should now be the minimum size’s body that we could detect around another star. Not sure Earth would yet qualify.

That’s utterly silly. It depends not only on the size of the planet, but on the size of the star, the distance between star and planet, the orientation relative to us of the plane of the planet’s orbit and the distance to the star. Not to mention disqualifying planets in our solar system than absolutely no one thinks should be excluded from the ranks of planets.

Somewhat off topic, but I really do not understand all the fuss about Pluto.

The concept of planet needed to be redefined. The number of extra solar planet candidates is in the hundreds. We are finding more and more trans-neptunian objects. To consider Pluto a planet and somehow not Eris or other similar objects makes no sense. Those that cite tradition put an awful lot of importance on the 75 years between the discovery of Pluto and Eris.

It is a silly thing to get all fundamentalist about.

  1. (See Lincoln on how many legs a dog has if you call its tail a leg.)

Why? Do we need to redefine how many days are in a week? Is Rudolph really one of Santa’s reindeer? If there is some signficant commonality to the innermost 8 planets why wouldn’t we create a new term to define them instead of redefining an existing term to suit that rather arbitrary classification. Why would we categorize Jupiter and Mercury as being similar while excluding Pluto?

This thread needs to move to GD so I can unleash my rage. The answer to the OP is simple and factual. There are nine planets by the 20th century definition, which is the arbitrarily designated set of bodies orbiting the Sun which we were able to identify. There was no need to redefine the term planet, and the new definition does nothing to advance science. We are talking about terminology here, not physical principles.

We lump balls of rock like Earth and Mars together with huge balls of gas like Jupiter. The word planet can refer to anything we want it to. There is no reason to think that keeping Pluto as a planet would harm anything. There is no reason why you wouldn;t be able to refer to the 9 as the “Classical Planes”, and any other Pluto sized objects as dwarf planets.

Well then why don’t we go back to referring to Ceres as a planet? It is more of a “classical” planet than Pluto. It was classified as a planet from around its discovery in 1801 till around 1850. There is no particularly good reason to refer to Pluto as a planet other than the affection of folks like yourself.

You mention lumping rocky bodies with gaseous bodies. And yes that is pretty arbitrary. But by doing so we have a definition that includes all the major independently orbiting bodies. Or we do until we hit the mess that exists outside Neptune’s orbit. But adding a single icy-rocky body to the collection while excluding all the other very similar icy-rocky bodies just because we saw it first, takes that arbitrariness up to 11. It certainly doesn’t improve our definitional problem in any significant way, and it eliminates what small value there is in defining planets in any way. In my opinion, either we should add all the Plutinos or we should remove all the Plutinos. So either we have 8 planets or we have 200+ planets. There is no useful way to keep Pluto and exclude Eris, Sedna and the rest.

Sure there was, a formal definition of planet was required for the first time due to the discovery of new objects in the outer solar system. It would have been ridiculous to bumble along without making a decision on the status of Eris and other TNOs.

Which is why we also use the terms “gas giant” and “terrestrial planets”, depending on the context.

Some usages are more meaningful than others.

“Classical planets” is already used to describe the planets visible to the naked eye and known to ancient peoples. The IAU’s definition makes a lot of sense in the context of the formation and evolution of the solar system. The eight planets are the dominant objects in their orbits, while the dwarf planets are not. In those terms, the difference is very stark. The new definition has descriptive and educational value, because it makes it easier to talk and write about the solar system. It’s less clumsy to talk about planets and dwarf planets than to say something like “the solar system contains 2,000 planets, including 8 major ones”, or to say “there are 9 planets, but Pluto is only considered a planet for historical reasons”.

At the end of the day, it’s taxonomy, and it’s really not worth getting upset over. Pluto hasn’t changed, it’s still the same as it always was. Often it’s not possible to find a perfect definition, when nature presents a continuum rather than distinct classes. Personally, I prefer the new definition, as it is based on measurable parameters rather than history (read the clearing the neighbourhood article I linked to above). In that sense, Mercury is more like Jupiter than Pluto, even though it is closer to in mass to Pluto in both relative and absolute terms.

We had a formal definition a planet. It was based on the historical/traditional arbitrary designation of certain orbiting objects as planets. What we needed was new terms with more precise definitions. One of those new terms could have satisfied the new IAU definition. I seriously doubt we’ve run out of new words.