What a planet is, revisited. Also Pluto.

IMHO a planet is a large round object that orbits a star and is not itself a star. The Earth is a planet. Jupiter is planet. The Earth resembles Pluto more than it resembles Jupiter. Therefore Pluto is a planet. Unless the Earth isn’t. Which it is. I suppose the rocky planets could be considered borderline cases. But that’s hard to imagine, if we consider Jupiter, Saturn et al to be true planets.

Q: What about comets, are they planets? They orbit stars. They are not stars.
A: Do they have a tail?
Q: Yes.
A: Then they are not round. Planets are round. Comets are not planets.
Q: “Large”. I don’t know what “Large” means. Say there’s a perfect spheroid the size of a marble. It orbits the sun. Is that a planet? It’s large relative to paramecium or most molecules.
A: Fair enough. Here’s the full definition: A planet is a near-spherical object orbiting a star which has undergone planetary differentiation and is not a star or a brown dwarf. Planetary differentiation occurs when the spherical object separates out into different layers: the Earth has a mantle, a crust and a metallic core for example. So piles of rubble typically won’t qualify. Nor would spheroid marbles.
Q: Is the death star a planet?
A: Does if have a mantle, crust and core? No. It’s not a planet. It’s a death star, very much unlike the ordinary kind.
Q: Is Ceres a planet?
A: Is it round? Does it have layers? Yes and yes. It’s a planet.
Q: Is 4 Vesta a planet?
A: It has layers: it has a core, mantle and crust. But it is not spherical: it looks too much like a potato. It’s an asteroid. Not a planet.
Q: What about Charon?
A: Charon orbits Pluto, which is a planet, not a star. Charon is therefore a moon or satellite. Charon also hasn’t undergone planetary differentiation. Charon is not a planet.
Q: But Pluto hasn’t cleared it’s neighborhood!
A: Not my problem. If the Earth was located where Pluto was, it wouldn’t clear it’s neighborhood either. No points for good housekeeping.
Q: What about Eris? Is that a planet?
A: It’s thought to be spheroid. It’s thought to have a mantle and core and might have an internal ocean, so it has layers. It’s a planet.
Q: But that means there might be a lot of planets orbiting Sol!
A: And this is a problem why?
Q: Is Jupiter a brown dwarf?
A: I don’t know. I accept it’s general designation as a planet. I don’t know what the line between brown dwarf and big honking gas giant is.

Older threads:
2005: What is a planet? http://boards.straightdope.com/sdmb/showthread.php?t=331463
2011: How many planets would there be if Pluto were still a planet? http://boards.straightdope.com/sdmb/showthread.php?t=628897
2008: Revived question: Is Pluto a planet?

Pluto and Charon orbit a common barycenter that lie outside the surface of either body. While this doesn’t make or break Pluto as a planet, the system should be considered a doublet which is orbited by another four moons. Pluto is also significant out of the plane of the ecliptic and at a high eccentricity which marks it as a Kuiper Belt object (KBO). Whether we regard some KBOs as planets or not is a matter of semantics, but it is clearly in a separate category than either the inner rocky worlds (Mercury, Venus, Earth, Mars) or the outer hot or ‘icy’ gas giants (Jupiter, Saturn; Uranus, Neptune). As we get more information on extrasolar planets we’ll no doubt have to further refine the categories, as it is already apparent that the distribution of planets in our solar system is neither generically representative nor unique.

It probably makes sense in terms of categorization to designate some kind of threshold of mass or equivalent surface gravity such that it has a measurable impact upon other bodies as qualifying as a planet, and Pluto is likely not to make the cut (at least, not without admitting several other small bodies and transitory moons). This doesn’t reduce the importance of Pluto in astronomy either as a milestone of discovery (by Clyde Tombaugh), nor as an useful data point in understanding of the evolution of our solar system, but it most likely should never have been regarded as a planet.

Regardless, by the time it is necessary to make a practical distinction, e.g. whether central governance based on ‘planets’ are necessary to establish some kind of autonomy, we’ll likely be able to live in artificial solar orbiting habitats indefinitely without any need for planets as resources, and thus planets will become about as necessary as keeps and tenshu are today. So the debate of whether Pluto or any other body qualifies as a planet is pointless semantics.


From your Eris link:

I’m just telling it like it is.

I notice the first word of your OP is IMHO. Do you want to lobby to get the definition of ‘planet’ changed? Or is there some way to establish that objects are planets regardless of the decrees of the IAU?

Not really. I accept their definition, though I don’t think it’s a good one. (Frankly, it isn’t a bad one either: I can live with “Dwarf planet”.) I suspect the issue is still in play on some level. In the end this is a semantic issue that doesn’t matter too much.

I was wondering whether planetary differentiation and/or “Has an atmosphere of x bars, or at least it would at 1 AU” might be a decent criteria for size. Pluto has less mass than the moon, so it might fail the atmospheric criteria even though both objects actually have a very thin atmosphere. Assuming I’m reading wiki correctly. But I sort of like the planetary differentiation criteria, as it jives roughly with my conception of objects that are “Like Earth and the gas giants”. Then again, we might discover an extrasolar supersized homogenous non-star, which could prove awkward.

That Pluto doesn’t orbit along the Earth’s plane is interesting, but while I accept it’s a Kuiper Belt object, that doesn’t imply to me that it’s not a planet.

In the same theme, what exactly is a “moon”?

If its just any natural object that orbits a planet then earth has at different times, 2-3 moons. But then that also depends upon what exactly is an “orbit”.

So does a moon have to have a particular size, shape, and orbit?

This astronomy major thinks the “planet” and “dwarf planet” categorizations are fine. There’s a reasonable case to be made for more categories – such as “gas giant” (Jupiter through Neptune), “rocky planet” (Mercury through Mars), and “dwarf planet” (Pluto, Eris, Ceres, probably many other very distant objects), or even “gas supergiant” for Jupiter and Saturn, “gas giant” for Uranus and Neptune, and the same as above for the rest.

(I admit up front to a lack of astronomical knowledge, but here’s what I’m wondering…)

So, how common are spheroids? Prior to New Horizons, based on earlier and less detailed pictures, I thought Pluto was an odd-shaped lump.

I assume spheroids are the result of gravitational accumulation of matter, aided by the occasional collision.

As such these would be different than ‘just lumps of rock’ as asteroids seem to be.

Doesn’t this make spheroids deserving of some special classification?

Is there any reason why a spheroid isn’t either a planet or a moon of a planet?

I personally prefer the term ‘ice giant,’ but that term doesn’t seem to have a particularly big following, although you do see it occasionally.

I don’t like it because, presumably, there are Uranus-sized planets out there close enough to their suns to not be particularly “icy”. I think size is a better distinguisher for these largest planets, if they must be grouped separately (though I think “rocky with small/no atmosphere” vice “mostly gas/ice/volatiles with a rocky core” is probably enough).

Personally I like “world” as a broad super category holding planets, dwarf planets and large moons that have achieved hydrostatic equilibrium inside it.

Splitter!! :slight_smile:

I think all this means is that some high-mass object blew through our system in the past and wobbled Pluto off-kilter. It doesn’t magically turn a planet into a non-planet just because of a little orbital shift. Sure, some will say that it’s Pluto’s fault for being in a vulnerable zone in the first place and was just asking for it being dressed up all spherically, but that’s just victim blaming and orbit shaming.

There’s some girl around the internet I’ve seen many times who trolls all the science blogs, searching for any mention of Pluto, then leaves a giant brick of a comment about all the reasons why Pluto is definitely a planet, don’t you dare call it a dwarf planet, etc. She’s been doing this for many years now.

With people like these, I wonder why they are so passionate that we MUST refer to Pluto as a planet, without any other qualifiers. I suspect some of the passion is a result of “Hey, when I was in elementary school, I learned that Pluto was a planet, so it IS a planet- nyah nyah nyah!” I don’t understand why something someone learned in school decades ago has far more weight than the modern opinions of astronomers. Plus Pluto honestly doesn’t give a fuck what we call it- why so defensive?

I think Pluto was “demoted” in 2006. That means that there almost all the kids who are in elementary school today have no memory of Pluto being a planet, no questions asked. I hope the teachers these days are teaching that Pluto is one of many Kuiper belt objects out there beyond Neptune. I wish I’d learned about Sedna and Nix and Hydra and Eris and all that other crazy stuff out there when I was a kid.

I feel like the “This is what I learned in elementary school and I don’t want it to change!” sentiment would be like ancient people insisting that the body contains four humors or something.

IMO the real problem with defining a planet as having differentiated layers and a spherical shape is that in order to determine those things we have to spend billions and decades to send a probe to see it up close.

All the big easy stuff (e.g. Jupiter) has already been found, at least in this star system. So the stuff we’re finding now that would be either just-barely-a-planet or not-quite-a-planet is small and far away. Not being able to know which it is for decades or centuries would lead to the need for a third category: Identifiable, but not otherwise categorizable; it’s just an orbiting space-thingy.

Part of the wisdom of the IAU definition is it takes care of this problem pretty efficiently. By the time you can characterize the orbit well enough to confidently know it *is *orbiting, you know how to classify it.

Lumper!! :wink:

As quoted above, in post 3:

It appears to me that the number of planets has been increased to 13.

Not a bug: feature. Discovering planets is a good thing.

Also, couldn’t differentiation and spherical shape be presumed at certain masses? If so, only dwarf sized objects would be of unclear provenance. Admittedly I’m not clear about whether planetary differentiation is sufficiently important that it should define a planet. I just came across the term this week.

Turpentine: I suspect that some formed an emotional attachment to Pluto because it was a) the smallest, b) the furthest away and c) had a cute name. The first two aren’t as applicable now.

ISTM that we’re defining a term inclusive of apples and hub caps while ruling out strawberries. Not that this is a huge issue: Phil Plait speaks for many when he asserts what matters are the properties of Pluto, not what label we stick to it. For this general interest message board though, it’s not a bad entry point to planetary science.

I only half-jokingly offer this definition–

Planet: Something of which this solar system has nine.

Turpentine, while I can’t speak for others, I have a strong tendency to feel that the original of anything is the best. The original definition of Pluto was that it was a regular planet. It was a planet before it was discovered, and it is a planet now. Calling it something else does not and CANNOT change the fact that it is still a planet, and always will be.

When the first asteroids were discovered, they were all initially counted as planets until the numbers started getting just utterly ridiculous, and it was clear that the area between Mars and Jupiter is just full of all kinds of junk. (For a somewhat broad definition of “full of”; it’s still nothing like The Empire Strikes Back.)

Personally, I do still have a sneaking fondness for the idea of ditching the “cleared its orbit” criterion and counting anything in “hydrostatic equilibrium” as a planet. The drawback is that we’d wind up with a Solar System with scores or hundreds or even thousands of new planets (including Ceres, a once-and-future planet, and maybe a few other of the classical Asteroid Belt bodies). That, to me, would also be the advantage, as I kind of think a Solar System with a bunch of new planets would be shiny, but I guess it would get kind of unwieldy. It would also be a little strange that a thing that’s mostly made of rock (and thus less likely to be in hydrostatic equilibrium) might not be counted as a “planet” even though it’s clearly larger (albeit irregularly shaped) than another thing that’s mostly made of ice (which, being less rigid than rock, tends to assume an essentially spherical shape more easily) and therefore is counted as a planet. That, of course, is true of both the Official IAU Definition and of my own “Screw ‘clearing its orbit’, bring on the planets!” alternative.

IMO the best alternative for defining “planet” is to go back to original intent. The word was coined by the Greeks to denote the wandering points of light they observed. It was a statement about what they saw, not about what was or wasn’t out there.

The planets are therefore simply and for always just the 6: Mercury through Saturn since those are the only ones the ancient Greeks could see with their eyes. Which is also all that any Earthbound human will ever see with the naked eye.

Any other criteria will suffer from boundary conditions and arbitrary exceptions. Or, we simply stop trying to have “planet” be a word with a strict scientific meaning. Just admit that it’s really a soft category with a deliberately informal definition. Just like “fish”.