Why (if anything) is a planet clearing its neighborhood important?

[Peelee]

For me personally, the word "planet" isn't that important either, it's the concept "planet" that matters, and the current definition of that is well and truly wonky, and should be replaced with something more sensible. The word "planet" could be replaced with "arglebargle" or "geronimo" and that wouldn't matter very much, though both of those would be much sillier than what we have.

Dwarf Planet doesn't seem any odder to me than "gas giant", even if the designation is a bit different.

[AvatarVecna]

It doesn't seem that weird to me. The definition for "planet" was already a kinda wonky wiggly vague thing. Always has been. Some people for thousands of years have assigned value to particular dots in the sky because they moved different than the other dots in the sky. At some point, we got a better look at those dots and realized they weren't just moving different, they were moving different for the same reason: they're pretty close, and they're orbiting the sun just like us! We're a planet, so that makes them planets.

Fast forward a few more centuries, and we're finding lots of things that don't feel like planets, but meet a lot of the same criteria as existing planets. Part of the point of science is setting definitions so that more precise conversations can be held about a subject. The guy 20 years ago might've been 99% correct when he talked about moons, but thanks to new definitions, you're gonna be 99.9% correct! "Science" as a concept doesn't care if Pluto, or Jupiter, or even Earth counts as a technical "planet", it's just about improving the width and depth of our understanding of the universe in small measured steps. And so a choice had to be made - do we let these things that meet the definition of "planet" count, even though they don't feel like planets...or do we change the definition to be a bit less vague?

Years later, around a hundred thousand dinner tables, parents would have conversations with their kids about school. The subject turns to science class, and the kid says they're learning about the eight planets in the solar system. "Nine." "No, pluto's not a planet anymore." You feel that? That's the sound of a hundred thousand parents having a brief moment of existential dread - Big Science changed the rules and made them look foolish in front of a child! And so a kernel of hatred is planted. It's not really based in anything. It's a tiny thing that barely matters to anybody outside of astronomy. Pluto hasn't changed, it's the same lumpy space rock it ever was, but the way the scientific community thinks about Pluto has changed very slightly. But to the average person, it just means that something they were taught as truth is no longer true, and that makes them a little mad.

(You see this same attitude occur if you've ever talked to somebody who learned Math prior to New Math.)

[halfeye]

Dwarf Planet doesn't seem any odder to me than "gas giant", even if the designation is a bit different.

It doesn't seem that weird to me. The definition for "planet" was already a kinda wonky wiggly vague thing. Always has been. Some people for thousands of years have assigned value to particular dots in the sky because they moved different than the other dots in the sky. At some point, we got a better look at those dots and realized they weren't just moving different, they were moving different for the same reason: they're pretty close, and they're orbiting the sun just like us! We're a planet, so that makes them planets.

Fast forward a few more centuries, and we're finding lots of things that don't feel like planets, but meet a lot of the same criteria as existing planets. Part of the point of science is setting definitions so that more precise conversations can be held about a subject. The guy 20 years ago might've been 99% correct when he talked about moons, but thanks to new definitions, you're gonna be 99.9% correct! "Science" as a concept doesn't care if Pluto, or Jupiter, or even Earth counts as a technical "planet", it's just about improving the width and depth of our understanding of the universe in small measured steps. And so a choice had to be made - do we let these things that meet the definition of "planet" count, even though they don't feel like planets...or do we change the definition to be a bit less vague?

Years later, around a hundred thousand dinner tables, parents would have conversations with their kids about school. The subject turns to science class, and the kid says they're learning about the eight planets in the solar system. "Nine." "No, pluto's not a planet anymore." You feel that? That's the sound of a hundred thousand parents having a brief moment of existential dread - Big Science changed the rules and made them look foolish in front of a child! And so a kernel of hatred is planted. It's not really based in anything. It's a tiny thing that barely matters to anybody outside of astronomy. Pluto hasn't changed, it's the same lumpy space rock it ever was, but the way the scientific community thinks about Pluto has changed very slightly. But to the average person, it just means that something they were taught as truth is no longer true, and that makes them a little mad.

(You see this same attitude occur if you've ever talked to somebody who learned Math prior to New Math.)

As I said before, it's not about the name it's about the concept. The part of the definition that is currently being used that irks me is the bit about clearing its orbit. There are two significant effects from this part of the definition. The first is that there is a graph of the mass of the smallest possible planet at a given distance from a given star, and that graph goes up, as the distance from the star increases (it probably goes up exponentially, but I can't prove that it doesn't go up linearly, it certainly goes up). The second is that the previously described graph is different for every different mass of star, if the star is less massive, the minimum mass of a planet at a given distance is higher.

So, to tell whether or not an object is a planet, you have to know how massive its star is, how massive the object is, and how far from its star it orbits (and gerd help you if the orbit is extremely elliptical). Some objects will be clear cut even so, but for the ones that aren't, that's a lot of "if"s.

[Mechalich]

As I said before, it's not about the name it's about the concept. The part of the definition that is currently being used that irks me is the bit about clearing its orbit. There are two significant effects from this part of the definition. The first is that there is a graph of the mass of the smallest possible planet at a given distance from a given star, and that graph goes up, as the distance from the star increases (it probably goes up exponentially, but I can't prove that it doesn't go up linearly, it certainly goes up). The second is that the previously described graph is different for every different mass of star, if the star is less massive, the minimum mass of a planet at a given distance is higher.

So, to tell whether or not an object is a planet, you have to know how massive its star is, how massive the object is, and how far from its star it orbits (and gerd help you if the orbit is extremely elliptical). Some objects will be clear cut even so, but for the ones that aren't, that's a lot of "if"s.

Generally I agree. For now it might be better to define 'planet' as a subset of non-stellar objects in hydrostatic equilibrium above some size minimum that orbit their primary. Mercury is roughly 10x the mass of even the largest known dwarf planet, so things are pretty safe there (a 0.05 Earth cutoff would serve nicely). This would be extremely arbitrary, but so what, it would work, and I think the public would generally be accepting of a a 'must be this big to count' definition of 'planet.'

[hamishspence]

As I said before, it's not about the name it's about the concept. The part of the definition that is currently being used that irks me is the bit about clearing its orbit.

Just as there's "protostars" there can be "protoplanets". Both can cross the boundary from "proto" to "actual" by doing something.

During the period when Earth hadn't yet cleared most of the debris out of its orbit, and there were multiple Earth-sized bodies around, it can be thought of as a protoplanet rather than a planet.

Dwarf planets are protoplanets that are not going to achieve the status of "planet" by clearing out all comparable-sized objects.

A protostar achieves the status of "star" when hydrogen-hydrogen fusion begins - a protoplanet achieves the status of planet, by removing nearby, comparable-sized objects.

The idea of calling Ceres and a few other large asteroids, and various big Kuiper Belt Objects "protoplanets" does have precedent:

https://en.wikipedia.org/wiki/Protoplanet

[Chronos]

Quoth AvatarVecna:

(You see this same attitude occur if you've ever talked to somebody who learned Math prior to New Math.)

Or someone who just thinks they learned math prior to New Math. These are the folks who learned math at any time since the late 70s, but don't remember any of it, and rather than admit that they don't remember it, they instead complain about all of the nonexistent changes in math pedagogy since then. These are the same people who are likely to complain about "the Millennials" ruining everything, even though they, themselves, are the Millennials.

You want to get their heads to really explode, show them some of the actual Old Math. There's a reason we stopped doing it that way; it was genuinely harder to understand.

[Radar]

As I said before, it's not about the name it's about the concept. The part of the definition that is currently being used that irks me is the bit about clearing its orbit. There are two significant effects from this part of the definition. The first is that there is a graph of the mass of the smallest possible planet at a given distance from a given star, and that graph goes up, as the distance from the star increases (it probably goes up exponentially, but I can't prove that it doesn't go up linearly, it certainly goes up). The second is that the previously described graph is different for every different mass of star, if the star is less massive, the minimum mass of a planet at a given distance is higher.

So, to tell whether or not an object is a planet, you have to know how massive its star is, how massive the object is, and how far from its star it orbits (and gerd help you if the orbit is extremely elliptical). Some objects will be clear cut even so, but for the ones that aren't, that's a lot of "if"s.

Now that I think about it, this contextual character of planet definition actually makes sense, since the very concept of planets and moons is all about the star system dynamics - so relations between the stars and all the surrounding objects are at the core of it.

Why Ganimedes is a moon and Mercury a planet if the former is even larger than the latter? Well, Mercury orbits the Sun, while Ganimedes orbits Jupiter. The difference between the two is basically about where they are in the solar system - not about any intrinsic qualities of those two celestial bodies. Since that distinction is useful in of itself, the added distinction between planets and dwarf planets might have some reasonable meaning as well in the context of solar system dynamics. As it turns out, it is not exactly new that we have astronomical definitions that are position-dependent.

Also, with pretty much any kind of categorization ever you have a problem of edge-cases and where to put them. In some fields there will be more of those than in others, but the point still stand.

[gomipile]

The first is that there is a graph of the mass of the smallest possible planet at a given distance from a given star, and that graph goes up, as the distance from the star increases (it probably goes up exponentially, but I can't prove that it doesn't go up linearly, it certainly goes up).

Instead of saying something wild like "it probably goes up exponentially" you could instead look it up:

https://en.m.wikipedia.org/wiki/Clea..._neighbourhood

There are two notions shown for predicting whether a body is capable of clearing its neighborhood. One due to Stern and Levinson, the other due to Jean-Luc Margot. Let "m" be the mass of the orbital body, and "a" be the semi-major axis of its orbit. You can just think of a as the "distance from the star."

Stern and Levinson's equation gives m proportional to a raised to the 3/4 power. That's a bit slower than linear growth.

Margot's equation gives m proportional to a raised to the 9/8. That's a bit faster than linear, but a lot slower than quadratic growth.

In either case, they're in the realm of slow polynomial growth. Absolutely nowhere near exponential growth.

[halfeye]

Instead of saying something wild like "it probably goes up exponentially" you could instead look it up:

https://en.m.wikipedia.org/wiki/Clea..._neighbourhood

There are two notions shown for predicting whether a body is capable of clearing its neighborhood. One due to Stern and Levinson, the other due to Jean-Luc Margot. Let "m" be the mass of the orbital body, and "a" be the semi-major axis of its orbit. You can just think of a as the "distance from the star."

Stern and Levinson's equation gives m proportional to a raised to the 3/4 power. That's a bit slower than linear growth.

Margot's equation gives m proportional to a raised to the 9/8. That's a bit faster than linear, but a lot slower than quadratic growth.

In either case, they're in the realm of slow polynomial growth. Absolutely nowhere near exponential growth.

They do consistently grow though? That's the thing that I object to, obviously near to the Sun in the Solar system it is a slow rate of growth until you get out somewhere like Neptune, but I have no idea where I can see a graph of the functions (it's a little strange to me that there's more than one function, but I suppose that is science).

[gomipile]

(it's a little strange to me that there's more than one function, but I suppose that is science).

If you go to the link I provided, it briefly explains the rationale behind each of the criteria.

An earlier one was more or less adjusted by hand to work for our solar system. A later one was developed to work with observable quantities for exoplanets.

[Radar]

They do consistently grow though? That's the thing that I object to, obviously near to the Sun in the Solar system it is a slow rate of growth until you get out somewhere like Neptune, but I have no idea where I can see a graph of the functions (it's a little strange to me that there's more than one function, but I suppose that is science).

There is more than one function as there are different ways to model the same complicated phenomenon and on top of that, different ways to define a number describing how well the orbit was cleared - even more so, if one wants to quantify a potential of a given planet to clear its orbit.

[Jay R]

A planet clearing its neighborhood is important because it is a clear unambiguous measure of how much effect it has on the solar system. A planet dominates its area; a dwarf planet doesn’t. That’s why nothing in the asteroid belt matters as much as Earth, Mars, or the others – because no single asteroid has a major effect.

Also, don’t confuse the measurement being used with what it’s being used for. There is a huge gap between the eight planets and the known dwarf planets. Ability to clear the orbit is a tool for defining where the limit should be, but there’s no question that these are very different groups.

Pluto is vastly smaller than any of the eight current planets, and has vastly less gravitational effect on the objects in its neighborhood. Pluto is about the same size as many other bodies. It’s larger than Ceres or Sedna or Haumea, but smaller than Eris. These are clearly not the same kind of thing as the eight current planets.

The measurement used for a planet’s ability to clear its neighborhood is Λ (lambda). If a body has a Λ >1, it will eventually clear out its neighborhood. So planets have a Λ greater than 1, and dwarf planets have a Λ less than one. And it isn’t close. There is a gap of five orders of magnitude between the Λ of the smallest planet and the Λ of the largest dwarf planet.

The planet with the smallest Λ is Mars, with Λ= 942. By contrast, Pluto has a Λ of 0.00295.

Or to use a more common measurement, Pluto has a mass slightly less than 4% the mass of the lightest planet (Mercury). The heaviest (known) dwarf planet is Eris, about 28% more massive than Pluto, but still only slightly more than 5% of the mass of Mercury.

Λ measures the effect a planet has on its area. That’s a useful distinction, but it isn’t the only difference. There is a vast difference between the planets and the dwarf planets. It isn’t merely about clearing its orbit – that’s just the measurement used to identify the border between them. But the crucial thing to realize is that the border is nowhere near the planets, and nowhere near the dwarf planets. There is a gulf of five orders of magnitude between them. We could put the defining border at Λ=500, or at Λ= 0.005, and it would make no difference in identifying any (known) object.

[TaiLiu]

A planet clearing its neighborhood is important because it is a clear unambiguous measure of how much effect it has on the solar system. A planet dominates its area; a dwarf planet doesn’t. That’s why nothing in the asteroid belt matters as much as Earth, Mars, or the others – because no single asteroid has a major effect.

Also, don’t confuse the measurement being used with what it’s being used for. There is a huge gap between the eight planets and the known dwarf planets. Ability to clear the orbit is a tool for defining where the limit should be, but there’s no question that these are very different groups.

Pluto is vastly smaller than any of the eight current planets, and has vastly less gravitational effect on the objects in its neighborhood. Pluto is about the same size as many other bodies. It’s larger than Ceres or Sedna or Haumea, but smaller than Eris. These are clearly not the same kind of thing as the eight current planets.

The measurement used for a planet’s ability to clear its neighborhood is Λ (lambda). If a body has a Λ >1, it will eventually clear out its neighborhood. So planets have a Λ greater than 1, and dwarf planets have a Λ less than one. And it isn’t close. There is a gap of five orders of magnitude between the Λ of the smallest planet and the Λ of the largest dwarf planet.

The planet with the smallest Λ is Mars, with Λ= 942. By contrast, Pluto has a Λ of 0.00295.

Or to use a more common measurement, Pluto has a mass slightly less than 4% the mass of the lightest planet (Mercury). The heaviest (known) dwarf planet is Eris, about 28% more massive than Pluto, but still only slightly more than 5% of the mass of Mercury.

Λ measures the effect a planet has on its area. That’s a useful distinction, but it isn’t the only difference. There is a vast difference between the planets and the dwarf planets. It isn’t merely about clearing its orbit – that’s just the measurement used to identify the border between them. But the crucial thing to realize is that the border is nowhere near the planets, and nowhere near the dwarf planets. There is a gulf of five orders of magnitude between them. We could put the defining border at Λ=500, or at Λ= 0.005, and it would make no difference in identifying any (known) object.

This is great! Thank you for your expert input, Jay R.

[halfeye]

A planet clearing its neighborhood is important because it is a clear unambiguous measure of how much effect it has on the solar system.

Rubbish. This is ad hoc at most.

Also, don’t confuse the measurement being used with what it’s being used for. There is a huge gap between the eight planets and the known dwarf planets. Ability to clear the orbit is a tool for defining where the limit should be, but there’s no question that these are very different groups.

Mass or even size (so long as we're not talking about undifferentiated gas clouds) would be better.

Pluto is vastly smaller than any of the eight current planets, and has vastly less gravitational effect on the objects in its neighborhood. Pluto is about the same size as many other bodies. It’s larger than Ceres or Sedna or Haumea, but smaller than Eris. These are clearly not the same kind of thing as the eight current planets.

Pluto was a planet for 50 years of my life, but I'm not that bothered that its status has been changed.

The measurement used for a planet’s ability to clear its neighborhood is Λ (lambda). If a body has a Λ >1, it will eventually clear out its neighborhood. So planets have a Λ greater than 1, and dwarf planets have a Λ less than one. And it isn’t close. There is a gap of five orders of magnitude between the Λ of the smallest planet and the Λ of the largest dwarf planet.

The planet with the smallest Λ is Mars, with Λ= 942. By contrast, Pluto has a Λ of 0.00295.

Or to use a more common measurement, Pluto has a mass slightly less than 4% the mass of the lightest planet (Mercury). The heaviest (known) dwarf planet is Eris, about 28% more massive than Pluto, but still only slightly more than 5% of the mass of Mercury.

So why didn't they pick the mass of Mercury, or some fraction of that to avoid complications, as the limit between planets and not fully planets?

The problem I have with this Lambda is that the mass required for an object to be a planet increases with distance from the relevant star or stars (notice that Mars, which is considerably more massive, scores lower than Mercury), and is also dependant on the mass of the star.

We could put the defining border at Λ=500, or at Λ= 0.005, and it would make no difference in identifying any (known) object.

I am not interested in any known Solar system objects in respect to this lambda. My problem is that an object with the mass of Jupiter, that was in orbit far enough out from the particular star (we know nothing like that exists in the Solar system), which almost certainly exists somewhere in this galaxy, and multiple examples of which must exist in other galaxies, would be a minor planet if this definition continues to be used.

[warty goblin]

The problem I have with this Lambda is that the mass required for an object to be a planet increases with distance from the relevant star or stars (notice that Mars, which is considerably more massive, scores lower than Mercury), and is also dependant on the mass of the star.

It's a definition concerned with the behavior of bodies in orbit. Of course it's going to depend on mass and distance, anything to do with orbital mechanics varies with mass and distance.

If you use the a "things that mass x or more kg are planets" definition, you impart no information about that body except that it masses at least x kg. This is a pointless definition, not even worth the semantic bother of existing. The orbital mechanics definition at least carries something about its behavior in system in question.

This seems entirely straightforward to me, it's a useable definition that describes a particular class of bodies in orbital mechanics according to their behavior. Using mass does not define anything about the behavior of the body in question as it relates to the system as a whole. So I gotta ask, why are you so set on a pure mass definition? What's the gain here?

[halfeye]

If you use the a "things that mass x or more kg are planets" definition, you impart no information about that body except that it masses at least x kg. This is a pointless definition, not even worth the semantic bother of existing. The orbital mechanics definition at least carries something about its behavior in system in question.

Why would it be good for a definition to tell you stuff about the defined object?

This seems entirely straightforward to me, it's a useable definition that describes a particular class of bodies in orbital mechanics according to their behavior. Using mass does not define anything about the behavior of the body in question as it relates to the system as a whole. So I gotta ask, why are you so set on a pure mass definition? What's the gain here?

The gain is simplicity. You have an object, it masses x, it's a planet. Simple. The edge cases are ones near the defining mass, and it's relatively easy to determine which side of the line they fall.

The definitions of the physical constants are simple because simplicity is a virtue, simplicity lets you work out combinations that couldn't be achieved with more complex definitions.

The "clears it's orbit" definition requires you to know two masses, the elipticality of the orbit and (if the orbit isn't too eliptical) the average radius of the orbit. Edge cases can be because the mass of the star is too low, the mass of the object is too low, the orbit is too far out or the orbit is too eliptical. That's a lot of things that can all be difficult to determine especially at long distances.

I'm not saying that most cases will be edge cases, but when the edge cases start to mess up, you know you're dealing with a badly organised system.

[georgie_leech]

It seems like you want a definition completely divorced from any relationship between stellar bodies. Is there any particular reason you want that? I mean, science is *full* of relational definitions, especially astronomy. A moon is by definition a smaller body orbiting a larger one; meteor, meteorite, and meteoroid all tell you different things about where this particular chunk of rock is headed. Why should Planet in particular be divorced from that?

[warty goblin]

Why would it be good for a definition to tell you stuff about the defined object?

Because the reason you have a definition is to convey useful information about the thing in question. Otherwise why have the definition?

The gain is simplicity. You have an object, it masses x, it's a planet. Simple. The edge cases are ones near the defining mass, and it's relatively easy to determine which side of the line they fall.

The definitions of the physical constants are simple because simplicity is a virtue, simplicity lets you work out combinations that couldn't be achieved with more complex definitions.

The "clears it's orbit" definition requires you to know two masses, the elipticality of the orbit and (if the orbit isn't too eliptical) the average radius of the orbit. Edge cases can be because the mass of the star is too low, the mass of the object is too low, the orbit is too far out or the orbit is too eliptical. That's a lot of things that can all be difficult to determine especially at long distances.

I'm not saying that most cases will be edge cases, but when the edge cases start to mess up, you know you're dealing with a badly organised system.

The fact you need to know more to say something is a planet is what makes it a useful, meaningful, definition. Scientific/mathematical definitions for complex phenomena tend to themselves either be complex, or require complexity to verify, because otherwise they don't say anything interesting and there's no point in having them. The two problems with a masses X definition seem to me to be that it conveys very little information, and doesn't tell the audience anything all that useful about the body in question as a member of the solar system. The only advantage I can see to your definition is that it requires very little thought, which is a pretty bad reason to go with it.

[Mechalich]

It seems like you want a definition completely divorced from any relationship between stellar bodies. Is there any particular reason you want that? I mean, science is *full* of relational definitions, especially astronomy. A moon is by definition a smaller body orbiting a larger one; meteor, meteorite, and meteoroid all tell you different things about where this particular chunk of rock is headed. Why should Planet in particular be divorced from that?

In order to conduct comparative analysis on astronomical objects based on their physical properties, it is necessary to classify them according to those properties. In such a classification, objects with highly similar properties ought to be classified as the same thing. When you bring position into the classification as well, it means that objects which are nearly physically identical may be classified as different things. That's bad, because it robs the classification of functional utility. The problem with the current official definition of a planet is that it's trying to do too much. It attempts to organize objects both by physical properties (primarily mass, but also composition) and by their position within a stellar system.

The solution is simple: multiple classifications running in parallel. This is well-established in biology, in which species are classified via their evolutionary heritage - the classic binominal names now organized via cladistics - and will also be classified via their ecosystem role (autotroph, heterotroph, parasite, decomposer, etc.) and by physical structure as well (ex. herbaceous vs. woody grow forms).

The problem in astronomy is that for centuries it was able to cheat planetary classification through the special case of n=1 for the sample size. Exoplanets were nothing but theory until the 1990s and therefore no need existed to define the properties of planets for any case other than that of our own solar system. And yes, the current definition also works effectively for our own solar system, but since we now have data on thousands of other such systems that is no longer sufficient.

[hamishspence]

That's what prefixes are for.

"Object bigger than (asteroid) but smaller than (brown dwarf), orbiting star, = planet"

"Object of this kind, not orbiting star, = rogue planet"

"Object of this kind, in a belt of similar-sized objects, = dwarf planet"

[halfeye]

Because the reason you have a definition is to convey useful information about the thing in question. Otherwise why have the definition?

I would say the reason to have a definition is so that you can check something against the definition and say "is this an x?" if it is put it in the 'x' pile, otherwise put it in the "not x" pile.

The fact you need to know more to say something is a planet is what makes it a useful, meaningful, definition. Scientific/mathematical definitions for complex phenomena tend to themselves either be complex, or require complexity to verify, because otherwise they don't say anything interesting and there's no point in having them. The two problems with a masses X definition seem to me to be that it conveys very little information, and doesn't tell the audience anything all that useful about the body in question as a member of the solar system. The only advantage I can see to your definition is that it requires very little thought, which is a pretty bad reason to go with it.

The basics need to be simple so you can build more complicated things from them, a triangle has three straight sides and three angles, and that is all there is to a triangle, but from trianges come sine, cosine and tangent and all the wonders that flow from those.

[factotum]

The gain is simplicity. You have an object, it masses x, it's a planet. Simple.

OK, so Pluto's mass is about 1.3 x 10^22 kg. Presumably you want this included as a planet or you wouldn't be complaining about the current definition? So, what do we do about (for example, there are quite a few like this) Ganymede, which is a moon of Jupiter but has ten times the mass of Pluto? What about Eris, which is slightly smaller but more massive than Pluto? And what arbitrary value of mass are you choosing as the cutoff point anyway?

[Mechalich]

OK, so Pluto's mass is about 1.3 x 10^22 kg. Presumably you want this included as a planet or you wouldn't be complaining about the current definition? So, what do we do about (for example, there are quite a few like this) Ganymede, which is a moon of Jupiter but has ten times the mass of Pluto? What about Eris, which is slightly smaller but more massive than Pluto? And what arbitrary value of mass are you choosing as the cutoff point anyway?

A purely mass based setup relies on certain natural cutoffs. Under such a scheme everything bigger than a dust particle but not large enough to reach hydrostatic equilibrium are asteroids. Everything large enough to reach hydrostatic equilibrium but not large enough to trigger deuterium fusion are planets. Everything large enough to trigger deuterium fusion but not large enough to trigger hydrogen fusion are brown dwarfs. Everything large enough to trigger hydrogen fusion are stars.

In such a scenario Ganymede would be a Planetary Moon, while Phobos would be an Asteroidal Moon, and you could even talk about something like Proxima Centauri as a Stellar Moon.

This sort of arrangement, which really doesn't have much to do with the issue of Pluto, is favored by astronomers who study exoplanets.

However, this is not the only option. Astronomy could prioritize stellar system dynamics and position over composition and mass, in which case there would be planets, and moons, and belt objects that haven't cleared their orbits. The caveat here is that under such a scheme you could have a planet, a moon, and a belt object that were all structurally more or less identical defined as different things.

Either method is possible, but the current official definition tries to do both things at once.

[Radar]

Either method is possible, but the current official definition tries to do both things at once.

Only to some small degree as it is pretty much impossible for an object that is not massive enough to reach hydrostatic equilibrium to clear its orbit. So we can expect every object that did clear the orbit to be in hydrostatic equilibrium. So the current definition of a planet contains both qualities, but one of them is pretty much a requirement for the other.

[halfeye]

OK, so Pluto's mass is about 1.3 x 10^22 kg. Presumably you want this included as a planet or you wouldn't be complaining about the current definition?

No.

I already said I didn't care about Pluto. How many times do I have to repeat that before anyone listens?

The current definition being absurd has very little to do with Pluto, except for Pluto apparently being the trigger for its selection.

Only to some small degree as it is pretty much impossible for an object that is not massive enough to reach hydrostatic equilibrium to clear its orbit. So we can expect every object that did clear the orbit to be in hydrostatic equilibrium. So the current definition of a planet contains both qualities, but one of them is pretty much a requirement for the other.

Considering that some of the rubble pile asteroids appear to be nearing hydrostatic equilibrium, you are right that including hydrostatic equilibrium in the definition of planets was totally redundant, which is another thing wrong with this definition.

[Radar]

Considering that some of the rubble pile asteroids appear to be nearing hydrostatic equilibrium, you are right that including hydrostatic equilibrium in the definition of planets was totally redundant, which is another thing wrong with this definition.

I would not say it is redundant because it is important to relate to the exception of the current definition of a planet. We have three points:
1. It must orbit a star (and not something else like moons do).
2. It must be big enough to have enough gravity to force it into a spherical shape.
3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

What is now called a planet, checks all three boxes. Dwarf planets check boxes 1 and 2. Rogue planets only check the second box.

So you can picture it like this: you have a wide group of different kind of planets which contains all objects that are big enough to obtain hydrostatic equilibrium. Within that big set you have more narrow subgroups: rogue planets (that do not orbit any star), dwarf planet (orbit a star, but did not clear their neighborhood) and regular planets (orbit a star and did clear the neighborhood). For the sake of brevity and keeping with the way the word "planet" was used up to now, the last category was shortened to just "planet".


It does seem that you mostly have a problem with that whether something is a planet or not depends on relative things like where it is in a solar system and other dynamical factors. Why according to you it is wrong to account for solar dynamics in the definition of a planet? After all, research dynamics and evolution of star systems is where the definition of a planet would be used the most and those dynamic qualities are very important there. And what about the definition of a moon? At its core it is all about the relative placement of a given object. In fact, any definition that includes a requirement that a given object has to orbit something is obviously relative to where a given object is at the moment.

[Bohandas]

Or to use a more common measurement, Pluto has a mass slightly less than 4% the mass of the lightest planet (Mercury). The heaviest (known) dwarf planet is Eris, about 28% more massive than Pluto, but still only slightly more than 5% of the mass of Mercury.

Which brings us back to the main question of why they didn't just use a size or mass based cutoff

[Lord Torath]

Which brings us back to the main question of why they didn't just use a size or mass based cutoff

For which answer I will direct you to Radar's post immediately above yours.

[Jay R]

Rubbish. This is ad hoc at most.

It's not enough to call it "ad hoc". You have to both:
a. explain in what way it is "ad hoc", and
b. show that besides being "ad hoc", it is incorrect.

In fact, this isn't ad hoc. The parameter Λ was proposed to quantify a crucial aspect of planetary behavior on a solar system, in a paper published in 2002. It wasn't initially a tool to divide objects. Then they measured Λ and found the five levels of magnitude between the eight main planets and other bodies in the system, and then the division between main planets and dwarf planets was proposed. Finally, in 2006 the IAU decided that the term "dwarf planet" would refer to a body with hydrostatic equilibrium but Λ < 1.

Mass or even size (so long as we're not talking about undifferentiated gas clouds) would be better.

People who actually study planets disagree with you. They see mass as an important distinction primarily based on
(a) its ability to force a body to be round with hydrostatic equilibrium, and
(b) its ability to affect the solar system by dominating an orbit.

The distinction between planets and dwarf planets is based on how they affect space in their neighborhood of the solar system. The distinction between dwarf planets and small Solar System bodies (SSSBs) is whether or not an object is massive enough to have hydrostatic equilibrium -- which is to say, having sufficient internal pressure that its own gravity forces it into an ellipsoid shape (which included near-spherical).

There is no exact mass definition because we don't yet know what mass that requires. And it might vary due to composition; icy bodies may have hydrostatic equilibrium at a smaller size than rocky or metallic bodies.

In any event, at least one astronomer (sort of) agrees with you. Dr. Alan Stern thinks that planets should be divided into two subtypes, one of which is "dwarf planet". But even he sees the ability to dominate an orbital neighborhood as a crucial distinction. In fact, he proposed the term "dwarf planet" for the bodies that don't dominate their neighborhood.

But mass alone is not a crucial measure. The two distinctions that all planetary astronomers believe are most crucial are hydrostatic equilibrium (based primarily on mass, but possibly also on composition), and the ability to dominate its orbit, based on mass relative to the star it orbits, and on distance from that star.

Pluto was a planet for 50 years of my life, but I'm not that bothered that its status has been changed.

Fair enough; I'll ignore it. But lots of people do get emotional about this so, other people are likely to keep talking about it.

So why didn't they pick the mass of Mercury, or some fraction of that to avoid complications, as the limit between planets and not fully planets?

Because they believe the mass of a planet matters primarily by how it affects the body itself (hydrostatic equilibrium) and how it affects the solar system (orbital dominance). Ceres is considered a dwarf planet, not simply because of its mass, but because it's in the asteroid belt, and not its own unique orbit.

The problem I have with this Lambda is that the mass required for an object to be a planet increases with distance from the relevant star or stars (notice that Mars, which is considerably more massive, scores lower than Mercury), and is also dependant on the mass of the star.

Exactly. That's the point. A body out in the Oort Cloud would have to be much larger to dominate that orbit as much as Jupiter, or Earth, or even Mercury, dominate theirs.

A body that is a minor planet in our system might be the same size as a planet orbiting a smaller star -- because in that position, it would be large enough to dominate its orbit. Similarly, if there is a Mercury-sized object deep in the scattered disc, it wouldn't dominate its orbit, and would not be the obviously most influential body in that location. Therefore it would not be a planet.

Professional astronomers think that a body's behavior and influence in its orbit is a more important factor than its mere mass.

I am not interested in any known Solar system objects in respect to this lambda. My problem is that an object with the mass of Jupiter, that was in orbit far enough out from the particular star (we know nothing like that exists in the Solar system), which almost certainly exists somewhere in this galaxy, and multiple examples of which must exist in other galaxies, would be a minor planet if this definition continues to be used.

That is correct. A body the mass of Jupiter, in orbit around a really massive star like R136a1, might be in an equivalent of our Kuiper belt or scattered disc, one of billions of bodies in that orbit. It wouldn't dominate the space around it, its orbit might be unstable or eccentric, and it would not be a planet.

A planet is defined by its effect on the solar system it's in.

[Excession]

Which brings us back to the main question of why they didn't just use a size or mass based cutoff

Because both of those are very hard to measure, especially for smaller bodies that are a long way from the sun. The roundness of a body can be checked by looking at how the brightness changes over time as it spins. Getting either size or mass from brightness requires making assumptions about both surface (albedo) and interior (density) composition, which are likely to be inaccurate. Pluto and some others have moons that can help for measuring mass, but not all.