Planet/Dwarf Planet Classification - Regarding Rogue Bodies

[Madi Araly]

Hi there,
I was reading up on how planets are classified based on their size, but ran into a problem when I began to consider rogue celestial bodies.

If a body has significant enough mass to maintain a hydrostatic equilibrium, yet doesn't orbit a star, I understand that it's classified as a rogue planet. The information taught in my current Earth & Space Science course states that the only difference between a regular planet and a dwarf is that regular planets have cleared the neighborhood around their orbits, but how are the two differentiated between if not explicitly orbiting any stars?

Is there a standard mass that we use to do so? I can't imagine this would be the case since we can't accurately measure mass for those light years away, but I don't see another clear way that we could categorize a planet.

 

[|Glitch|]

Hi there,
I was reading up on how planets are classified based on their size, but ran into a problem when I began to consider rogue celestial bodies.

If a body has significant enough mass to maintain a hydrostatic equilibrium, yet doesn't orbit a star, I understand that it's classified as a rogue planet. The information taught in my current Earth & Space Science course states that the only difference between a regular planet and a dwarf is that regular planets have cleared the neighborhood around their orbits, but how are the two differentiated between if not explicitly orbiting any stars?

Is there a standard mass that we use to do so? I can't imagine this would be the case since we can't accurately measure mass for those light years away, but I don't see another clear way that we could categorize a planet.

Planets are not classified based on their size. Which is part of the problem with the definition of a "planet."

Q: What is the exact wording of the official IAU proposed definition of planet?

A: A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Source: https://www.iau.org/public/themes/pluto/

Based upon the above definition, there is no maximum size. Furthermore, no other star can have planets since they do not meet the first definition. Lastly, none of the "planets" in our solar system meet the IAU definition since there are objects in the Lagrange Points 4 and 5 of every "planet" in our solar system. Hence, no "planet" has "cleared the neighbourbood around its orbit." This definition by the IAU is purely political and has absolutely nothing to do with science.

In order to properly define what a planet is, or what it is not, there needs to be a minimum and maximum size or mass. The minimum size has already been defined as having sufficient mass to achieve hydrostatic equilibrium. There must also be a maximum mass, which should be less than what is required to begin deuterium fusion, or less than ≈13 Jupiter masses.

Whether they orbit our sun, another star or stars (including brown dwarfs), or are not orbiting any other object, they should still be considered a "planet," as long as they fall within the minimum and maximum mass requirements. The definition of a "moon" can also be defined as any object that orbits a "planet," with no minimum size requirement. The maximum size requirement for a "moon" is automatically determined by the object it orbits. If it was larger than the "planet," then it cannot be a "moon" since the "planet" would be orbiting it, not the other way around.

However, that would make Ceres, Pluto, Eris, Makemake, 90377 Sedna, and several other objects "planets." That does not mean we cannot use additional qualifiers to better identify them. We can still call Ceres a "dwarf planet" or Pluto, Eris, Makemake, and 90377 Sedna "Trans-Neptunian Objects." All this definition does is allow us to define a group of objects that are big enough to achieve hydrostatic equilibrium, yet small enough so that there is no fusion taking place. Whether they orbit a star, more than one star, a brown dwarf, white dwarf, neutron star(s), black hole(s), orbit any other object more massive than a "planet," or not orbit anything at all, should make absolutely no difference. Nor should the definition change depending on whether these objects have "cleared the neighbourbood around its orbit."

 

[mfb]

Based upon the above definition, there is no maximum size. Furthermore, no other star can have planets since they do not meet the first definition. Lastly, none of the "planets" in our solar system meet the IAU definition since there are objects in the Lagrange Points 4 and 5 of every "planet" in our solar system. Hence, no "planet" has "cleared the neighbourbood around its orbit."

They have. It is not necessary to get rid of every speck of dust. There are various metrics for "cleared the neighborhood", and they all see a huge gap between the 8 planets and all other objects - it does not matter which metric you use, the result is always the same. Objects in Lagrangian points or orbiting the secondary object ("planet") are gravitationally bound to the secondary object orbit anyway.

I agree that (a) of the definition is odd, it would be handy to generalize the definition to all stars - but then a maximum mass (or similar) has to be introduced, otherwise binary stars are also planets.

The definition of a "moon" can also be defined any object that orbits a "planet," with no minimum size requirement. The maximum size requirement for a "moon" is automatically determined by the object it orbits. If it was larger than the "planet," then it cannot be a "moon" since the "planet" would be orbiting it, not the other way around.

What about objects with very similar masses, e.g. one object 5% lighter than the other? Would you call them "planet plus moon"? It is not so easy.Concerning the original question: I think if we have the technology to discover "rogue dwarf planets" (whatever that will mean in the future) then some categorization system will establish. Maybe everything in hydrostatic equilibrium that is not doing fusion will stay categorized as rogue planet: it could be one, if it gets a proper orbit around a star and cleans it.

 

[|Glitch|]

They have. It is not necessary to get rid of every speck of dust. There are various metrics for "cleared the neighborhood", and they all see a huge gap between the 8 planets and all other objects - it does not matter which metric you use, the result is always the same. Objects in Lagrangian points or orbiting the secondary object ("planet") are gravitationally bound to the secondary object orbit anyway.

I agree that (a) of the definition is odd, it would be handy to generalize the definition to all stars - but then a maximum mass (or similar) has to be introduced, otherwise binary stars are also planets.

I suppose it depends on one's definition of "neighbourhood." I would call the Trojan and Greek Asteroids that orbit in Jupiter's Lagrange Points 4 and 5 to be more than just a "speck of dust." Furthermore, the 67 known moons that orbit Jupiter would imply that it has not "cleared the neighbourhood around its orbit." An object that has "cleared the neighbourhood around its orbit" would not have moons.

I would argue that "planets" do not necessarily need to orbit stars. "Planets" that have been ejected out of their solar systems due to a sudden mass change of their parent star, for example, would still be "planets," albeit rogue "planets." A "planet" that orbits a black hole, or a brown dwarf, would still be a "planet." The definition of a "planet" should be determined by mass alone: Large enough to achieve hydrostatic equilibrium, yet small enough so that there is no fusion taking place.

What about objects with very similar masses, e.g. one object 5% lighter than the other? Would you call them "planet plus moon"? It is not so easy.

If they are both large enough to achieve hydrostatic equilibrium, yet small enough so that there is no fusion taking place, you could call them both "planets" or a "binary planet system." Much like our moon and Earth. If it was not for the fact that our moon orbits the Earth, it could easily fit the definition of a "planet."

 

[mfb]

I suppose it depends on one's definition of "neighbourhood." I would call the Trojan and Greek Asteroids that orbit in Jupiter's Lagrange Points 4 and 5 to be more than just a "speck of dust." Furthermore, the 67 known moons that orbit Jupiter would imply that it has not "cleared the neighbourhood around its orbit." An object that has "cleared the neighbourhood around its orbit" would not have moons.

Compared to Jupiter they are specks of dust. But we don't need words, you can check the numbers. No matter which metric you use, there is a gap of a factor of 1000 between the least planet-like planet and the most planet-like non-planet, while the planets are closer together.

If they are both large enough to achieve hydrostatic equilibrium, yet small enough so that there is no fusion taking place, you could call them both "planets" or a "binary planet system." Much like our moon and Earth. If it was not for the fact that our moon orbits the Earth, it could easily fit the definition of a "planet."

You can certainly do that, but I think most people won't like a definition that adds Moon and various other moons and asteroids to the list of planets.

 

[|Glitch|]

Compared to Jupiter they are specks of dust. But we don't need words, you can check the numbers. No matter which metric you use, there is a gap of a factor of 1000 between the least planet-like planet and the most planet-like non-planet, while the planets are closer together.
You can certainly do that, but I think most people won't like a definition that adds Moon and various other moons and asteroids to the list of planets.

The point I was trying to make was that the IAU definition of "planet" is purely political, not based upon science. They were determined to keep Eris, Makemake, and other newly discovered trans-Neptunian objects off the list of official "planets." As a consequence of that effort, and due to a very poor political definition, Pluto happened to also be excluded from the official list of "planets."

Celestial objects that have achieved hydrostatic equilibrium, but not so large as to begin the fusion process, need a label. I would suggest that the label should be "planet." That doesn't mean we cannot use other adjectives to further identify them. There can be rogue planets, dwarf planets, gas giant planets, trans-Neptunian planets, Keiper Belt or Oort Cloud planets, etc., etc. All we are really doing by using the label "planet" is establishing both a minimum and maximum mass of the object.

 

[mfb]

I would suggest that the label should be "planet."

And others would like to use that label in different ways. Pointless to argue about labels.

 

[|Glitch|]

And others would like to use that label in different ways. Pointless to argue about labels.

I disagree. Labels are how we communicate. We all know the definition of the label "star," therefore, when you call an object a "star" I know exactly to which you refer. The same should also be true for the label "planet." If everyone has a completely different definition for "planet" then communication breaks down, and we might as well be talking a foreign language.

 

[mfb]

If everyone has a completely different definition for "planet" then communication breaks down, and we might as well be talking a foreign language.

Right. So can we please use the definition nearly everyone uses - the IAU one? And not argue about it? That was my point.

 

[|Glitch|]

Right. So can we please use the definition nearly everyone uses - the IAU one? And not argue about it? That was my point.

The IAU definition of "planet," as has already been established in this thread, is wholly inadequate. Nobody, other than politicians, uses the IAU definition. Actual scientists use completely different definitions, which is why we have exoplanets, rogue planets, and numerous other celestial bodies (such as binary stars) that do not fit the political IAU definition.

 

[Madi Araly]

Wow, I really wasn't expecting this detailed of a response, haha. Thanks to both of you guys! I wasn't aware of some of the concepts you mentioned during your debate, but I find them really interesting nonetheless.

Celestial objects that have achieved hydrostatic equilibrium, but not so large as to begin the fusion process, need a label. I would suggest that the label should be "planet."

I agree with you here, this seems like a fair definition. It also accounts for the rogue planets I was wondering about since by IAU's defintion, they aren't planets at all. Hopefully we'll have a better classification system in the future... we really need it.

Right. So can we please use the definition nearly everyone uses - the IAU one? And not argue about it? That was my point.

I think what Glitch was trying to get at was the fact that IAU's definition is flawed. Although we're essentially forced to use it at the time, it can easily lead to flawed categorization of celestial bodies since (as you two figured out), it's very subjective. I understand your reluctance to use something else, however I do agree with him that we need a more rigid definition in the future.

 

[Vanadium 50]

See https://www.physicsforums.com/threads/why-is-february-short.712505/#post-4517711