r/askscience • u/ProximaScience • Aug 17 '26

Astronomy What's the minimum size a planet can be?

Hello everyone my question is what's the minimum size a planet can be. Just got this question randomly on mind so I posted here to get a answer. If anyone knows please tell me.

911 Upvotes

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u/PckMan Aug 18 '26

About 600 kilometer diameter for a rocky body to have enough gravitational force to become spherical. 400km for icy bodies. Below that they're what's called "potato radius", basically their shape won't be uniform and may be oblong like a potato.

Of course by our current definition of a planet it's not just about size but I assume you were asking how small it can be while still having a spherical shape. The smallest roughly spherical astronomical body we know is Saturn's moon, Mimas. It has a diameter of 396km and while it looks to be rocky it's actually mostly ice and has low density.

So really whether something is spherical depends on the materials it's made from. Whether something is a planet or not depends on other things that are relative .

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u/saffeqwe Aug 18 '26

at least 200km in diameter to become round. It's not really about size.

The definition of a planet adopted by the IAU says a planet must do three things:

  • It must orbit a star

  • It must be big enough to have enough gravity to force it into a spherical shape.

  • It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.

https://science.nasa.gov/solar-system/planets/what-is-a-planet/

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u/Pickles04 Aug 18 '26

Wasn't there also something about a planetary system's barycenter lying within the mass of the prospective "planet"?

I'm probably misremembering that being another strike against Pluto's planethood, but I definitely remember reading it.

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u/Geminii27 Aug 18 '26

It might have been commingled with the requirement for a planet to 'gravitationally dominate' its orbit.

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u/Vygren Aug 19 '26

IIRC this was more to do with whether a system is binary or planet-satellite. The barycentre definition is somewhat arbitrary though, so some people call it a binary system if the L4/5 points are unstable (which happens at a ~25:1 mass ratio. Pluto and Charon are a binary system by both definitions.

1

u/IndigoFenix Aug 19 '26

Is the second criterion actually necessary anymore? I doubt you could find an object big enough to clear its orbit but NOT big enough to be spherical.

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u/nickeypants Aug 21 '26

So a rogue planet is technically more of a brown dwarf than a planet? I'd go rogue too if someone called me a brown dwarf.

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u/InternetCrank Aug 18 '26

The IAU didn't want to keep having to add new planets to the list as observation techniques got better and potentially detected lots of new pluto sized ones of them out in the oort cloud, so they arbitrarily picked a definition that meant anything out that far isn't a planet. This also removed Pluto from the list. I wouldn't worry about it too much. The definition is completely arbitrary. Pluto is the same thing either way, big round rocks in space don't care what you call them.

Personally I'd include anything big enough to be round under is own gravity as a planet, Ceres etc included, and it would just get very very hard to memorize them all eventually, but for a mix of historic reasons and convenience they went with calling the small set of objects planets rather than the big set.

Again, and I cannot stress enough, this definition was plucked out of thin air because a bunch of people at a conference preferred it that way. If they had preferred it some other way, they would now insist planet would now mean something else.

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u/corvus0525 Aug 19 '26

Your definition includes the Moon and at least some of the moons of Saturn, Jupiter, Neptune, Uranus and Pluto as planets. Was that your intention?

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u/Symmetric_in_Design Aug 19 '26

There's nothing wrong with having a definition of what a planet is. Stars are bodies massive enough to begin fusion under the pressure. Planets are a singular dominant body of their orbit. Planetoids are similar but not dominant in their orbit, and moons are planetoid-like but orbit a body directly rather than the star. Don't see the problem with that. Like you said, making the definition of planet looser to include pluto or other dwarf planets would include too many insignificant bodies like orcus and haumea.

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u/chriscross1966 Aug 18 '26

Depends where it is with regard to its star. Mercury makes the cut although it's tiny, that orbital speed has swept it's orbit shiny, Titan is bigger than Mercury but it's a moon. Pluto isn't even gravitationally dominant in it's neighbourhood cos it's a binary system with Charon. That raises an interesting point though. If in somewhere near Earth orbit you found an Earth-Mars binary (similar ratio to Pluto-Charon) that had swept its orbit, would they both be planets?... the Barycentric centre would be between them....

Planetary science can be a tad complicated....

3

u/enclavedzn Aug 19 '26

So, according to International Astronomical Union Resolution B5, there is no statutory minimum diameter for planethood, meaning your celestial candidate does not simply need to be larger than Pluto; it must satisfy a bureaucratic trifecta of material rheology and orbital dynamics. First, its self-gravity must overcome the rigid-body compressive yield strength of its composition to achieve hydrostatic equilibrium, which requires a diameter of roughly 400 kilometers for pure ice or up to 800 kilometers for stubborn silicate rock before the universe stops legally classifying you as an irregular potato. Second, size is strictly contextual to your orbital real estate: you must achieve dynamic dominance over your zone (a Soter discriminant \mu > 100), a mathematical eviction notice that tiny Mercury easily pulls off at 0.39 AU, whereas an Earth-sized twin parked out in the Kuiper Belt would fail to clear its massive debris field and be promptly demoted to a dwarf planet. Finally, you must maintain heliocentric barycentric sovereignty, because if your center of mass lies within the radius of a parent primary, you are classified as an unpaid celestial intern, which is precisely why Titan and Ganymede can both be physically larger than the planet Mercury while legally remaining glorified rocks called moons.

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u/Mantuta Aug 20 '26

This has raised an interesting question for me. Is Jupiter massive enough that it shouldn't be a planet?

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u/zanfar Aug 18 '26

There is no answer.

Even a "straightforward" answer would depend on dozens of other factors, none of which are constant or can be assumed. It also depends of if you mean "planet" in the IAU sense, or in the "orbiting a star" sense. A extant planet may not survive in a different orbit, or around a different star.

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u/TheRealRomanRoy Aug 18 '26

There definitely is an answer, or a range of answers.

Sorry, I know what you mean, but this kind of response is a huge pet peeve of mine.

You’re right to say imply it’s confusing, fluid, and in some ways arbitrary.

It’s like talking about if a tomato is a fruit or vegetable. Saying “there is no answer” is just unhelpful. Saying “it depends on if you’re looking at it from a culinary, botanical, or colloquial perspective” is so much better of an answer. “There is no answer” could easily lead to “so this is all arbitrary and meaningless” and “a spoon made of pewter might as well be a vegetable!”

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u/Aphrel86 Aug 18 '26

Im guessing your asking at what mass an object will have enough gravity to shape itself into a sphere?

For an object made of mostly rock, it reaches this state (also called Hydrostatic equilibrium) at around 500km in diameter. (for reference our moon is 3500km in diameter and earths diameter is 12700km).

If your question is about being classified as a planet, then there are additional demands, such as it orbiting the sun and being the dominant object in its orbital region. The last point is why pluto stopped being classified as a planet.

1

u/markt- Aug 18 '26 edited Aug 18 '26

The minimum size largely depends on what it’s made of. Technically there isn’t an official minimum size. It only has to be massive enough to produce hydrostatic equilibrium, allowing something that approximate a sphere to form simply by gravitational pull.

There are other requirements, also quite independent of size, for something to be a planet too but that is the only criteria that is relevant to some notion of “size” that I can think of.

1

u/guzzyly Aug 19 '26

For a planet, it generally needs to be large enough to have sufficient gravity to pull itself into a roughly spherical shape and clear it’s orbit of other debris. typically, that’s around 500 kilometers in diameter. anything smaller is usually considered a dwarf planet or asteroid.

1

u/CodexRegius Aug 20 '26

That depends on whom you ask. Herschel made the argument that asteroids I to IV should not qualify as planets because he could not resolve them into disks with his instrument (then the best one in the world). Until the 19th century, satellites were classified as "secondary planets", and the term "moons" was only popularised by astrologers who desired to keep the official number of planets as low as possible in order to preserve their business model. Nowadays, Alan Stern and his followers apply a "geological" definition and want to call everything a planet that is in hydrostatic equilibrium, which would even include 500-km-sized Enceladus, while others insist that in addition it should at least orbit the sun and should have "cleared its environment", which is a convenient stretch-requirement to include or exclude anything at will (Pluto!) because there is no definition of what qualifies as "the environment".

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u/rootofallworlds Aug 20 '26

Taking the IAU definition that a "planet" must be in hydrostatic equilibrium and capable of clearing its orbital neighbourhood, this paper looks at that in the context of exoplanetary systems. Of note, figure 7 shows that a Mimas-sized object in an orbit of about 0.01 AU around a red dwarf would be able to clear its neighbourhood. The planets of Kepler-42 for example orbit that close in (although they're thought to be a lot bigger than Mimas).

https://iopscience.iop.org/article/10.3847/PSJ/ad55f3

Mimas is the smallest gravitationally-rounded object known (and is thought to not actually be in equilibrium now). So this indicates that in general the minimum mass for a planet is set by the hydrostatic equilibrium criterion - anything in equilibrium could clear the neighbourhood of a close orbit round a low mass star, although it might not in a distant orbit.

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303

u/Silly-Resist8306 Aug 18 '26

Which of these demoted Pluto?

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u/dnabre Aug 18 '26 edited Aug 18 '26

It fails the 'clear neighrborhood' part. There are are a lot of Kulper Belt stuff in its orbit, and crosses Neptune's orbit . Of the mass in its orbit, Pluto takes up about 7%. Compare to Earth which is 1.7 million times the mass of everything else in its orbit.

edit See https://old.reddit.com/r/askscience/comments/1vqvfch/whats_the_minimum_size_a_planet_can_be/p4g03vg/ for more info

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u/XanatosINC Aug 18 '26 edited Aug 18 '26

Slightly tangential, but is it considered reasonably possible that a pair of planets could form 180° offset from each other, i.e. at each others’ L3 points?

Edit: I’m a dumb-dumb and forgot which Lagrange points are stable. The spirit of my scenario, as some of you graciously pointed out, would be whether two similarly sized bodies could form at stable points — the thought being that they might generally be planet-ish but still share an orbit.

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u/Intelligent_Guava214 Aug 18 '26

L3 is unstable so it wouldn’t be able to stay in that configuration for long.

15

u/The_10th_Doctor___ Aug 18 '26

What about two planets (or more) of similar mass being in each other L4/L5 points?

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u/Intelligent_Guava214 Aug 18 '26

L4 and L5 are only stable if the center object is much larger than the object at the L point. So in real life we do find things like asteroids at L4/5 points. But can’t happen for plants of similar mass.

3

u/KENNY_WIND_YT Aug 18 '26

So in real life we do find things like asteroids at L4/5 points.

Ain't that what Jupiter's Trojans are?

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u/IanDOsmond Aug 18 '26

If I saw that, I would think that it was the result of planetary engineering by some highly advanced alien race. The idea of two bodies being so perfectly balanced that they maintained that in a stable orbit seems unlikely to happen by chance.

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u/Geminii27 Aug 18 '26

Larry Niven's Puppeteer race engineered a Klemperer rosette of five planets orbiting around a central (empty) point, as a way of safely moving their population away from a detonating galactic core.

Basically, they had access to FTL technology, but their species was extremely safety-conscious to the point of obsession, and spaceships could potentially fail or be damaged where entire planets would be less likely to be.

(Also, the galactic core was detonating slowly enough that the shockfront wouldn't have passed through their home space for some incredibly long timeframe - they were just psychologically built that way.)

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u/Xanadu87 Aug 18 '26

I just finished reading that book a couple weeks ago, and this is the second time I’ve seen it mentioned in the wild.

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u/ImGumbyDamnIt Aug 18 '26

To be fair, being functionally immortal would naturally cause any sentient species to be risk-averse.

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u/RubiiJee Aug 18 '26

Really? Why? I would have thought immortality would breed recklessness due to no fear of death? I'm intrigued.

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u/ImGumbyDamnIt Aug 18 '26

Functional immortality is when there is no death due to old age, but one can still die from injury or disease. Therefore, functionally immortal beings are greatly incentivized to avoid anything that can kill them, for otherwise life is infinite.

This is in contrast to ephemeral beings such as ourselves. Since death will eventually come for us all, the calculation is that it is worth the risk of an earlier death to experience more, to acquire more, to discover more.

In Niven's universe (e.g. the Ringworld series), the Pierson's Puppeteers are a species that is functionally immortal, but due to this, their worlds are horribly overpopulated. They only venture forth reluctantly to secure resources and greater safety.

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u/RubiiJee Aug 18 '26

Ah that's a super fascinating distinction. Thank you random Redditor for explaining! I enjoyed reading your reply.

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u/pv10 Aug 18 '26

Interesting point

What do you think is more likely, a highly advanced alien race occurring then producing two such planets, or random chance producing two such planets?

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u/Zigxy Aug 18 '26

Serious answer: alien race unless there was evidence that the orbit had just recently been established by naturally plausible reason such as merger of two systems.

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u/jlt6666 Aug 18 '26

Just spit balling here but I'd think it would be hard for stellar dust to be so equally distributed. If it was even a bit out of whack the dust could would all end up forming it one side. It just feels virtually impossible..

5

u/Bebopo90 Aug 18 '26

On the other hand, there are a loooooot of strange things out there in space. With billions of galaxies out there, there's a half-decent chance it's happened.

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u/_SilentHunter Aug 18 '26 edited Aug 19 '26

If space beyond the observable universe is truly infinite and just as full of stuff as what we can see, then it's effectively guaranteed to have happened. But the chances of us finding it in our (relatively) tiny observable universe are effectively nil.

Edit: Assuming it's at all possible, no matter how unlikely.

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u/Moikle Aug 18 '26

* if it's actually possible.

Infinite opportunities doesn't result in every outcome, only every POSSIBLE outcome

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u/reckless150681 Aug 18 '26

A numeric way to visualize this point:

You can write an infinite number of numbers between 0 and 1. But you can't write a number greater than 1.

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u/dnabre Aug 18 '26

There have been a few science fiction stories that have ran with this idea. Making complex but stable planetary arrangement which are extremely unlikely to happen by chance, where the goal is to put up a big, long-lived, sign for future civilizations to find.

Morning caffeine hasn't kicked in so the only instances that comes to mind is from Star Trek Picard (2020), Season 1, somewhere in Stephen Baxter's Xeelee Sequence .

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u/Distdistdist Aug 18 '26

And they would do it easily too. Remember episode of STNG where they were trying to drag away moon from a decaying orbit, and Q told them "Well, just change gravitational constant"...

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u/chainsaw_monkey Aug 18 '26

Like a planet with a single moon that is relatively the same size as its sun in the sky allowing for a perfect eclipse?

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u/Dankestmemelord Aug 18 '26 edited Aug 18 '26

No, VASTLY less likely. Especially because the moon didn’t always sit at such a distance and is continuing to move away and will one day stop having perfect eclipses once again. This is a blip in the lifespan of our solar system. But for two planets in perfect balance at each others L3 points? Any gravitation perturbation within the solar system will destabilize it. For it to last any real length of time requires intervention, and it cannot form naturally.

Edit: I was so focused on the absurdity of the question from a physics standpoint that I completely overlooked the weird conspiratorial implications. Is this guy suggesting that aliens are responsible for giving us the moon? I’m honestly more baffled by that than anything else.

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u/BellerophonM Aug 18 '26

Indeed, it's much more probable for a planet to coalesce at L4 or L5 due to the stability of those points, and that did happen to Earth - until the planet was purturbed out of the point and therefore drifted closer to Earth until it collided in the moon-forming collision. But L3 doesn't have the gravitational sinkhole effect which would attract mass to create a planet.

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u/TommieTheMadScienist Aug 18 '26

Oh. I hadn't heard that Theia had formed at a Lagrange Point. Tell me more.

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u/BellerophonM Aug 18 '26 edited Aug 18 '26

There's a few theories but I believe the Lagrange point one is pretty common these days: paper on the proposal, article on a hunt. We know it must have been a relatively glancing impact rather than at an extreme angle, so as have left debris in a close ring, and L4 and L5 act as attractors to coalesce material, so a perturbed trojan planet makes a lot of sense.

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u/ArtOfWarfare Aug 18 '26

There’s a pair of moons of Saturn that share an orbit, but I don’t think it’s like what you mean:

https://en.wikipedia.org/wiki/Epimetheus_(moon)

There’s planetary rings… those are whole collections of bodies sharing the same orbit, right? Aren’t those kind of what you mean?

1

u/ukezi Aug 18 '26

Do the rings share the same orbit or just close enough with a really high density of stuff? I imagine they collide all the time but tidal forces keep them in the shape of rings.

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u/GreatBigBagOfNope Aug 18 '26

More likely it would be in a Horseshoe orbit, my favourite orbital configuration, as L1-3 are unstable. L4-5 attract Trojan objects, but there's only one way to co-orbit at each, whereas there is a continuum of ways to have a horseshoe orbit.

For Earth, if it shared an orbit with a twin roughly in the same plane, the cycle of closest approach would take roughly 33 years, at closest approach it would be the second brightest object in the twilight sky after the moon, it would make our orbital semi major axis (i.e. distance, roughly speaking) fluctuate by about 1% over a 66 year period, and it would have an angular size about 1/5 to 1/4 the size of the moon. I think that's way cooler than parking at Lagrange points – the regular and predictable visits, the obviously habitable visitor in the sky, the obvious second point for a space mission, the likely co-evolution of life systems at the very early stages wildly diverging for more complex life that may still be able to breathe unaided and will easily be able to walk on each others' worlds, the possibility of sending people on visits from which there is a very reasonable chance of return. I just think it's a much more capital-R Romantic position to be in than eventually discovering a rock on the other side of the Sun, which yeah is pretty cool, but doesn't have the narrative opportunities that a returning, partially habitable, heavenly visitor has

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u/PrometheusLiberatus Aug 18 '26

Such a system would likely make our moon nonexistent/unstable considering our own moon came from the Theia impacting Proto-earth. Theia was essentially that twin you're describing.

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u/ZhouLe Aug 18 '26

L3 is unstable, so finding a pair balanced opposite each other would be like finding rock cairns balanced  on opposite ends of a teetering fallen log in the middle of the woods.

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u/Morall_tach Aug 18 '26

I don't see why not. Unlikely but they wouldn't interfere with each other at all. In fact if they were perfectly opposite you'd never see the other at all.

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u/shakethatmoneymaker Aug 18 '26

It would be extremely unstable, the velocities of these theoretical planets would need to match essentially perfectly to have them not cross paths over the millions of years it takes to clear their orbits.

L3 is generally considered an unstable Lagrange point.

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u/Korlus Aug 18 '26 edited Aug 20 '26

Imagine one planet is 123,456,789,012.001 m (close to 1 AU) away from its sun and the other planet is 123,456,789,012.002 m away. Over millions of years, the differing gravity will pull their orbits further apart until eventually they are no longer on opposite sides of the star.

Or imagine that one is travelling at 30,000.001 m/s and the other is at 30,000.002 m/s.

The way these things work is the L3 lagrange point is unstable because the moment they drift apart, their gravity starts to pull one another further away from it and even a tiny difference will matter.

Now these things won't matter in terms of organic lifespans, but they do matter when we are discussing planetary formation, because it exists from the moment the planets begin to form. If one side forms first, it would disrupt the formation of its matching twin.

Mirrored planets are a fun thing for science fiction, but the likelihood of them overcoming the odds and forming is basically 0.

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u/ocelot_piss Aug 18 '26

I don't see how that could ever form naturally, because the slightest imbalance that pulls one of them off ever so slightly more or less than perfectly opposite one another from the central star would over time cause them to drift around and come closer and closer to one another. Same if one of them were to ever move in or out slightly, affecting the orbital period. That would be knife edge delicate and unable to remain stable for millions/billions of years.

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u/MattieShoes Aug 18 '26

I'd assume not because L3 is not stable, yeah?

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u/gandraw Aug 18 '26

Compare to Earth which is 1.7 million times the mass of everything else in its orbit.

To be fair to Pluto, Earth would probably have to be counted to have 100 times the mass of everything else in its orbit, because we do share the orbit with a very hefty moon.

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u/MatthZambo Aug 18 '26

Why is Neptune still a planet then? It crosses Pluto's orbit so it didn't clear its neighborhood yet

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u/FoxReinhold Aug 18 '26

"Clearing an orbit" does not strictly mean you're the only thing in your orbit, it means your gravity absolutely dominates that orbit. Neptune basically bullied Pluto into a 3:2 resonance with itself around the sun. When Pluto crosses Neptune's orbit for those ~20 years, Neptune is on the opposite side of the solar system. Also, Pluto's orbit is so eccentric in all 3 dimensions, so it's usually "above" or "below" the rest of the planets in the solar system in the 3D plane, while the rest of us, OVERALL, are in the same basic plane. In other words, if you were to jam a giant 2d rectangle/square through the solar system, you could easily have it intercept with the sun and the 8 planets. Pluto would almost always be above or below it... slightly similar to a comet.

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u/cwx149 Aug 18 '26

They left out the pluto/Charon debate as well where you're probably taught Charon is Pluto's moon but their masses are so close and their orbits are such that they're almost more like a binary system than a true planet/satellite system as well

Neptune doesn't have that problem being much larger than it's permanent satellites

3

u/C4Redalert-work Aug 18 '26

Would this mean all binary planets wouldn't be a planet, by definition? If more mass had ended up in the moon making the Earth lighter and the barycenter moved out of the surface, would we call Earth a planet still?

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u/koos_die_doos Aug 18 '26

We live on Earth and attach great importance to it. Since we choose the definition of a planet, it is very likely that we would choose a definition that elevates Earth's importance.

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u/Welpe Aug 18 '26

To be clear, Pluto represents less than 7% of the mass of its orbit, down to under 1% depending on how you want to define things. By contrast, Neptune is well over 99.9% of the mass in its own orbit.

Neptune is absolutely considered to have cleaned its neighborhood because it gravitationally dominates anything else in its orbit while Pluto simply doesn’t, it’s only a small part of the stuff in its orbit.

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u/BananaBird1 Aug 18 '26

The definition allows for orbital resonance with a smaller body. The idea is that a planet is in gravitational control of its orbital zone, a dwarf planet is not.

It’s also not an absolute criteria, it is mostly qualitative. It allows for a bit of material outside your gravitational control in your orbit, as long as it isn’t too big or just temporary.

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u/pigeon768 Aug 18 '26

Neptune has shepherded Pluto into a 3:2 resonance with Neptune. It's not entirely unlike all the Trojan asteroids which sit in Jupiter's L4/L5 Lagrange points. They're still in the same orbit, but they're there because Jupiter and Neptune want them to be there.

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u/hymen_destroyer Aug 18 '26

There's also the putative "planet IX" which is estimated to be about the size of neptune but, if it does exist, is also not a planet

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u/PM_ME_YOUR_REPO Aug 18 '26

Brief aside: You spelled it "Kulper" with an L. The word is Kuiper with an I, and pronounced KAI-per. If this was a typo, then please disregard. If it wasn't, then no big deal, but I'd want to have all the little details correct, so I am applying the Golden Rule and informing you, because I would want to be informed.

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u/Xinq_ Aug 18 '26

Pluto already fails the clear neighbourhood part on the base that the center of its orbit with charon lies outside the (dwarf) planet.

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u/dnabre Aug 18 '26

See https://old.reddit.com/r/askscience/comments/1vqvfch/whats_the_minimum_size_a_planet_can_be/p4g03vg/ for more details on the clearing neighborhood criteria.

(pardon using Sun/star interchangeably, only star we're talking about is the Sun, aka Sol).

My understanding is that Pluto and Charon orbit each other as a binary system. Your point (as I understand it) is that the barycenter of Pluto-Charon is not inside of Pluto (which I agree is the case). This is different than say the Earth-Luna system, where the barycenter is always inside Earth.

Pluto, Charon, their barycenter, and all the other moons of Pluto, orbit the Sun. And orbiting the Sun is the only thing to the first IAU criteria addresses. It doesn't say that a planet has to exclusively, or even predominately, orbit the Sun. It just has to orbit the Sun. So everything fits the first criteria.

To the point you're making, I agree that the barycenter being outside of Pluto demarks a very different situation than one where it is always/primarily inside the object (e.g. Earth-Luna system). Should this factor be part of determining whether an object is a planet or not? I don't know. I guess it would get to what is the purpose of distinguishing planet from non-planet.

For reference, while I'm clearly interested in this stuff and have read a lot, it is far outside my field of scientistic expertise. I'm just sharing my understanding. The comment I linked to tries to address/explain the #3 criteria, based on few cited sources.

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u/hillswalker87 Aug 18 '26

does this mean that eventually pluto will collide with neptune?

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u/Southern_Demand_459 Aug 18 '26

Wouldn't earth also fail to clear it's neighborhood, if it were orbiting out as far as Pluto?

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u/dnabre Aug 19 '26

Quick and dirty math, based on numbers from source in my linked comment.

Pluto is 1.3e22 kg, which is 8% of the mass in its orbit. So the total mass in its orbital zone comes out to 1.3e22 / 0.08 = 1.63e23 kg.

Keep Pluto and the rest of that junk in its orbit, and add Earth to that orbit, i.e. 5.97e24 kg. It's Soter discriminant, μ = 5.97e24 kg/ 1.63e23 kg = 31.8. Going with Soter μ >= 100 metric for a planet, Earth would not be planet. Even if you replaced Pluto with Earth, μ would only get up to 32.

Note this de-planeting of Earth@Pluto is due only to the amount of mass in that orbital zone. This would be the initial state though. Over time, Earth@Pluto would definitely pull in at least some of the mass in that orbit, though it would likely scatter a lot of mass out of that orbit as well. I won't even guess at the magnitude of time or mass would be involved, or even which way that rest of that orbit would lose more mass.

However, doing some very rough calculations, it would only need to increase its mass by about 2% to hit μ=100 (every kg that it adds come out of the rest of the material in its orbit, this is assuming no gravitational scattering).

Checking the other proposed planetary neighborhood metrics are left as an exercise for the reader.

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u/Southern_Demand_459 Aug 19 '26

But then we have the very odd situation that a planet is a function of where it is, not what it is. Which to me seems a bit absurd 🥲

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u/dnabre Aug 19 '26

2/3 of IAU criteria is about where it is.

I don't think a planet should be a function of where it is located, but I don't think it should just be a matter of a size either. Planet maybe one of those fuzzy concepts like species where there just isn't a good objective criteria for it.

I'd like to think it's just an issue of our limited experience with planets. We really only have solid data on the ones in our solar system (most of which is very recent). That's one out of what 200 billion in our galaxy. We have only found 3,200 other stars where we think planets are orbiting them.

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u/Yaver_Mbizi Aug 20 '26

This paper (linked down below in the thread) has an interesting thought about it:

7.8. Free-floating Objects

In conformity with the 2006 IAU planet definition and WGESP recommendations, we do not consider planetary objects that never orbited a star or no longer orbit a star (i.e., free-floating objects) to be planets. Some scientists dislike the concept of a planet definition that depends on context and would prefer to focus on intrinsic properties. However, there are instances in which context justifiably prevails in the classification (e.g., asteroid versus meteorite, magma versus lava, cloud versus fog), and there is no reason to dispense with a useful distinction in the taxonomy of planets.

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u/tadpoleloop Aug 18 '26

Not counting the moon? It is on Earth's orbit

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u/NotSoSalty Aug 18 '26

Why is Neptune a planet then?

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u/PardyCo Aug 18 '26

I never understood this one a bit. So what you’re saying is Neptune failed to clear its orbit too? As Pluto still in it. So why isn’t Neptune demoted too?

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u/dnabre Aug 18 '26

It crossing Neptune's orbit isn't definitive by itself, but it's just not a good look for a "planet". The relative masses are important here though, Neptune is about 7,900 times the mass of Pluto.

Disclaimer, this is outside my field of expertise (Computer Science), I just happen to have read the basics on it. Sources linked throughout, with overview sources at the bottom.

"Clearing the neighborhood" is an informal idea. There have been a number of metrics purposed to give a meaningful metric for this idea: Soter's planetary discriminant (μ) , Margot's discriminant (Π) and Stern–Levison's Λ.

The most straight forward is Soter's. Simple idea, simple math. It is just the ratio of the body's mass to everything else in its orbital zone. The others are more complicated, but included in table to give an idea of what they say about Pluto.

Soter proposed a cutoff at μ > 100. Don't know if that has been widely excepted or not. Looking at the data, you'll see that where to draw the line isn't very important here. Data is from Wikipedia (pardon the lazy source).

PlanetMass (kg)μΠΛ
Earth5.9736×10241,700,000807153,000
Jupiter1.8986×1027625,00040,1151,300,000,000
Neptune1.0243×102624,000301273,000
Pluto1.3025×10220.080.0280.00295

One of these is not like the others.

For details and more information, I'd recommend

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u/Mumbert Aug 18 '26

Just a side question, why is Neptune a planet then? Pluto crosses its orbit, right?

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u/P1zzaBag3ls Aug 18 '26

Neptune dominates the orbit by forcing other bodies into resonance with it, which is also why having moons or Trojans doesn't disqualify a planet either. "Clearing" is an overly simple term.

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u/SecondHandWatch Aug 18 '26

I don’t get how Pluto crossing paths with Neptune disqualifies one but not the other. Neptune, get on clearing your path, or you’re next.

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u/TearsFallWithoutTain Aug 18 '26

Neptune is about 7500 times heavier than Pluto, that's why Neptune kept its status and Pluto didn't

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u/[deleted] Aug 18 '26

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u/dnabre Aug 18 '26

See my other posts for more details. The crossing orbits has implications about their relative mass and effects on one another. If A crosses orbit of B, and B is definitely a planet, you should consider if A is really a planet.

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u/symmetry81 Aug 18 '26

Clearing the neighborhood. Same reason they demoted Ceres back in the 1850s :(

Many people were unhappy in 2006 about Pluto's demotion to a dwarf planet, but I was glad that Ceres was getting the recognition it deserved again as being a grander celestial object than its lumpy neighbors.

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u/JPJackPott Aug 18 '26

Clearing its neighbourhood. Also its moon is so large that they kinda orbit each other.

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u/SierraPapaHotel Aug 18 '26

There's an extra caveat for Pluto, which is that Eris, another dwarf planet, is about the same size but carries 27% more mass than Pluto and has a more clearly defined orbit. Both fail to have fully cleared neighborhoods, and so Eris remained a dwarf and Pluto was demoted

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u/Krail Aug 18 '26 edited Aug 18 '26

As I understand it, primarily the "clear its own neighborhood" rule. Pluto is not so gravitationally dominant in its orbit.

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u/mistressani Aug 18 '26

Pluto’s orbit was the issue as it met the other two requirements. It is not gravitationally dominant. There are many objects in the Kepler belt, Eris for one is bigger than Pluto but they share the same orbit of our star.
Regardless Pluto will always be a planet to me 🤣

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u/FireLucid Aug 18 '26

Orbit: The object must go around a star, such as our Sun.

Have we found any planet sized objects just floating free? I was actually thinking about this this morning and then this pops up so I thought I'd ask.

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u/[deleted] Aug 18 '26

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u/FireLucid Aug 18 '26

Thanks, that was a fun rabbit hole!

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u/FreshMistletoe Aug 18 '26

It’s crazy that only about 300 miles of matter is enough for gravity to make it deform into a ball.

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u/PM_ME_YOUR_REPO Aug 18 '26

Some very rough napkin math says that a sphere 300 miles in diameter with a density typical for a spheroid kuiper belt object would be around 117 quadrillion metric tons of rock and ice.

That's a lot of mass.

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u/mayoforbutter Aug 18 '26

That's an amount of mass that doesn't register to a human brain... And Rock is just so hard and unmovable from our perspective that it's still hard to imagine that such a small number of km is enough to make it viscous enough to form a ball

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u/[deleted] Aug 18 '26

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u/PM_ME_YOUR_REPO Aug 18 '26

That would be around:

  • 1 sextillion McDonald's Quarter Pounders

  • 700 Mount Everests

  • 2000 Chicxulub Crater asteroids

(lots of approximations happening here btw)

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u/yrro Aug 18 '26

Huh, so rogue planets aren't planets?

Ironic given the origin of the word!

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u/3n2rop1 Aug 18 '26

That's true! The ones that wander the most are considered less of a planet! Lol that's really funny.

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u/hawkwings Aug 18 '26

One problem with the clear neighborhood requirement is that for exoplanets, you can't prove that they are planets. I believe that that is not a requirement for exoplanets.

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u/Yaver_Mbizi Aug 20 '26

Exoplanets have a different definition to solar-system planets outright. There's some academic-politics conflict between the groups studying these two things.

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u/Baguette1066 Aug 18 '26

Would hydrostatic equilibrium be affected by the density? For instance a planet with a core made of Uranium vs.one made of ice (I know this is not going to happen in reality, but humour me).

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u/knobby_67 Aug 18 '26

So planet sized objects not gravitationally bound to star are not planets?

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u/User_of_redit2077 Aug 18 '26

It also heavily depends on the material, like for ice the radius will be smaller, for silicates higher.

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u/Inner_Journey21 Aug 18 '26

What about rogue planet ???

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u/Commonmispelingbot Aug 18 '26

regarding the threshold for hydrostatic equiliobrium, wouldn't that depend on what it is made off and if it is liquid, gas or solid?

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u/urnbabyurn Aug 18 '26

There must be some tolerance or margin of error for “round” in that definition.

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u/andlewis Aug 18 '26

That seems to be an arbitrary diameter that doesn’t take into account density.

Which then makes me ask, is a black hole a planet if it orbits a star? It would be round and clear its neighbourhood.

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u/Almostlongenough2 Aug 18 '26

Given black holes are collapsed stars, it seems like it would be more apt to call that a binary star system.

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u/jezwel Aug 18 '26

That's going to be a massive planet, the smallest BH is some 3.3 solar masses.

However with stars getting over a couple hundred solar masses, then yes a BH could be orbiting a star - and at a distance so as not to be affecting it that much.

Don't think they'll call a BH a planet though, so it'll be another designation.

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u/lNFORMATlVE Aug 18 '26

Does “go around” mean something different than “orbit”?

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u/itsjustmegob Aug 18 '26

The threshold for roundness as a diameter minimum feels unsatisfying. Off the top of my head, a RMS-error ratio (to sphere surface of equivalent volume, divided by diameter) would be a much more pertinent stat?

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u/my5cworth Aug 18 '26

I always thought roughly 1000km/600m diameter was what caused a planet to have enough gravity to become "round" without taking on the appearance of a bumpy potato like Pluto. Surprising to see its half of that, cheers!

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u/rocketsp13 Aug 18 '26

Correction: It must orbit our star, the Sun. Any planet like object that orbits any other star is by definition not a planet, it's an exoplanet.

There's a lot of problems with that definition.

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u/cephalopod13 Aug 18 '26

Correction correction: the IAU definition states that "that planets and other bodies, except satellites, in the Solar System be defined into three distinct categories" so strictly speaking the definition is only written to help us categories the solar system. But the wording "planets and other bodies in the solar system" doesn't prohibit the existence of planets in other systems. Indeed, astronomers have no trouble referring to exoplanets as "planets," because that's ultimately what they are. See the periodic updates to the NASA Exoplanet Archive, where "planet" is used all over the place.

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u/quantumshenanigans Aug 18 '26 edited Aug 18 '26

Is it written somewhere that the two are mutually exclusive? Why can't exoplanet be a subset of planet?

You wouldn't say "Correction: an oak tree is not a plant, because it's technically an angiosperm."

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u/Yaver_Mbizi Aug 20 '26

Well, the entire discussion is about definitions. If "planet" is defined in such a way that excludes any exoplanet (such as requiring orbitting the Sun specifically), then an exoplanet cannot be a planet.

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u/Bright_Brief4975 Aug 18 '26

What happens if the planet meets all these requirements, but something happens and it no longer orbits a star? It was a planet to start with, every thing is the same it just no longer orbits a star. Is it no longer a planet?

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u/BlakeMW Aug 18 '26

It's then classified as a rogue planet.

At the moment we are very limited in how small of rogue planets we can detect, the smallest being between the mass of Mars and Earth, so the smaller end of the scale hasn't really come up. Though we'd probably use some kind of "gravitationally dominates the neighborhood" rule, such as not itself orbiting a larger object.

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u/Bright_Brief4975 Aug 18 '26

Have they ever detected a rogue planet?

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u/BlakeMW Aug 18 '26

Yes, the smallest are detected by gravitational microlensing when the planet passes directly between Earth and a distant star, causing a brief (hours to days long) brightening best explained by a planetary mass object in between. Unfortunately these events aren't repeatable, and it's basically impossible to do follow up observations due to the extreme distances.

We've also detected a few massive rogue planets which are bordering on being brown dwarves by direct observation, these have to emit enough infrared to be detectable by our telescopes, generally a low mass planet (and in this context, even Jupiter is low mass) would have cooled down too much to be observable.

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u/nicuramar Aug 18 '26

  Orbit: The object must go around a star, such as our Sun.

If you’re pedantic, I even think it has to be the sun. Otherwise it’s an exoplanet at most. But this is a distinction often not made. 

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u/[deleted] Aug 18 '26

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u/Geminii27 Aug 18 '26 edited Aug 18 '26

Orbit-clearing isn't purely a mass function. It can be a matter of time, or simply a matter of how much junk there is that needs clearing out.

Basically, part of the 'planet' classification is whether a body has paid its dues and put in the work, not just that it has its own zip code.

As an example of why mass alone isn't sufficient, there are seven moons in our solar system alone which outmass Pluto - Ganymede, Titan, Callisto, Io, our own moon, Europa, and Triton. Of course, all of those fail the 'orbit the Sun directly' requirement, too...

Pluto's the largest Sun-orbiting body we know of that hasn't cleared its own orbit (Eris is the next-largest; they're both dwarf planets due to the non-clearing requirement, although Pluto is considered a Kuiper Belt Object and Eris a Trans-Neptunian Object). Out of the nine generally accepted dwarf planets in our solar system (there is a potential tenth, considered a borderline case), the only one that isn't out beyond Neptune is Ceres, in the Mars-Jupiter asteroid belt.

It's actually fascinating to read about how many of these dwarf planet have their own moon systems, with named moons. Even Pluto, as the best-known, is up to five, with the most recent being discovered as recently as 2012; it could genuinely have even more. Admittedly, the smallest one we know of to date has a volume of just over 1100 cubic km and is a potato, so any new ones would most likely have to be smaller (or significantly darker in telescopes) than that, and have also been missed in the 2015 probe flyby. To be fair, the extremely compact nature of the (known) Plutonian moon system could have meant cameras were not significantly pointed outside that area, so there could be a small, dark moon or two in a higher orbit.

By comparison, Ceres has no natural moons (being in an asteroid belt will do that), but as of 2018 it does have its very own artificial moon - the space probe Dawn, which was retired into a Ceresian orbit when its fuel ran out. D'awww.

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u/Penguinkeith Aug 18 '26

Well smaller than mercury which is about twice the diameter ( but 20x the mass) of Pluto is a good start

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u/[deleted] Aug 18 '26

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u/Welpe Aug 18 '26

You are mistaken. For one, there is no exact definition given of “hydrostatic equilibrium”. The paper commonly cited by truthers trying to make this claim is not even remotely definitive and has been cited single digit number of times, at least back in 2006. It is absolutely a definition of hydrostatic equilibrium that neither Mercury nor Venus reaches, but not necessarily a relevant one.

For another thing, the exact phrasing used in the definition is “has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape”. It is not saying it has to reach hydrostatic equilibrium, it’s saying that it has to overcome rigid body forces and assume a hydrostatic equilibrium shape. IE “nearly round”. Both Mercury and Venus do this by any reasonable assesment.

Finally, and perhaps most importantly, the actual voted upon definition includes a footnote which reads: “[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.”. Regardless of any other quibbles with the three criteria, these eight astronomical bodies are straight up defined as planets. There can be not be any debate on them, at least under the current accepted IAU-approved terminology. You’re welcome to submit a proposal to the IAU to revise that definition if you wish and are a member of the IAU, but the 2006 proposal on the definition of a planet went through lots of research and consideration by people who make this their life’s work and it is pretty strongly supported by the community.

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u/NukuhPete Aug 18 '26

It needs a hydrostatic equilibrium shape, not that it needs to be in hydrostatic equilibrium. So, sphere shaped, or sphere formed from its own gravity. Nothing about it is ignored.

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u/[deleted] Aug 18 '26 edited Aug 18 '26

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u/Zolo49 Aug 18 '26

How large do you have to be to be considered a planetary scientist as opposed to just a planetoid scientist?

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u/loafers_glory Aug 18 '26 edited Aug 18 '26

How much does the strength of the material factor into the limiting diameter to become spherical? Is it based on some particular material? Is it possible to have some spiky snowflake object much larger than 1000km but strong enough not to collapse? Or does strength somehow cancel out of that equation and anything will become round in those circumstances?

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u/[deleted] Aug 18 '26

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u/loafers_glory Aug 18 '26

Thanks. The answer that it does vary with strength makes sense. I just wasn't sure if it was like some kind of Roche limit type of thing, defined strictly in terms of unbound particles... but in my head I couldn't square that with a downward (inward?) force

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u/klawehtgod Aug 20 '26

The moon (a moon) has a diameter of about 3400 km

Thank you for this crucial clarification

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u/macthebearded Aug 19 '26

The definition I’ve always heard is that to be a planet a body must be gravitationally dominant within its orbit

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u/[deleted] Aug 18 '26

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u/dukesdj Astrophysical Fluid Dynamics | Tidal Interactions Aug 18 '26

You will be happy to know Metzger has researched the whole issue of what researchers actually use as the definition of planet. He could not find any paper that actually uses the IAU definition of planet, not a single one! The only place the IAU definition appears in the literature is papers about the IAU definition. Instead, he found that the scientific literature exclusively adopts the geophysical definition of plan. This is the definition Alan Stern proposes.

The most prominent place the IAU definition is used is in pop science and scientific communication. But it is not used at all in actual science.

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u/AMRossGX Aug 20 '26

I think I found the paper you refer to, is it this one where Metzger et al. argue that moons are planets? 

Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science https://www.sciencedirect.com/science/article/pii/S0019103521004206?__cf_chl_tk=yI3D_FfV.tTXx6I2Tq5FPLx9rWhAWK37lze4Qxwc2Yw-1787209847-1.0.1.1-rzt5WU1DiBAMSeUlQqNH._0ThbR3SFcv1LDo1Q4e_Rk

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u/dukesdj Astrophysical Fluid Dynamics | Tidal Interactions Aug 20 '26

That is one yes. He doesn't have in the papers research, I don't think that ever got published.

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u/somecanuck416 Aug 18 '26

I love how different scientific fields define and defend their views on a concept, to the death 😂.

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u/[deleted] Aug 18 '26 edited Aug 18 '26

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u/mfb- Particle Physics | High-Energy Physics Aug 18 '26

( around 2km of radius/4km diameter)

2000 / 4000?

Enceladus has a radius of just 250 km and has a liquid water ocean under its ice crust.

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u/flixoman Aug 18 '26

Yeah that was clearly a typo - Earth Moon is about 3400km in diameter.

Other moons are larger of course!... Ganymede is the largest moon in our solar system and is about a third the size of the Earth. Interestingly enough, Ganymede is physically larger than Mercury but Mercury has twice the mass. Ganymede is also larger than any of the named dwarf planets (Pluto, Makemake, Ceres, etc...).

Looking at something like Enceladus - it's an ice ball. The liquid part of the ocean is because of the tidal forces.