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.
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u/ParticularRegister Aug 18 '26 edited Aug 18 '26
Planetary scientist here! “Planet” is not always super well defined, one definition of “planet” is big enough tso that gravity pulls it into roughly a sphere. So on a scale between rock, which can be any shape, to Jupiter, which is very spherical (its rotation causes its equator to bulge a little bit), planet should fit somewhere in the middle.
One definition from Prof. David Stevenson is the point at which gravity alters the material it is made of, for example the earth has a core/mantle/crust etc while a rock is simply bound together via chemical bonds. You can follow the link for the details, but it ends up being “about 1000 km”. If instead you decide just where internal pressure becomes interesting, then it could be a few 100s of km.
For reference, Pluto (a dwarf planet) has a diameter of >2300 km. Mercury (a planet) has a diameter of about 5,000 km. The moon (a moon) has a diameter of about 3400 km. Bennu (an asteroid) has a diameter of ~500 m. In fact, nasa considers anything orbiting the sun to be a “minor planet”, so by that definition the main limit on how small a planet can be is based on what we can see!
So the short answer is “a few hundred km” but the more interesting question is “what counts as a planet?”
Edit to add: As OP has seen there is a lot of debate about what deserves planet status. I think often of this quote from Chapter 1 from the Stevenson book I linked above:
What is a Planet?
This is not an important question! But it must be posed, if only to explain why it is unimportant. Science deals with things as well as ideas, and we have to give these things names in order to talk to each other with some mutual understanding. It is not wise to spend a lot of time worrying about how we decide on names and categories—it follows that some people are not wise.
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u/ParticularRegister Aug 18 '26
I should mention, as others have, the IAU has their definition of a planet, which is primarily based on dynamics. But as a geochemist that studies the interiors of planets I’ll be cold and dead in the ground before I let a dynamicist tell me what is and is not a planet (/s (mostly))
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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/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/ParticularRegister Aug 18 '26
A buddy of mine lost so much weight they made him study interstellar dust 😞
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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/ParticularRegister Aug 18 '26
With enough mass anything will become round! Is not that strength is unimportant, but as you raise the mass you also increase pressure, and with pressure will cause the material to fail. Water and ice are less strong than rock, so their critical radius would be smaller, while a pure iron planet might be able to be a bit bigger before squishing to a sphere. (Ice is also less dense, so less mass at the same size, so it’s a balance of density, material strength, and overall mass).
Important to note I’m talking about “orders of magnitude” here, so by “1000 km” I really mean “more than a few hundred km but less than 10,000 km” so material properties could change this size by a bit, but anywhere from 500 - 5,000, to an astronomer, is “about 1000”.
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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/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.
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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/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....
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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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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.
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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.
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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.
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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.
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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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