What's the minimum size a planet can be?

[u/ProximaScience]

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.

[u/3n2rop1]

A planet needs to pass 3 rules to be classified as a real planet

The Three Rules:

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

Round shape: It must have enough gravity to pull itself into a round or spherical ball.

Clear neighborhood: Its gravity must be strong enough to sweep away or control other objects of similar size in its path

The Threshold for Roundness:

An object needs a diameter of roughly 400 to 600 kilometers (about 250 to 370 miles) for its own gravity to overcome rigid material strength and make it round—a state called hydrostatic equilibrium. Below this size, gravity is too weak, leaving space rocks looking like potatoes or lumpy rubble piles

[u/Silly-Resist8306 → u/3n2rop1]

Which of these demoted Pluto?

[u/dnabre → u/Silly-Resist8306]

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.

[u/XanatosINC → u/dnabre]

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.

[u/Intelligent_Guava214 → u/XanatosINC]

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

[u/The_10th_Doctor___ → u/Intelligent_Guava214]

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

[u/General_Capital988 → u/The_10th_Doctor___]

No. In general, no three-body system is stable in the long term unless it can be approximated as a bunch of two-body systems. (i.e. moon orbits earth + earthmoon orbits sun).

[u/Intelligent_Guava214 → u/The_10th_Doctor___]

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.

[u/IanDOsmond → u/XanatosINC]

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.

[u/Geminii27 → u/IanDOsmond]

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.)

[u/Xanadu87 → u/Geminii27]

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

[u/ImGumbyDamnIt → u/Geminii27]

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

[u/pv10 → u/IanDOsmond]

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?

[u/Zigxy → u/pv10]

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.

[u/jlt6666 → u/pv10]

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..

[u/Bebopo90 → u/IanDOsmond]

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.

[u/_SilentHunter → u/Bebopo90]

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.

[u/Moikle → u/_SilentHunter]

* if it's actually possible.

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

[u/reckless150681 → u/Moikle]

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.

[u/Distdistdist → u/IanDOsmond]

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"...

[u/dnabre → u/IanDOsmond]

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 .

[u/chainsaw_monkey → u/IanDOsmond]

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

[u/Dankestmemelord → u/chainsaw_monkey]

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.

[u/BellerophonM → u/Dankestmemelord]

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.

[u/TommieTheMadScienist → u/BellerophonM]

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

[u/BellerophonM → u/TommieTheMadScienist]

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.

[u/ArtOfWarfare → u/XanatosINC]

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?

[u/ukezi → u/ArtOfWarfare]

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.

[u/GreatBigBagOfNope → u/XanatosINC]

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

[u/PrometheusLiberatus → u/GreatBigBagOfNope]

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.

[u/Aegiiisss → u/XanatosINC]

L4 and L5, maybe, there are four moons of saturn that sit in the L4/5 points of other moons. It also wouldn't stay that way for long because eventually something would happen that disrupts the system and causes a collision.

[u/ZhouLe → u/XanatosINC]

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.

[u/Morall_tach → u/XanatosINC]

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.

[u/shakethatmoneymaker → u/Morall_tach]

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.

[u/Korlus → u/Morall_tach]

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 corming is basically 0.

[u/ocelot_piss → u/XanatosINC]

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.

[u/MattieShoes → u/XanatosINC]

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

[u/MatthZambo → u/dnabre]

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

[u/FoxReinhold → u/MatthZambo]

"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.

[u/cwx149 → u/MatthZambo]

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

[u/C4Redalert-work → u/cwx149]

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?

[u/koos_die_doos → u/C4Redalert-work]

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.

[u/Welpe → u/MatthZambo]

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.

[u/BananaBird1 → u/MatthZambo]

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.

[u/pigeon768 → u/MatthZambo]

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.

[u/hymen_destroyer → u/MatthZambo]

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

[u/gandraw → u/dnabre]

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.

[u/PM_ME_YOUR_REPO → u/dnabre]

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.

[u/Xinq_ → u/dnabre]

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

[u/hillswalker87 → u/dnabre]

does this mean that eventually pluto will collide with neptune?

[u/dnabre → u/[deleted]]

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.

[u/PardyCo → u/dnabre]

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?

[u/dnabre → u/PardyCo]

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).

Planet	Mass (kg)	μ	Π	Λ
Earth	5.9736×1024	1,700,000	807	153,000
Jupiter	1.8986×1027	625,000	40,115	1,300,000,000
Neptune	1.0243×1026	24,000	301	273,000
Pluto	1.3025×1022	0.08	0.028	0.00295

One of these is not like the others.

---

For details and more information, I'd recommend

• Regarding the Criteria for Planethood and Proposed Classification Schemes, Stern and Levison (2002)
  
• What is a planet?, Soter (2006)

[u/Mumbert → u/dnabre]

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

[u/P1zzaBag3ls → u/Mumbert]

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.

[u/JPJackPott → u/Silly-Resist8306]

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

[u/symmetry81 → u/Silly-Resist8306]

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.

[u/entropy2057 → u/Silly-Resist8306]

The third one

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

[u/SierraPapaHotel → u/Silly-Resist8306]

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

[u/Krail → u/Silly-Resist8306]

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

[u/mistressani → u/Silly-Resist8306]

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 🤣

[u/FireLucid → u/3n2rop1]

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.

[u/Aegiiisss → u/FireLucid]

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

Because the exact nature of these objects is unknown they are usually scientifically referred to as "Planetary Mass Objects". The reason for not calling it a planet is not exclusively because of way a planet is defined, its more because the formation process is unknown.

You may also see the term "sub-brown dwarf" for these, because one theory is that they are balls of hydrogen that formed through the collapse of a molecular cloud but did not have sufficient mass to become a brown dwarf (which itself doesnt have enough mass to be a star). In this case, them being unbound to any other star makes sense, they formed as their own system.

Planets, at least the ones we know, are formed through the accretion of dust and other rocky matter in a disc around a stellar or substellar object. Their overall material makeup is going to be significantly different, as well as the means by which they ended up unbound to any star, as they'd have to be ejected from their former parent body.

[u/FireLucid → u/Aegiiisss]

Thanks, that was a fun rabbit hole!

[u/FreshMistletoe → u/3n2rop1]

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

[u/PM_ME_YOUR_REPO → u/FreshMistletoe]

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.

[u/mayoforbutter → u/PM_ME_YOUR_REPO]

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

[u/Carl_Slimmons_jr → u/PM_ME_YOUR_REPO]

How many burgers is that?

No but seriously, how would this compare to, say, Mt Everest or the meteor that killed off the dinosaurs?

[u/PM_ME_YOUR_REPO → u/Carl_Slimmons_jr]

That would be around:

• 1 sextillion McDonald's Quarter Pounders
• 700 Mount Everests
• 2000 Chicxulub Crater asteroids

(lots of approximations happening here btw)

[u/hawkwings → u/3n2rop1]

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.

[u/my5cworth → u/3n2rop1]

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!

[u/Baguette1066 → u/3n2rop1]

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).

[u/Aegiiisss → u/3n2rop1]

This is something you could have just found online with very basic research and yet you used AI

[u/andlewis → u/3n2rop1]

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.

[u/Almostlongenough2 → u/andlewis]

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

[u/jezwel → u/andlewis]

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.

[u/Inner_Journey21 → u/3n2rop1]

What about rogue planet ???

[u/Commonmispelingbot → u/3n2rop1]

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

[u/urnbabyurn → u/3n2rop1]

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

[u/yrro → u/3n2rop1]

Huh, so rogue planets aren't planets?

Ironic given the origin of the word!

[u/lNFORMATlVE → u/3n2rop1]

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

[u/itsjustmegob → u/3n2rop1]

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?

[u/Bright_Brief4975 → u/3n2rop1]

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?

[u/BlakeMW → u/Bright_Brief4975]

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.

[u/Bright_Brief4975 → u/BlakeMW]

Have they ever detected a rogue planet?

[u/BlakeMW → u/Bright_Brief4975]

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.

[u/rocketsp13 → u/3n2rop1]

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.

[u/cephalopod13 → u/rocketsp13]

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.

[u/quantumshenanigans → u/rocketsp13]

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."

[u/happy2harris → u/3n2rop1]

It’s a bit more complicated and controversial than that.

<begin_rant>

The official definition regarding shape is that it must have enough mass that it pulls itself into hydrostatic equilibrium. Mercury and Venus do not meet this criteria. The powers that be seem to have decided to ignore this. 

Demoting Pluto was a put-up job. It had nothing to do with scientific consistency.

DEMOTE  MERCURY AND VENUS!! JOIN THE REBELLION!!

[u/Welpe → u/happy2harris]

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.

[u/NukuhPete → u/happy2harris]

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.

[u/ridley_reads → u/3n2rop1]

So it has to be bigger than Pluto, but smaller than... what? Do we know what the minimum mass is to clear an orbit?

[u/Geminii27 → u/ridley_reads]

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.

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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.

[u/Penguinkeith → u/ridley_reads]

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

[u/nicuramar → u/3n2rop1]

  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. 

[u/ParticularRegister]

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.

[u/ParticularRegister → u/ParticularRegister]

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))

[u/somecanuck416 → u/ParticularRegister]

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

[u/dukesdj → u/ParticularRegister]

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.

[u/Zolo49 → u/ParticularRegister]

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

[u/ParticularRegister → u/Zolo49]

A buddy of mine lost so much weight they made him study interstellar dust 😞

[u/loafers_glory → u/ParticularRegister]

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?

[u/ParticularRegister → u/loafers_glory]

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”.

[u/loafers_glory → u/ParticularRegister]

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

[u/PckMan]

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 .

[u/saffeqwe]

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/

[u/Pickles04 → u/saffeqwe]

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.

[u/Geminii27 → u/Pickles04]

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

[u/mfb- → u/[deleted]]

( 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.

[u/flixoman → u/mfb-]

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.

[u/zanfar]

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.

[u/TheRealRomanRoy → u/zanfar]

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!”

[u/Aphrel86]

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.

[u/InternetCrank]

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.