I know that some of the asteroids are quite large (hundreds of miles across? I forget how big) – at what point do they get big enough to start looking like planets? Taking on the form of a sphere? Or is the material that they are composed of preventing them from being spherical?
Aren’t some of the moons in our solar system pretty tiny? Are some asteroids bigger than the smallest moons? What is the smallest natural sphere that we have found?
Wikipedia’s list of solar system objects by size.
Of course there’s no clear dividing line between “spherical” and “non-spherical”. But looking at that list, Mimas at 198.3 km radius seems to be the smallest object that looks nearly spherical, except for the huge crater. But there are a couple of objects that are slightly larger, but non-spherical. Most notably Vesta at 263 km mean radius.
This may come down to an opinion on what is considered spherical.
As a rough rule of thumb, objects smaller than 300 km in diameter don’t collapse in on themselves. That leaves Ceres, Pallas, and Vesta in the asteroids as large enough. But even Ceres has two different diameters: it’s 960 x 932 km. Does that make it spherical?
There are huge numbers of moons smaller than Vesta, which has a diameter on that page of 530 km. This Wikipedia page lists all of those between 200 and 400 km radius (400 - 800 km diameter) and claims that 200 km radius is the cutoff.
That would put Proteus, a moon of Neptune, as the smallest spherical moon and a few trans-Neptunian objects as slightly smaller. They also list Vesta as the smallest asteroid.
I’m pretty sure that the cutoff is arbitrary, since they go from a radius of 199 to one of 200.5. Not enough to make the body behave differently. If Vesta is borderline, then most of those below it also are.
Slightly ninjaed.
Ceres might be better described as an oblate spheroid.
That applies to most, if not all, sizable objects in our system.
Fascinating! Thanks for the link and the info.
I was going to answer “my balls”, but thought better of it.
Obviously there are a lot of objects that are almost, but not quite spherical due to their rotation (e.g. oblate spherioids such as the Earth and the other planets) and there are also some objects that don’t even look spherical, but would be if they weren’t spinning so fast (e.g. the dwarf planet Haumea). Objects that are in hydrostatic equilibrium will settle down to be very close to spherical if they are not rotating and the smallest known object in the solar system to be in hydrostatic equilibrium is Mimas and certainly describing it as spherical would certainly be no great liberty.
What’s a sphere, anyway?
Ask Science!
Computing the Sphericity and Roundness of Rocks
Heather Dunlop
Dec. 1, 2005
(Interestingly, for a robotics study. Mars, I guess.)
Math, charts, for the taking.
See also Wiki “Sphericity.” Wiki “Sphericity scale” is empty; use Dunlop for leads.
From the context of the question, OP is apparently referring to astronomical objects.
Otherwise, I was about to mention that I have a small jar of protons on my desk, that I bought as a souvenir at a bazaar I visited in Marrakech a while back. I was wondering if those are actually considered sperical?
They’re made of three quarks, as we now understand. Are quarks spherical? I know they’re really really small.
And that’s assuming we actually have a meaningful definition of “size” at those quarcical scales. Isn’t there a law that, while you can measure an object’s size or shape, you can never determine both at once?
(ETA: I can’t picture how you would put together three spherical objects to make a larger spherical object. Try it will billiard balls sometime. It doesn’t work.)
I asked this a while back…
Can’t remember search terms or I’d find it…
The Usual Suspects get this question a lot.
Also, Senegoid, that’s a fine question but a textbook example of what should be a freestanding thread. I’ll leave the honors to you…
Hey, those are mine!
Signed:
Muster Mark
Could arbitrarily small objects have formed from collisions that produced melts that slowly cooled?
Which means that the OP should have been “What’s the smallest object in dynamic equilibrium?” That is, the shape is determined mainly by the effects of gravity and rotation.
It turns out that the answer depends on the composition of the object. Rock is stronger than ice, so rocky bodies have a larger diameter at which they can be in non-dynamic equilibrium.
Take Vesta, for example. It’s not in dynamic equilibrium or else it would be spherical. Instead, it’s definitely oblate, but that oblateness does not come from rotation. Originally it was spherical, but that was when it was warmer and the rocks a bit softer. After it cooled off, it took a couple large hits from other objects that happened to be at almost the exact same place. These hits knocked off about 1% of Vesta’s mass and left it out of round. The craters are centered at the south pole, although the south pole was not necessarily where the hits took place.
So Mimas is probably about as small as an icy body can be and be in dynamic equilibrium, while rocky bodies will have to be somewhat larger than Vesta.
Hmm, the context of your question is questionable. If it orbits the sun, its a planet. If it orbits a planet, its a moon. Size doesn’t matter. This is essentially the crux of the argument.
Replacing “is spherical” with “is in dynamic equilibrium” doesn’t help much, since just in the same way we can dispute how spherical is “spherical enough”, we can also dispute just how close to dynamic equilibrium is close enough. After all, an object with a mountain can’t be perfectly in dynamic equilibrium.
There are many objects, some fairly large, that orbit the sun and are not planets.
And, altho there’s no agreed upon defining line* in theory, there are millions of objects that orbit planets that are not “moons”- for example the Rings of Saturn.
Some searching on wiki seems to show the moon Methone as the smallest that may be in hydrostatic equalibrium.
Too bad we don’t have a better picture of Polydueces, but it is smaller at 1.3 ± 0.4 km.
I didn’t really realize how many moons were in the Saturnian system.
Only about a mile across. Normally such a small object would not be in equilibrium, but if its interior has been melted by tidal heating, then of course it would be.
As we can see by comparing glaciers to mountains (one moves, the other does not) It would seem there are two different answers depending on the composition. Ice would have a substantially lower mass of spherocity. If we add gas giants into the mix, there’s another critical mass, at which a ball of gas will coalesce into a sphere rather than dissipate into space.
I think we’re going to have to dump the idea of planets having a discrete number of moons, and simply accept that many planets are surrounded by rubble of all sizes.