Interstellar asteroids

Could interstellar asteroids not orbiting a particular star exist? Could such a thing pass through our solar system?


I believe so, though I can’t find a cite on it. I don’t believe we’ve spotted one, since interstellar space is so far away and dark, asteroids are small and don’t shed light of their own.

Probably rocky/metal asteroids can’t form on their own away from a solar system, but there’s a slim chance that gravitational effects between larger planets could accelerate an asteroid to escape velocity, which would let it leave its home star’s gravity behind and start hurtling through interstellar space.

A variety of interstellar objects are expected. Those would include rocky or icy fragments, planet-sized objects and even brown dwarf stars that could be even a little larger than Jupiter (essentially, stars that didn’t quite make it).

Since it’s believed that stars may pass near each other, this could disrupt the orbits of asteroid-like objects and send them out into interstellar space. I suppose a star going nova might do the same. I’m not sure whether they would have been formed out there all by themselves.

So far as I know, we’ve never seen anything that was clearly interstellar in nature, but there are a lot of small space objects we’ve never seen at all.

Your question is a bit nonsensical; Strictly speaking, an ‘asteroid’ would come from the asteroid belt between Jupiter and Mars, so there can be by definition no ‘asteroids’ arriving from outside our solar system; an asteroid has to be in our solar system to be…well, an asteroid.

But as for meteors, comets and other space bric-a-brac, sure. This paper discusses the probability of such objects. Here’s another on finding the little sods and projecting their trajectories, among other things.

Oh, the links are .pdf.


Astronomically speaking the boffins apparently also use the term Minor planet, there’s also crossover with the term Small Solar System body, which includes many objects in the asteroid belt (except Ceres, due to the face that comparatively speaking it ain’t that small).

I am being a bit of a pedant, though;
Asteroid belts could exist around other stars, of course

Are there any upper limits on the velocity of such an object (besides the speed of light, of course)? I was curious if we could suddenly get the smack-down by some large interstellar object.


Sure, it’s possible, but very unlikely. Large chunks of solid metals are only going to coalesce around Population I (young, metal-rich stars), so a body would have to be ejected, either by interaction with another orbiting body or by some passing influence. Once it did, it would have to happen to fly through our Solar system, which occupies a very tiny fraction of local space.

If your plan is to take a ride to another system, you’re better off flagging down a passing spaceship. Just don’t let them read poetry to you.


Ok, that doesn’t support your claim.

Neither does this, although it gets close, if read favorably. Doesn’t speak to the origin of the object, though.

Do you have anything that indicates all asteroids must come from the asteroid belt, and that if it didn’t come from the belt, it’s not an asteroid? Because that’s what you said. Where did you find the definition that in order for an object to be an asteroid, it must originate with our solar system?

Is this not a contradiction of your previous claim? How you do reconcile asserting that, by definition, an asteroid must originate in our solar system (between Jupiter and Mars, no less), and then claiming asteroids may have been found in another system?

How does it not? "one of many rocky objects, varying in width from over 900 kilometres to less than one kilometre, which circle the sun. "
“1. Also called minor planet. Astronomy. any of the thousands of small bodies of from 480 miles (775 km) to less than one mile (1.6 km) in diameter that revolve about the sun in orbits lying mostly between those of Mars and Jupiter.”

Ergo, interstellar asteroids cannot exist. Unless you use the word ‘interstellar’ very liberally, in which case the term becomes nonsensical.

Note that I used the disclaimer “strictly speaking”; all definitions of the word ‘asteroid’ define them as coming from the asteroid belt of our solar system. This does not exclude them from existing outside it, but the parlance in common usage (and current definitions) definitely suggests it.

Well that’s definitely wrong. There are the trojan asteroids which orbit the sun in the same orbit as Jupiter (actually, they really orbit Jupiter’s Lagrange points, not the sun directly at all), and there are at least some (and probably a ton) of Neptunain trojan asteroids. The point is that “asteroid” isn’t a particularly precise term because it was coined when we knew a lot less about the solar system than we do now. So it’s really not an appropriate candidate for this type of pedantry.

Furthermore, although they’re much too small to see with current technology, I assume that there are probably uncountable similar objects in orbit around other stars, and they’d be classified as asteroids, just as we call extrasolar planets, planets.


Well, it’s irrelevant with respect to the OP, but IAmNotSpartacus said your definition said nothing about origin. So, a rock that was created around Alpha Centauri but that escaped orbit only to be caught by our sun’s gravity and settled down between Mars and Jupiter, would fit your definition of asteroid, but didn’t* come from* the asteroid belt.

As I said, it’s all tangential to the OP. An orphan rock could certainly exist not orbiting any star, but orbiting the galaxy’s center of gravity.

I’d be very leery of using the dictionary definition of a scientific term. Here’s the definition of “pterosaur”, to pick a random example:

So… the many members of the order Pterosauria that lived in the Triassic period don’t count as pterosaurs then?

Don’t worry; I was just pointing out a peculiarity in English vocabulary, we’re singing from the same hymn sheet. In any case I’m not going to argue about it.

Ignoring all the dictionary argueing.

Could a large chunck of rock or ice be passing between stars than being bound to one particular star?

Hell yes. Due to close encounters with more massive objects, stuff gets flung totally out of ours and other star systems “regularly”. When these systems were full of stuff and still in the early stages of forming, I would expect a good bit of stuff was flung out.

I seem to recall an Astronomy or Sky and Telescope article about 10 years ago that discussed such a thing. IIRC something NOT from our solar system is thought to pass through it with decent regularity.

We know there are interstellar comets since we have observed comets in our solar system on hyperbolic (greater than solar escape velocity) paths, usually after passing near enough to Jupiter to get a gravitational kick. The chances of something passing anywhere near us are very small however. To give you an idea of how vast interstellar space is, the nearest (known) star to our solar system, Proxima Centauri, is 7000 times further away from our sun than Pluto is. A solar system is a vanishingly tiny point compared to interstellar space.