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B What do you call rocks that don't orbit the sun?

  1. May 6, 2016 #1
    I understand the difference between comets, asteroids, and meteoroids. I was just reading about asteroids recently, and it occurred to me that when I read anything about these three types of objects the implicit (and sometime explicit) assumption is that they only apply to things that orbit the sun. I realize that this is probably because we are not (yet) able to detect them elsewhere. At least, not that I know of.

    From what I understand, asteroids always refer to things in the asteroid belt. Comets refer to things that come from the Kuiper belt or Oort cloud. Meteoroids I'm not sure about other than they refer to things smaller than asteroids I believe.

    I have tried to find the "official" definitions for these things. And by that I mean what an astronomer would call them, but I keep finding conflicting information.

    What are the official definitions of comet, asteroid, and meteoroid?

    Are there different names for space rocks that are like these three, but that orbit other stars, or that don't orbit a star at all?
  2. jcsd
  3. May 6, 2016 #2


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    No, an asteroid is any large lump of rock orbiting the sun and not confined to the asteroid belt
    The asteroid belt just happens to contain a large number of them

    There are 2 main styles of meteors
    1) randomly moving ones, they may or may not have origins in the solar system
    2) meteors that are mainly dust to small objects that belong to the dust trails from comets
    The comets and dust trails do orbit the sun in highly elliptical paths
    The earth is passing through one of those trails at the moment from Halley's Comet
    Eta Aquarid meteor shower

    does that help ?

  4. May 7, 2016 #3
    Comets and asteroids circling other stars are just comets and asteroids, though some people use redundant terms like 'exocomet'. There are plenty of stars with dust disks or asteroid belts visible as an excess of infrared radiation around the star, so it is certain that such objects exist. They are mostly too distant to be resolved by any planet-hunting techniques, though there are remarkable exceptions. In beta Pictoris system astronomers have counted ~500 individual comets: http://www.eso.org/public/news/eso1432
  5. May 7, 2016 #4
    Comets are small objects that emit gases due to solar radiation and their orbits are generally highly eccentric. But a comet may run out of gas, and its orbit may be circularized by interactions with other objects. The dividing line between comets and asteroids is actually quite blurry. There are comets that do not come from the Kuiper belt or (still hypothetical) Oort cloud, but from between the outer planets.

    You can find the current definitions here: https://www.iau.org/public/themes/naming/

    By the way it is dumb that 'asteroid' ("star-like") is still the most common term in English for chunks of rocks that couldn't be less similar to stars.
  6. May 7, 2016 #5
    Thanks for the answers.

    I have done some more research, and it looks like the definitions of these things maybe aren't as firm as I thought. Or at least some people use the terms loosely. I read Ratman's link, and they call them exocomets, as he pointed out. I don't know if the term is redundant though. I think maybe they are just trying to be specific. Because some people, like at the below link, specifically say comets and asteroids are objects that orbit the sun:


    The woman that wrote the article worked for NASA for a while, and seems to know a lot about astronomy. Then again, maybe she didn't mean it as literally as I am taking it, since the following link from NASA uses asteroid to describe rocks orbiting other starts too:


    I guess Ratman is right. Comets and asteroids are just comets and asteroids no matter where they are. So the only difference seems to be comets are big, and have a high ice content. Asteroids can be as big or bigger than comets, but are mostly rock or metal, and meteoroids are just small asteroids.

    As a bonus to myself I now not only have a better understand of what these things are, but I know we have found them in other places in the galaxy, which I had no idea about before. It always good to ask questions. You never know what you might find out.

    Thanks again, both of you!
  7. May 9, 2016 #6
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