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Interstellar Exocomets

  1. Jun 14, 2015 #1
    I'm trying to find reliable research on the density of interstellar exocomets per cubic lightyear beyond our solar system, particularly in the direction of the Hercules constellation if possible. From this, I want to extrapolate the statistical probability of one being captured by our solar system, and then from there the probability of it impacting Earth. Has this been done?

    Side Question: I'm finding that it's more common that these bodies are labeled as "exocomets," is it not possible for asteroids to be ejected into interstellar space as well?
  2. jcsd
  3. Jun 14, 2015 #2
    'Exocomet' usually refers to comets orbiting a star other than the Sun, and a few of these have been discovered.
    Wandering interstellar comets are likely to be very rare, though not an impossibility, but as far as I know, none have yet been detected.

    Most (probably all) of the comets presently in the solar system formed from the same condensing cloud of gas and dust as did the Sun and other larger bodies.
    Today they mostly populate the Kuiper belt and the more distant Oort cloud, with only a few of them becoming gravitationally perturbed so that they develop an orbit which periodically takes them into the inner solar system.
    Other perturbations could result in an occasional comet getting kicked out of the solar system, but that's not likely to be a commonplace occurrance.
    Indeed much larger objects right up to the scale of small stars can become ejected from multiple body systems if the gravitational circumstances are just right, it's possible, but very unusual for this to happen.

    Why are you particularly interested in constellation Hercules?, I can't think of any reason why there should be more of such (very rare) wandering interstellar bodies in that particular direction.
    Bear in mind that the stars making up a constellation are not all at a similar distance from Earth.
    A star in any given constellation can be thousands of times more distant than another star of the same constellation.

    I don't think a reliable source for the density of small bodies wandering in interstellar space will be found.
    I doubt that the extremely sensitive and high resolution technology that would be required to do such a survey exists at present.

    Oh, and the chance of the Earth being impacted by such a body is miniscule, several orders of magnitude less likely than an impact with a regular comet or asteroid which originated inside the solar system.
    Last edited: Jun 14, 2015
  4. Jun 14, 2015 #3
    Thank you for the information. I brought up the Hercules constellation because I thought our solar system is headed in that direction. If that's the case, then we should be seeing more wandering interstellar comets in that direction, correct? Like your car hitting bugs on the road, easier to detect when you are heading right into them.
  5. Jun 14, 2015 #4
    Based on the current detection rate of such objects being zero, I'd say the probability of colliding with one would be more akin to your car hitting one specific insect that is somewhere within a 50km radius.
  6. Jun 14, 2015 #5
    I'd say that it would be (thread starters scenario) several times smaller odds... less than that to the 'nth' power. Not impossible but as close to that term as one can subjectively describe.
  7. Jun 14, 2015 #6


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    I've seen this paper referenced while reading some other papers on arxiv:
    Exchange of meteorites (and life?) between stellar systems. Melosh HJ
    It's paywalled, and I can't access it, but those other papers referred to capture and impact probabilities in it. Maybe you could ask somebody with a subscription to send you a copy, or email the author for one.
    From what I understand the paper is pessimistic (it's discussing viability of lithopanspermia), with low capture probabilities calculated.

    The issue here is that you don't want the incoming object to have much velocity w/r to the solar system, as this reduces the chances of a capture in a three-body interaction - the object must be able to bleed enough of its relative velocity so as to go below the escape velocity.
    The Sun having large velocity w/r the local standard of rest (assuming cometary material to be at rest w/r to the LSR) makes such a capture unlikely, leading to one-off hyperbolic orbits.

    The situation appears to be quite different in clusters of stars, where the low relative velocities and tightly packed neighbourhood allow for much easier transfer.

    I don't think I'd agree with rootone's assertion that ejection events are rare. Young systems especially eject material like crazy, with one paper estimating some 10^19 ejection events for >10kg cometary bodies over the lifetime of a cluster (~300 My), and another estimating as much as 90% of Oort cloud material being of extrasolar origin.
    There is also a question of what happens to the Oort cloud once a star leaves the main sequence. This paper:
    The Great Escape: How Exoplanets and Smaller Bodies Desert Dying Stars Veras et al.
    concludes that only a small fraction survives, suggesting a growing population of free-floating interstellar comets and even planet-sized bodies.

    Having said that, I don't know of any studies specifically estimating the density of cometary material in interstellar space.
    Last edited: Jun 14, 2015
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