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Are comets a feature unique to our Solar System? Or, are comets/cometary clouds detected around discovered/observed extra-solar systems too? If they were detected elsewhere, how do such cometary clouds affect discovery by perturbation of planets in that system ?

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Scale, again. The lightest exoplanet yet discovered is just over 1 Earth mass and close in. How big is your typical comet? –  dmckee Oct 17 '12 at 19:55
    
A fraction of Earth's mass. In other words, then, given the state of technology the question cannot be answered one-way-or-the-other ? –  Everyone Oct 17 '12 at 20:20
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Well, you asked "Are they known to exist?", which is answerable in the negative, but there was no reason to expect that it might be answerable in the positive. –  dmckee Oct 17 '12 at 20:42

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It is unlikely that comets are a feature unique to our Solar System. Since comets are simply remnants of star and planetary formation, then anywhere stars and planets have formed would be fertile ground to expect comets.

Their individual masses are relatively very small compared to discovered planets. For example, Halley's Comet has a mass of roughly $2.2\times10^{14} kg$ compared to roughly $6\times10^{24} kg$ for the Earth. That's a factor of 30 billion times smaller... so it is also unlikely that the same techniques used to discover Earth-sized or larger planets would find comets, too.

However, although they're not likely to be detected, given their prevalence in our planetary system, and given that they form from natural processes, and given observational evidence of other planetary systems, it is not unreasonable to infer their existence in other systems.

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For a bit more scale, the recently discovered planet around Alpha Centauri B, which is at least earth-sized and orbits a sun-size star closer than Mercury would, imparts on its star orbital velocities only slightly faster than a toddler. Detecting a comet would require measuring velocity shifts with angstroms-per-second precision on a star light-years away. –  Emilio Pisanty Oct 17 '12 at 22:21
    
I'm sorry I was unclear in my reference to perturbation in this context. The question is updated now to clarify. –  Everyone Oct 18 '12 at 9:59
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My answer wandered a bit as well. I hope this more specifically answers your updated question: Are comets unique? No. Ours is an average system, nothing terribly unique going on. Are they detected in other systems? No. Because they have too little mass individually, and are too spread out generally. Do they affect the discovery by perturbation of planets? No. For the reasons above. –  OrangeWombat Oct 18 '12 at 13:09

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