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How can an extrasolar comet be detected - this is based on an article regarding the influence on a extra solar gas giant on these comets causing changes in their orbit and leading to them being swallowed by the star .. Still stuck on understanding how we detect them .. they certainly have no measurable effect on the movement of the planet or the star .. and the chemical make-up of the comet seems irrelevant given the size of the star and observations we can make ? Just missing something Simple I think ?

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The chemical make-up of the comet is indeed relevant. From Wikipedia (Exocomet):

The exocomets can be detected by spectroscopy as they transit their host stars. The transits of exocomets, like the transits of exoplanets, produce variations in the light received from the star. Changes are observed in the absorption lines of the stellar spectrum: the occultation of the star by the gas cloud coming from the exocomet produces additional absorption features beyond those normally seen in that star, like those observed in the ionized calcium lines. As the comet comes close enough to the star, cometary gas is evolved from the evaporation of volatile ices and dust with it.

This is not to say that you can determine anything else (besides their existence and chemical composition) about the comets.

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