Mon. Not. R. Astron. Soc. http://doi.org/ngv (2013)

Comets are chunks of rock and ice that, when sublimated by the Sun's radiation, form a distinctive tail of dust and gas. Some have hyberbolic orbits that bring the comet into the Solar System before being sling-shot back into deep space. Other comets originate in the Kuiper belt beyond Neptune and have short-period orbits — the most famous of these is Halley's comet. A small population of active comets is also mixed in with the main asteroid belt between Mars and Jupiter. A solution to their mysterious origin is proposed by Ignatio Ferrín and collaborators.

Certain comets are classed as 'extinct', having sublimated all of their ice content, or 'dormant' when any remaining ice is buried deep within. The authors argue that millions of years ago there were thousands of active comets, which eventually lost their surface ice and fell dormant. Should a comet's position become perturbed by planetary influences, however, any change that would bring the comet closer to the Sun could cause thermal waves to reach the ice and therefore 'rejuvenate' the comet.