• noun an astronomical object that is a mass of ice and dust and has a long luminous tail produced by vaporisation when its orbit passes close to the Sun
  • Mass of dust, gas and solid matter in orbit around the Sun. Comets contain very little mass but are of interest to astronomers for several reasons. One is that they can be highly spectacular. More significantly, they are remnants of very early matter within the solar system. They also tie into other small objects in the solar system, like meteoroids, many of whose orbits resemble those of comets, and the asteroids, which in some cases seem to be comets attracted into the asteroid belt by Jupiter’s gravitation and stripped of their gaseous matter by solar radiation. Beyond the planets there may lie billions of comets in a zone called Oort’s cloud, occasionally tipped into the inner solar system by random gravitational effects or visiting it in long orbits which send the comets into deep space most of the time. If other stars have also formed like the Sun, leaving material not swept up around them into planets or the stars themselves, there must be incalculably large numbers of comets in the apparently bare space between stars.

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