彗星，俗称扫把星，是由冰构成的太阳系小天体（SSSB），当他朝向太阳接近时，会被加热并且开始释气，展示出可见的大气层，也就是彗发，有时也会有彗尾。这些现象是由太阳辐射和太阳风共同对彗核作用造成的。彗核是由松散的冰、尘埃、和小岩石构成的，大小从P/2007 R5的数百米至海尔博普彗星数十公里不等。

彗星的轨道周期范围也很大，可以从几年到几百万年。短周期彗星来自超越至海王星轨道之外的古柏带，或是与离散盘有所关联。长周期彗星被认为起源于欧特云，这是在古柏带外面，伸展至最近恒星一半距离上，由冰冻天体构成的球壳。长周期彗星受到路过恒星和银河潮汐的引力摄动而直接朝向太阳前进。双曲线轨道的彗星可能在进入内太阳系之前曾经被沿着双曲线轨迹被抛射至星际空间，则只会穿越太阳系一次。来自太阳系外，在银河系内可能是常见的系外彗星也曾经被检测到。

A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger and, if sufficiently bright, may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many cultures.