什么是 Asteroid, comet, trans-Neptunian object？

An asteroid is a small rocky or metallic body that orbits the Sun, too small to be a planet and without the gas and dust envelope of a comet; most orbit between Mars and Jupiter. A comet is a small body of ice and dust that, when its orbit brings it near the Sun, releases gas and dust that form a glowing envelope round it and tails pointing away from the Sun. A trans-Neptunian object is a body of the Solar System whose orbit lies, on average, beyond that of Neptune.