They are either comets, asteroids, or fragments of these bodies.
Comets are very old bodies of ice mixed with rock and organic material.
Most of them come from far beyond Pluto and Neptune and then swing around the Sun. Occasionally, the eccentric orbits of these bodies cause them to cross paths with a planet. Tom Herbst, MPIA Heidelberg. In 1994, the fragments of comet Shoemaker-Levy 9 struck Jupiter. The impacts occurred where you see the bright spots in the infrared image to the left.