[Gk. aster, a star, and eidos, form.] The asteroids are a group of very small planets between the planets Jupiter and Mars. Ceres, the first known of these, was discovered on January 1, 1801, the first day of the nineteenth century. Before the last day about 450 had been discovered. Nearly all of these are very small. Ceres is about 1200 miles in diameter, but many of them are only a few miles through. They may be fragments of a former planet, for they occupy the place in the solar system where, by the calculations of the astronomers, a planet should be.