Planetoids

Planetoids are the very small planets which have their orbits between those of Mars and Jupiter. They escaped discovery till quite modern times, but ever since their existence was first shown to be probable on theoretical grounds, they have been discovered in increasing numbers. The greatest number has been found since 1845, owing doubtless to improved instruments and methods of search. The inclination of the orbits of these bodies to the plane of the ecliptic varies considerably, that of Pallas being as much as 34° 42'; and their eccentricities are much greater than those of the planets proper. Their motions among themselves are extremely irregular, the paths interlacing in a most intricate manner. Being so extremely small, it is almost impossible to ascertain their size; but it is believed that the largest is less than 450 miles across, while Leverrier considered that the sum of all their masses would be less than one-third that of the earth. The first five planetoids which were discovered were Ceres, Pallas, Juno, Astrsea, and Vesta. It was once believed that the planetoids were portions of some planet, but the idea was not held for very long.