PLANETOIDS, or ASTEROIDS, the name given to that numerous group of very small planets which are situated in the solar system between Mara and Jupiter. Till the present century they remained undiscovered; but for some years before, their existence had been suspected, mainly owing to the remarkable hiatus in the series of the planetary distances when compared with the law of Bode (q.v.). On the first day of the present century the first of them was detected by Piazzi of Palermo, and his success roused his brother astronomers to search for more planets. Their search was successful, for Olbers (q.v.) discovered two in 1802 and 1807, and Harding one in 1804; but us all researches for some time subsequent to 1807 were unavailing, astronomers gradually allowed them selves to settle down into the belief that no more planetoids remained to he discovered, when the detection of a fifth by Ilencke in 1845 revived the hope of fresh discoveries, and from this period no year (excepting 1846) has passed without adding to the list. The number now known is more than 170. This remarkable success of the astronomers of our time is due to the systematic manner in which the zodiacal belt has been explored, and the place and apparent size of every star of this region distinctly determined; so that the nre2ence of a wandering body can at once be detected.
The magnitudes of these Celestial bodies have not been accurately ascertained, but it is certain that they are exceedingly small as compared even with Mercury, the least of the other planets; the diameter of the largest among them being generally believed not to exceed 450 miles, while most of the others are very much smaller than this. They also differ, generally speaking, from the rest of the planets in other respects; their orbits are of greater eccentricity, are inclined to the ecliptic at a greater angle, and are interlaced in a most intricate manner, crossing each other so frequently as to form. when viewed
perpendicularly, a kind of net-work. The consequence of this is that a planetoid which is nearest the sun at one part of its orbit is, when at another part of its orbit, further from it than are several of the others, and a mutual eclipsing of the sun at different periods by two planetoids must be of very frequent occurrence. Of the planetoids, of which the elements had in 1876 been satisfactorily calculated, Flora has the shortest period of revolution, 1193 days, and Sylvia the 2,374 days. The corresponding mean distances from the sun, expressed in parts of the earth's mean distance, are respectively 2.201 and 3.484. [But it is now known that Blida has a period of 2,868 days and a mean distance of 3.947.] The nearest approach to the sun is made by Phocea (perihelion distance, 1.787). Freia recedes furthest from him (aphelion distance, 4.002). Polyhymnia's orbit has the Frentest excentricity, amounting to 0.33998; Lomia's, the least, 0.2176. Massalia's orbit makes a smaller tingle—only 41' 7"—with the ecliptic than that of any other planet in the solar system, while the inclination of the orbit of Pallas is no less than 34" 42' 41". After the first two or three of these bodies had been discovered, the opinion was propounded by Others that they were but the fragments cif some large planet; and this hypothesis received corroboration from the intimate connec tion which was shown to subsist among them; but of late years it has fallen out of favor with astronomers. Some infer that the planetoids are best accounted for by the nebular hypothesis. It has been calculated that•the combined mass of all the planetoids cannot exceed one-fourth of the earth's mass. The number of planetoids discovered now amounts to 200; the following is a table of those known in Feb., 1877: