PLANETOIDS (from Gk. 71-Xamirns, plann wanderer, wandering star• planet. ± egos, color, form), or ASTEROIDS. A 1111111erOUS group of very small planets which, with one exception, travel in orbits lying between those of Mars and Jupiter. Till the first year of the nineteenth century they remained undiscovered; but for some years before their existence had been sus pected. mainly owing to the remarkable hiatus in the series of the planetary distances when com pared with the law of Bode (q.v.). On the first day of January, 1801, the first of them was de tected by Piazzi at Palermo, and his success roused the astronomers to search for more plan ets. Their search was successful, for (fibers (q.v.) discovered two in 1802 and 1807, and Harding one in 1804; but as all researches for some time subsequent to 1807 were unavailing, astronomers gradually allowed .i tl ..emselves to set tle down into the belief that no more planetoids remained to be discovered, when the detection of a fifth by Henske in 1845 revived the hope of fresh discoveries, and from this period no year (excepting 1S46) has passed without adding to the list. And since the application of photographic processes to observation, the number known to astronomers has increased with great rapidity. The first photographic asteroid was found by Wolf, of Heidelberg. Germany, December 20. 1891, and was named by him Brueia, in honor of Katharine Wolff Bruce. of New York. who con tributed to the endowment of the Heidelberg observatory. Brueia received the number 323 in the list of planetoids. Since 1892 only a few planetoids have been found visually, though the total number in the list has been extended from 323 to over 500. It will be seen. therefore, that photography has almost completely replaced the visual telescope in this kind of observation.
The magnitudes of the planetoids have not been very accurately ascertained, but it is certain that they are exceedingly small compared even with Mercury, the smallest of the other planets, the diameter of the largest among them having been measured by Barnard as 4S5 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 frequent ly as to form, when viewed perpendicularly, a kind of network. The consequence of this is that a mutual eclipsing of the sun at different periods by two planetoids must be of very frequent oc currence. After the first two or three of these bodies had been discovered, the opinion was pro pounded by Olbers that they were but the frag ments of some large planet, and this hypothesis received corroboration from the intimate connec tion which was shown to subsist among them : but this hypothesis is now rejected by most astrono mers, and their existence is explained by the nebular hypothesis. See NEBUL.E.
Decidedly the most interesting planetoid is the one discovered August 13. 1898. by Witt of Ber lin. and which has received the number 433 and the name Eros (q.v.). This small planet pos sesses the remarkable orbital peculiarity that at perihelion it passes considerably within the orbit of Mars. and thus approaches the earth nearer than any other known body except the moon, and is therefore especially well suited for the deter mination of solar parallax by the planetoid method. By means of long photographic exposure Wolf and Dnuan. during May and June. 1903, dis covered eight planetoids, provisionally indicated by 1903. LQ to LX. See PARALLAX.