But a year later, Dr. Olbers, a physician at Bremen in Lower Saxony, discovered a second planetoid in the same region, with a mean distance and a periodic time so nearly identical with that of Ceres that they were almost within hailing distance. This second planetoid, which was named Pallas, was not greeted as joyously as the first, for now a situation was created that was harder to explain than the great vacant gap. But this was not all. In 1804, still another little globe was discovered, which was called Juno, and another in 1807, which was named Vesta,--four little planets in less than six years! Subsequent discoveries have brought the number almost up to 1000. An explanation by Olbers suggested that these minute bodies might be fragments of an exploded planet, but since they do not lie in the same plane nor do their orbits have a common point of intersection, this theory was discarded. It was then thought that the great attractive force of Jupiter would not allow a planet to form in the space. This theory was also discarded, and even now, a century and a quarter later, no one theory bearing on the origin of the little worlds has found general acceptance. Following the custom of the ancients, the planetoids were given the names of Greek divinities, the next four being named Astræa, Hebe, Iris and Flora. But the names of Greek divinities at last ran out, and the planetoids were christened with more common names. Astronomers do not bother with any of these names, however, except in the case of a few. The rest they refer to by inclosing its number in a circle. Although these little globes are very small, many of them have been found to possess distinct individuality. Vesta, for instance, is exceedingly bright, while Ceres, in contrast, is gray and dull. An explanation is that Ceres may be composed "of rugged and sombre rock, unclothed by any vestige of air, while Vesta displays a brilliant shell of clouds." Ceres is the largest of the planetoids, although even Ceres is less than 500 miles in diameter. Eros, which comes closer to the earth than any other known celestial body with the exception of the moon, exhibits a rapid variation in the brilliancy of its light. It is thought that this may be due to a rough, uneven surface with unequal reflecting power, for Eros is no more than a huge rock only 20 miles across. It has also been suggested that perhaps Eros consists of two bodies revolving close together so that one body eclipses the other in certain parts of its orbit, or again, it may be an angular body, not round at all! Many of the planetoids are smaller than Eros, some being only a few miles across.