Herschel closely scrutinised the surfaces of the planets. Mercury alone was neglected by him. From 1777 to 1793 he observed Venus, with the object of determining the rotation period, but he was unable to observe any markings on the surface of the planet. He did not place reliance on Schröter’s value of the rotation period (about twenty-three hours). Meanwhile, Schröter announced the existence on Venus of mountains which rose to five or six times the height of Chimborazo. As to these, said Herschel, “I may venture to say that no eye which is not considerably better than mine, or assisted by much better instruments, will ever get a sight of them.” Herschel demonstrated the existence of an extensive atmosphere round Venus.
“The analogy between Mars and the Earth,” Herschel wrote in 1783, “is perhaps by far the greatest in the whole Solar System.” In 1777 he began, in his house at Bath, a series of observations on the red planet, which yielded results of the utmost importance. Fixing his attention on the white spots at the north and south poles,—discovered by Maraldi, nephew of Cassini,—he soon ascertained the fact that they waxed and waned in size, the north polar cap shrinking during the summer of the northern hemisphere, increasing in winter, and _vice versa_ in the southern hemisphere. He regarded the caps as masses of snow and ice deposited from “a considerable, though moderate, atmosphere,” a theory now generally accepted. Herschel gave an immense impetus to the study of Mars. He carefully examined the planet’s surface, and the dark markings were regarded by him as oceans.
During Herschel’s lifetime the four small planets, Ceres, Pallas, Juno, and Vesta, were discovered by Piazzi, Olbers, and Harding. The great astronomer was much interested in these small worlds. He commenced a search through the Zodiacal constellations for new planets, but failed. He was of opinion that many minor planets would be discovered. Accepting Olbers’ theory of the disruption of a primitive planet, Herschel calculated that Mercury might be broken up into 35,000 globes equal to Pallas. Meanwhile Herschel named the four new planets “Asteroids,” owing to their minute size. He estimated the diameter of Ceres at 162 miles and Pallas at 147 miles, but Professor Barnard’s measures have shown them to be larger.