Terrestrial planet from the original solar nebula. The Solar System has four terrestrial planets: Mercury, Venus, Earth, and Mars. Only one terrestrial planet, Earth, is known to have an active hydrosphere. During the formation of the Solar System, there were probably many more terrestrial planetesimals, but most merged with or were ejected by the four terrestrial planets. Dwarf planets, such as Ceres, Pluto and Eris, and small Solar System bodies are similar to terrestrial planets in the fact that they do have a solid surface, but are, on average, composed of more icy materials (Ceres, Pluto and Eris have densities of 2.17,
Wonders of the Solar System by John Herschel's actinometer. He also travels to Norway to observe and explain the defensive role of the Earth's magnetosphere against the sun's solar wind and its role in forming the Aurora Borealis. He also relates the Voyager missions and their continuing exploration of the massive reach of the sun's gravitational forces on objects in the farthest regions of the solar system. Finally, in the clear skies of the Atacama Desert, at the Paranal Observatory he is able to observe, with the naked eye, the myriad of stars on the Milky Way and relates the meaning of their diverse colours