Terrestrial planet Terrestrial planet A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth ("Terra" and "Tellus"), as these planets are, in terms of structure, "Earth-like". Terrestrial planets have a solid planetary surface, making them substantially different from the larger giant planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various
The Hitchhiker's Guide to the Galaxy Tertiary to Hexagonal Phases nods to the radio series such as a reference to Brontitall and the bird people from the Secondary Phase that were never put into the novels. The episode opens with a Grebulon spaceship suffering an accident, and losing nearly all records of what it is and what it should be doing, along with the crew's memories of why. Based on what little remains of their orders, they land on the tenth planet from the Sun, and start to 'monitor' Earth. After a year's travelling Arthur has returned to the co-ordinates ZZ9 plural Z alpha—where he is expecting to find Earth,