1.1 The planets of the solar system: Planetary Geology

When people first recognized that the planets resemble Earth more than the stars, excitement grew. Could intelligent life exist on these other planets, or elsewhere, in the universe? Space exploration has rekindled this interest. So far, no evidence of extraterrestrial life within our solar system has emerged. Nevertheless, we study the other planets to learn about Earth’s formation and early history. Recent space exploration has been organized with this goal in mind.

The sun is the hub of a huge rotating system of eight classical planets, their satellites, and numerous smaller asteroids, comets, meteoroids, and dwarf planets. An estimated 99.85 percent of the mass of our solar system is contained within the Sun. The planets collectively make up most of the remaining 0.15%. The planets, traveling outward from the Sun, are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was recently reclassified as a dwarf planet.

Under the Sun’s control, each planet is tethered by gravity in a nearby circular orbit. All of them are traveling in the same direction. The nearest planet to the Sun, Mercury, has the fastest orbital motion, 48 kilometers per second, and the shortest period of revolution around the sun, 88 “Earth-days”. By contrast, the distant dwarf planet Pluto has an orbital speed of 5 kilometers per second and requires 248 “Earth-years” to complete one revolution. Furthermore, the orbital planes of seven planets lie within three degrees of the plane of the sun’s equator. The exception is Mercury, which is inclined seven degrees.

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