Did you know...
...99% of the solar system's mass is concentrated in the Sun.
...that the Future's Museum in Sweden contains a scale model of the solar system? The Sun is 105 m in diameter and Pluto is 6km away. This particular model also contains the nearest star, Proxima Centauri, still to scale, situated in the Museum of Victoria...in Australia!
The region of the universe in which we live, the solar system, is but one of a great many clusters of stars and planets in space. Of the nine planets that orbit our nearby star, the Sun, Earth is the only one known to support life. Life forms on Earth have grown more complex over billions of years, yet it is only since 1962 that they have ventured forth in manned and unmanned spacecraft to explore the neighboring planets and their satellites.
Our solar system lies in a region of the universe called the Milky Way galaxy. When seen through telescopes, galaxies appear as fuzzy clusters of light created by numerous stars, some of which may have their own planetary systems. These other solar systems are so distant that even the most powerful telescopes on Earth cannot directly see whether planets exist. Our solar system is home to several types of objects, ranging from icy accumulations of dust and frozen water, to giant gaseous balls more than 11 times the diameter of Earth, without even a solid surface. Periodically, some of these smaller objects may collide with a planet, reminding us that the solar system is not a static place, but is governed by the laws of motion, and sometimes by chaos.
All of the planets in the Solar System move around the Sun in the same direction, and nearly in the same orbital plane. This common plane of planetary motion is roughly in the equatorial plane of the Sun's rotation. The planets' axes of rotation are nearly perpendicular to this plane, with the exceptions of Uranus and Pluto, which are tilted on their sides. Pluto is so distant that it takes 248 years to revolve around the Sun, whereas Mercury takes 88 days to make the same revolution.
Other small bodies such as comets travel in elliptical orbits. Some travel as far out as Jupiter and return close to the Sun, repeating these orbits until disturbed by the gravitational attraction of a passing planet. Halley's Comet, for example, returns to the inner part of the solar system every 76 years. Periodic meteorite showers are caused when Earth returns to a region of its orbit where a comet once passed, and are often spectacular examples of the regular motions in the solar system.
The masses and compositions of the individual planets are arranged in an orderly fashion. The terrestrial planets (Mercury, Venus, Earth, and Mars) of the inner solar system are low in total mass, and they are composed primarily of silicate rocks and metals. By contrast, the giant planets (Jupiter, Saturn, Uranus and Neptune) in the outer solar system are much more massive and are composed largely of gases (chiefly hydrogen and helium) and with satellites made mostly of ice. Some of these substances (such as water, methane, ammonia, and nitrogen) condense to solids at low temperatures, less than O° C; others (such as hydrogen and helium) remain gaseous under nearly all natural conditions.
Present theories about the origin of our solar system are all based on one generally accepted concept: the Sun and the planets formed nearly simultaneously about 4.5 billion years ago out of a single protosolar nebula, the result of the collapse of an interstellar dust cloud. The combination of gravitational attraction among the individual particles, angular momentum built up from the collapse, and other complex forces led to the original geometry and rotation of the solar system. The observed compositions of the planets suggest that planet formation was dominated by accretion of solid particles. The inner four terrestrial planets formed directly through a process of accretion beginning with silicate and metal dust particles in a region where ice particles were largely absent. In the outer solar system, the lighter elements condensed to ices, and the resulting solids apparently accreted to form massive ice-rich planetary cores. For Jupiter and Saturn, the massive cores attracted much of the original interstellar gas, creating the atmospheres now observed. Our current theories imply that the origin of the solar system was not a unique, or even a remarkable, event.
The conditions of solar system formation are similar to those associated with the formation of many other stars that we can now observe. Until the time when we can study other solar systems in detail, our theories on the origin of solar systems will be based only on our own neighbors in space.
Sun
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
 | the formation of the Solar System |
 | Solar System family portrait |
* This text was adapted from Our Solar System, a Geologic Snapshot by NASA.
