The Solar System may be defined as consisting of all those objects that are governed by the Sun's gravitational field. Other effects arising from the proximity of the Sun could equally well be used as criteria, such as radiation pressure or interaction with the solar wind. With any of these definitions the Solar System extends out to a distance of about two light-years.
The planets are the most massive bodies in the Solar System. In ancient times the motion of these objects relative to the sphere of fixed stars was noted by the Greeks, who gave them the name of 'wandering stars' (planets).
The planets of our solar system produce no visible light of their own; they are visible only by reflected sunlight. Mercury, Venus, Mars, Jupiter, and Saturn are all easily visible to the unaided eye and look like stars, but Uranus is usually too faint to be seen, and Neptune is never bright enough.
The solar system consists of the Sun; the eight official planets, at least three "dwarf planets", more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium.
The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator.
The inner Solar System includes the planets Mercury, Venus, Earth and Mars. Of all the known moons in our Solar System, only three are in the inner region. Mars has two. Earth has one. Mercury and Venus are the only planets without moons. The outer Solar System includes Jupiter, Saturn, Uranus, Neptune and Pluto and the rest of the moons.
A moon is a natural satellite rotating around a planet. While moons vary in size, each moon is much smaller than its planet. Almost 140 moons are known in the Solar System.
Several moons are larger than the planet Pluto and two moons are larger than the planet Mercury. There also are many small moons that may be asteroids captured by their planets.
At the heart of the solar system is our sun. The four planets nearest it are rocky, terrestrial worlds — Mercury, Venus, Earth and Mars. After that are four gas giants — Jupiter, Saturn, Uranus and Neptune. Between the orbits of Mars and Jupiter lies the asteroid belt, which includes the dwarf planet Ceres. Beyond the orbit of Neptune one finds the disk-shaped Kuiper belt, in which dwarf planet Pluto resides, and far beyond that is the giant, spherical Oort Cloud and the teardrop-shaped heliopause.
Our Milky Way Galaxy is unusual in that it is one of the most massive galaxies in the nearby universe. Our Solar System also seems to have qualities that make it rather unique. These qualities make the Sun one of the few stars in the Galaxy capable of supporting complex life.
There are billions of stars in the Milky Way Galaxy, and some are more metal-rich than others. Part of this is a condition of age: The older a star, the more metal-poor it tends to be. That's because the most ancient stars formed from just hydrogen, helium, and lithium. When the most massive of these stars exploded, nuclear reactions fused these light elements into heavier ones.