The solar system consists of the Sun and its large family of space objects. These include the planets and moons, and countless smaller bodies such as the asteroids and comets. The number of known members rises monthly as smaller, more distant objects are found.
structure of the solar system
The Sun, which is the largest and most massive member, dominates the system. Its central position and strong gravity keep the whole system together. Each other member moves along a path around the Sun. One complete circuit of this path is one orbit; each object spins as it travels. The planetary part of the system is disk-shaped; it is almost flat and nearly circular. Beyond this is the domain of the comets. They orbit the Sun in any plane, from close to the planetary plane, to above and below the Sun, or anywhere in between. They make a vast sphere around the planetary part, stretching to about 1.6 light-years from the Sun. Beyond is interstellar space.
The solar system formed about 4.6 billion years ago from a vast, spinning cloud of gas and dust termed the solar nebula. Material collapsed into the center and made inner rocky planets
The four inner planets—Mercury, Venus, Earth, and Mars—are collectively known as the rocky planets. In fact, this is something of a misnomer, since they are all rocky-metallic. A slice through any one of them would reveal a metal core surrounded by a rocky mantle and the Sun. A spinning disk of unused material surrounding it produced the planets. Rocky and metallic material near the Sun formed the rocky planets. In the cooler, outer regions, rock, metal, gas, and ice formed the outer gas giants.
the orbits of the planets
The planets orbit the Sun in roughly the same plane, and travel in the same direction, counterclockwise when seen from above the north pole. The length of an orbit and the time to complete one increases with distance. The planets and their orbits are not shown to scale.
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