Sun(a star) and all the bodies orbiting it: the eight (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), their moons, and smaller objects such as planet asteroids and comets. The Sun contains 99.86% of the mass of the Solar System. The planets orbit the Sun in elliptical paths, and in the same direction as the Sun itself rotates. The planets nearer the Sun have shorter orbital times than those further away since the distance they travel in each orbit is less, and their orbital speeds are higher.
The inner planets (Mercury, Venus, Earth, and Mars) have solid, rocky surfaces; relatively slow periods of rotation (Mercury takes 59 days to complete one rotation, Venus 243 days, Earth nearly 24 hours, and Mars 24.5 hours); very few natural satellites; and diameters up to 13,000 km/8,000 mi. Venus can be seen with the unaided eye, appearing in the evening as the brightest ‘star’ in the sky. In contrast, the outer planets (Jupiter, Saturn, Uranus, and Neptune) have denser, gaseous atmospheres composed mainly of hydrogen and helium; fast periods of rotation (Jupiter takes 10 hours for one rotation, Saturn nearly 10.5 hours, Uranus 11 hours, and Neptune 16 hours); and many natural satellites (Jupiter and Saturn have more than 30 between them, Uranus has 15, and Neptune has 13). Uranus and Neptune were discovered after the development of the telescope.Beyond the orbit of Neptune lies the kuiper belt, a disc of small bodies, of which Pluto may be the largest member. Still farther beyond this is the Oort cloud, extending into the interstellar regions in all directions, consisting of slow-moving dormant comets. Some of these are occasionally perturbed gravitationally and plunge into the inner Solar System.
The Solar System gives every indication of being a strongly unified system having a common origin and development. It is isolated in space. All the planets go around the Sun in orbits that are nearly circular and coplanar, and in the same direction as the Sun itself rotates. Moreover this same pattern is continued in the regular system of satellites that accompany Jupiter, Saturn, Uranus, and Neptune. It is thought to have formed by condensation from a cloud of gas and dust in space about 4.6 billion years ago.
