Solar System

Most of the objects in the Solar System lie close to the plane of the ecliptic. The planets are tiny compared to the Sun (not shown to scale). If the Sun were the size of a basketball, the planet closest to the Sun, Mercury, would be the size of a mustard seed 15 m/48 ft from the Sun. The most distant planet, Pluto, would be a pinhead 1.6 km/1 mi away from the Sun. The Earth, which is the third planet out from the Sun, would be the size of a pea 32 m/100 ft from the Sun.

The rings of Saturn, consisting of small pieces of ice and rock, are 62,000 km/38,000 mi wide and just 100 m/300 ft thick.

In this photograph of Earth from space, the African continent and southern oceans are clearly visible beneath white swirls of cloud. Earth is the only planet on which life is known to exist.

The planets of the Solar System in order and to scale. Surface detail is included for the smallest planets. To remember the order of planets moving away from the Sun, the mnemonic ‘My very educated mother just served us nine pies’ may be useful.

The orbits of the outer planets of the Solar System. The eccentricity of a circular orbit is zero. Pluto is the only planet with a large eccentricity.

Examples of ellipse eccentricities. Pluto has an eccentricity of 0.25, the most marked of all the planets in the Solar System. The Earth has an eccentricity of 0.0167.

Sun (a star) and all the bodies orbiting it: the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), their moons, and smaller objects such as 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.