The Solar System

Earth is a ball of rock that abounds with life, and it is a special place for humans because it is their home planet. But it is not the biggest, nor the most important object in its space neighbourhood. The Sun has that position. Earth is part of the Sun's family, the solar system, which consists of eight planets, several smaller bodies called dwarf planets, over 140 moons, and billions of asteroids and comets. They have existed together for about 4.6 billion years within the Milky Way galaxy, just one of the millions of galaxies that make up the universe.

The sun exerts its influence on a vast number of bodies and an immense volume of space. As the central, largest, and most massive member of the solar system, all other members orbit around it. Mercury, Venus, Earth, and Mars orbit closest to the Sun. The largest planets—Jupiter, Saturn, Uranus, and Neptune—are beyond Mars. A number of large objects have also been discovered in the Kuiper Belt. The best known of these objects is the world of rock and ice named Pluto, which was regarded as a planet when it was discovered but has recently been reclassified as a dwarf planet.

All the bodies within the system formed at the same time, about 4.6 billion years ago. Their origin was a nebula, a cloud of gas and dust many times larger than the present solar system. By the time the system was formed, just two-thousandths of the nebula's original mass remained. The rest had been blown, or pushed, out into space. The Sun was made first, followed by the planets. Tiny particles of nebula material clumped together to form larger and larger pieces, chunks, boulders, and

JUPiTER AND iO

In this image captured by the Hubble Space Telescope, the upper atmosphere of Jupiter is the backdrop to its moon, Io. The moon's shadow is cast onto the Jovian cloud tops.

finally huge spheres, the rocky planets. The gas giants formed a solid core first, then these captured a gas atmosphere. Material between Mars and Jupiter failed to make a planet, and became the "main belt" of asteroids. Remaining material beyond Neptune became objects in the Kuiper belt, and the comets.

view from earth

The closest objects seen in Earth's sky belong to the solar system. The Sun brightens Earth's day, and illuminates the Moon and planets to shine at night. All the planets, except Neptune, can be seen with the naked eye if conditions are good. The Sun, Moon, and six of the planets— Mercury, Venus, Earth, Mars, Jupiter, and Saturn— have been known since ancient times, as have comets, although their nature was not then understood. Other objects, such as moons, were discovered once the newly invented telescope was turned skyward in the early 17th century Uranus was discovered in 1781, the first asteroid in 1801, Neptune in 1846, Pluto in 1930, and the first Kuiper belt object in 1992. Space probes have been revealing close-up details of these remote, fascinating worlds to us for almost 50 years. They are set to continue, along with ever-improving Earth-based observing techniques, to reveal more, as well as new, members of the system.

LOCAL STAR, THE SUN

The Sun is the only star we can see in detail. The SOHO space probe has imaged this colossal prominence shooting out of its dynamic surface.

the solar system

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