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Solar System

Solar System

A photograph of the SunA photograph of the Sun The star we call the Sun lies at the centre of the Solar System, an array of objects of various sizes that move around, or orbit, it. These are: the eight major planets (in order of distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), their moons, dwarf planets, the asteroids, comets, meteoroids and vast amounts of gas and dust. The Sun’s massive size, compared to the rest of its family, gives it the gravitational pull that keeps all the planets and other objects in perpetual orbit.


The planets orbit the Sun in the same anticlockwise direction and in elliptical paths (oval, rather than perfectly circular). Seven of the planets, and most of their moons, all travel roughly on the same plane. Mercury is the exception: it has a slightly tilted orbit. Pluto, now classified as a dwarf planet, lies in the Kuiper Belt, a vast region beyond the orbit of Neptune, the outermost planet. Pluto's orbit is significantly more tilted and elliptical than the true planets.

The planets' orbits around the SunThe planets' orbits around the SunThe relative distances of the planets from the SunThe relative distances of the planets from the Sun

Diagram of the Oort CloudDiagram of the Oort Cloud

Beyond the planets

The Kuiper Belt is a region of the Solar System lying beyond Neptune. It consists mainly of fragments of frozen methane, ammonia and water. At least three dwarf planets lie in the Kuiper Belt: Pluto, Haumea and Makemake.

Overlapping the Kuiper Belt is another region, called the Scattered Disc. It consists of small icy objects. Eris is the largest known Scattered Disc object.
Kuiper Belt objects and orbitsKuiper Belt objects and orbits

Comet West, an Oort Cloud objectComet West, an Oort Cloud objectMuch more distant is the Oort Cloud, a spherical cloud of comets that surrounds the Solar System. It lies roughly a light year from the Sun, or about a quarter of the distance to Proxima Centauri, the nearest star. The outer edge of the Oort Cloud forms the limit of the Sun's gravitational pull.

Objects in the Oort Cloud are made of water, ammonia and methane ice. It may be the source of long-period comets (those with orbits lasting longer than 200 years) entering the inner Solar System.


Solar wind

The effect of solar wind on the EarthThe effect of solar wind on the EarthConstantly streaming away from the Sun in all directions are electrically charged particles that make up the solar wind. Travelling at an average speed of about 400 km/sec (250 miles/sec), they produce magnetic fields and electric currents across a vast “bubble” called the heliosphere. The edge of the heliosphere, the heliopause, lies up to 18 billion kilometres (11 billion miles) from the Sun and marks the boundary of the Solar System.

 Dave Hawksett

See also in Space

See also in Science

The region between the Kuiper Belt and the Oort Cloud, called the Scattered Disc, is still almost completely unknown.  

A large red object, Sedna, was discovered in the outer Solar System in 2003. Astronomers do not yet know how large it is, but it will probably be classed a dwarf planet. Apart from long-period comets, it is the most distant object known in the Solar System. It orbits the Sun once every 11,400 years.

The Sun's completes one revolution (orbit) of the Galaxy every 225–250 million years, travelling at about 250 km/sec (150 miles/sec). This revolution is known as the Solar System's galactic year.

The closest star to the Sun is the small red dwarf Proxima Centauri which lies about 4.24 light years distant. Proxima Centauri forms part of a triple star system. The other two members are Alpha Centauri A and B (both about 4.4 light years away).

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