The Oort cloud

The Oort cloud is an immense spherical shell surrounding the solar system between 1000 and 100 000 AU (30 trillion km) from the Sun. This region contains billions of small icy objects probably left over from the formation of the solar system. Sometimes the orbit of one of these objects gets disturbed by other bodies, causing it to come streaking into the inner solar system as a long-period comet (one with a period of around 2000 years). In contrast, short-period comets take less than 200 years to orbit the Sun and they come from the Kuiper belt. The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth.

One of the major Oort cloud objects is Sedna (2003 VB12), which was discovered in November 2003 by a team led by Mike Brown at Palomar

Observatory near San Diego, California, USA. The object was named after Sedna, the Inuit goddess of the sea, who was believed to live in the cold depths of the Arctic Ocean. Sedna has a highly elliptical orbit that is inclined at about 12° to the ecliptic. Its distance from the Sun varies between 76 AU and 975 AU, so it is best described as an inner Oort Cloud Object. Sedna will make its closest approach to the Sun (perihelion) about the year 2076, and will be furthest from the Sun (aphelion) in 8207. The shape of its orbit suggests it may have been captured by the Sun from another star passing by our solar system, or its orbit could be affected by another larger object further away in the Oort cloud.

Sedna is an odd body because astronomers thought they would not find an object like it in the empty space between the Kuiper belt and the Oort cloud. Sedna is the second reddest object in the solar system after Mars. Its size is estimated to be between 1200 and 1800 km (about three-quarters the size of Pluto), and it takes 12 000 years to orbit the Sun. Recent estimates put its rotational period at about 10 hours and its surface temperature at a cold -250°C. Sedna appears to have very little methane ice or water ice on its surface.

There is no doubt that more discoveries will be made in the Kuiper belt and the Oort cloud. Any new objects discovered will of necessity be faint,

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