New icy world resembles a comet

Space probes have told us much about the inner worlds of the solar system but we are only just getting to know our more distant neighbours. In particular, the realm of icy worlds around and beyond Neptune has never been examined by any craft – although some have flown through it.

Artist’s impression of Sedna. Credit: NASA

A new discovery announced this month reveals a strange new inhabitant of this far-flung empire which is a kind of “missing link” between them and comets.

This outer fringe of the solar system includes Pluto, the former ninth planet which lost its status when the International Astronomical Union reclassified it as a dwarf planet two years ago this month. It is now first of a type called Plutoids.

Although some – particularly in the United States from where Pluto was discovered in 1930 – still contest the IAU’s decision, it was always clear that this world had a character quite unlike the other planets.

Its orbit is greatly tilted to that of the other planets and it is also far more elliptical. Indeed, contemporary writings from the time of Pluto’s discovery show that its status was questioned even then.

Instead, in recent years we have learned that Pluto is just a major member of a vast swarm of icy bodies, which have been termed Trans Neptunian Objects. Many are in the vicinity of Pluto’s orbit – a region called the Kuiper Belt.

More than a thousand times further away is thought to be a huge zone of icy fragments encircling the solar system, called the Oort Cloud. Both zones produce the mysterious comets which occasionally have their orbits disrupted to send them flying into the inner solar system.

Between the two regions we find another class of ice world known as Scattered Disk Objects. The best known is Sedna which takes over 12,000 years to go round the Sun, its distance varying between 7 billion miles (11.5 billion km) and 90 billion miles (145 billion km).

Now a new icy body with the simple label 2006 SQ372 has been discovered with an even more extreme orbit. It is presently closer than Neptune and a little more than 2 billion miles (3.2 billion km) away. But it will recede to a distance of 150 billion miles (240 billion km) in a 22,500-year orbit around the Sun.

SQ372 was discovered by an automatic photographic patrol called the Sloan Digital Sky Survey and announced at a symposium in Chicago. Discovery team leader Andrew Becker of the University of Washington said that whereas the orbital paths of the major planets were nearly circular, the orbit of 2006 SQ372 was an ellipse four times longer than it is wide.

The new object is much smaller than Sedna – probably 30-60 miles (50-100 km) across instead of nearly 1,000 miles (1,600 km). Dr Becker said: “It’s basically a comet, but it never gets close enough to the Sun to develop a long, bright tail of evaporated gas and dust.”

A NASA probe called New Horizons currently racing to find out more about Pluto and the Kuiper Belt.

Picture: A NASA artist’s impression of Sedna.

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