Planet may be solar system’s final frontier
THE most distant object yet detected orbiting the sun could be the solar system’s tenth planet, the United States space agency NASA announced yesterday.
Sedna, named after the Inuit goddess of the ocean, is some eight billion miles from Earth and three times as far away from the sun as Pluto.
The “mysterious object”, or planetoid, is up to 1,250 miles across, making it the largest object found to be orbiting the sun since the discovery of Pluto in 1930. Pluto is 1,406 miles across and more than three billion miles away from Earth.
Sedna was spotted in the Kuiper Belt, where 400 such planetary bodies have already been found, but it is different because it could be the first one whose original orbit has remained unchanged.
Dr Michael Brown, an astronomer at the California Institute of Technology in Pasadena, who led the NASA-funded research, said: “The sun appears so small from that distance that you could completely block it out with the head of a pin.”
The temperature on Sedna never rises above -400C, making it the coldest known object in the solar system.
Sedna follows a highly elliptical path around the sun, a circuit that it takes 10,500 years to complete. Its orbit loops out as far as 84 billion miles from the sun, or 900 times the distance between the Earth and our star.
The team, which also includes Dr Chad Trujillo, of the Gemini Observatory in Hawaii, and Dr David Rabinowitz, of Yale University in Connecticut, first spotted Sedna last November, using a 48-in telescope at the California Institute of Technology’s Palomar Observatory, near San Diego.
They also have indirect evidence that a tiny moon may trail Sedna, which is redder than all other known solar-system bodies except Mars.
Follow-up work included measuring the thermal radiation being emitted from Sedna so that they could calculate how hot it is. They used this information to estimate Sedna’s size.
Sightings of Sedna have also been made by the Hubble and NASA’s new Spitzer space telescope, whose infrared detectors can peer through cosmic dust.
Details of the discovery come less than a month since the same team detected another frozen celestial body which was then thought to be the largest object to be found in the solar system since Pluto.
The object, named 2004 DW, is 4.4 million miles from Earth and is thought to be 10 per cent larger than Quaoar, an 800-mile diameter planetoid found in 2002. However, several leading British astronomers were sceptical about whether Sedna should be classed as a planet.
Dr Robin Catchpole, a senior astronomer at the Royal Observatory in London, said he expected most of his colleagues will prefer to view Sedna as a very large asteroid.
He said: “Some astronomers do not believe Pluto is really a planet because of its small size and the same debate is going to happen about Sedna. Sedna would be completely frozen and the sun would simply appear as a very bright star.
“But whether it is a planet or an asteroid it is exciting to discover such a large object so far away.
“It could offer a great deal more information about the formation of the solar system.”
Patrick Moore, the Sky at Night presenter, said: “The discovery is very interesting indeed – but [Sedna] is not a planet.
“I am sure there are similar objects out there that will also be discovered as time goes on.”
Graphic: The tenth planet