About 1 1/2 billion kilometres beyond the solar system's most distant planet and six billion kilometres from the Sun, astronomers have discovered a massive object never seen before.
"Quaoar" (pronounced kwah-o-wahr,) a frozen world measuring 1290km across, is the biggest mass found orbiting the Sun since the discovery of Pluto in
1930.
Quaoar is "about the size of all the asteroids put together", said Michael Brown, associate professor of planetary astronomy at the California Institute of Technology in Pasadena. "So this thing is really big".
But it is unlikely to win the accolade of "planet". Even Pluto has struggled to hold on to that title.
Quaoar orbits the Sun once every 288 years. It is about one-tenth the diameter of Earth and half the size of Pluto, though apparently larger than that planet's moon, Charon.
The discovery lies in the "Kuiper Belt", a distant swarm of ice and rock objects that orbit the Sun far beyond Neptune. They are reckoned to be leftovers from the process that formed the solar system five billion years ago.
Many comets are believed to originate there, falling towards the Sun as their orbit becomes unstable.
The belt contains as many as 10 billion objects at least 1km across; astronomers estimate five to 10 of those are as large as Quaoar (named after a creation force in Indian mythology).
"This discovery fits with our expectation that there should be a handful or two of objects as large as Pluto," said astronomer Dr David Jewitt, of the University of Hawaii. Dr Jewitt, with his then-colleague Jane Luu, discovered the first Kuiper Belt object a decade ago.
Unfortunately for the planet, this discovery weakens its case to be counted with larger siblings.
In 1999, Pluto narrowly survived being downgraded from a "planet" to "minor planet" by the International Astronomical Union.
One embarrassment for astronomers is that Quaoar could have been discovered 20 years ago.
The Caltech scientists found it when they checked film archives going back to 1982.
- INDEPENDENT
Space Telescope Science Institute
