And then there were ten
14th Oct 1999
A tenth planet even bigger than Jupiter has been orbiting the Sun unnoticed, claim two astronomers. They believe that the Sun captured the giant planet as it wandered through space.
The new planet is about 0.5 light years away, and lies within the Oort cloud, a chilled reservoir of billions of comets that extends to more than a light year out from the Sun. Gravitational tugs from passing stars, the disc of our Galaxy and other Oort cloud objects are thought to occasionally tip comets out of the cloud towards the Sun, which they orbit with periods of millions of years.
When John Matese of the University of Louisiana at Lafayette looked at the orbits of these long-period comets he found that more than expected came from a particular band around the Oort cloud. A planet with 1.5 to 6 times the mass of Jupiter must have deflected those comets towards the Sun, Matese told a meeting of the American Astronomical Society in Padua, Italy, this week. His analysis shows the new planet is about 0.4 light years away, roughly 100 times Neptune's distance from the Sun--and circles the Sun every 4 to 5 million years.
Independently, John Murray of the Open University in Milton Keynes found a similar effect. His analysis suggests that many comets were ejected from the Oort cloud after a close encounter with an object roughly the size of Jupiter. Murray estimates that the object orbits the Sun roughly every 6 million years at a distance of 0.5 light years (Monthly Notices of the Royal Astronomical Society, vol 309, p 31).
His study also reveals that the new planet's orbit is steeply inclined compared to those of the other planets and is in the opposite direction. An object in this orbit could not have kept its position for the 4.5-billion-year lifetime of the Solar System, he adds, which suggests it was wandering in space until captured by the Sun's gravity.
"It would be fascinating if this is there," says Murray, who knew nothing of Matese's work until last week. "The coincidence is extraordinary."
Many astronomers are cautious, however. "Just because two groups come up with the same effect doesn't make it real," warns Gareth Williams of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. His colleague Brian Marsden found no evidence of the planet when he analysed similar observations 30 years ago. "If the object were many Jupiter masses, it would be fairly bright, and it's surprising it has escaped detection," adds Adam Burrows of the University of Arizona in Tucson.
All the astronomers agree that the acid test will be whether the planet can be observed directly. It will be extremely faint at visible wavelengths, and may lie against the bright background of the Milky Way in the constellation Delphinius. But Burrows thinks the next generation of infrared telescopes would have a good chance of spotting it.
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