Massive planet may lie beyond Pluto

ROYAL ASTRONOMICAL SOCIETY NEWS RELEASE

Posted: October 7, 1999

Intrigued by the fact that long-period comets observed from Earth seem to follow orbits that are not randomly oriented in space, a scientist at the Open University in the UK is arguing that these comets could be influenced by the gravity of a large undiscovered object in orbit around the Sun. Writing in the issue of the Monthly Notices of the Royal Astronomical Society published on 11th October, Dr John Murray sets out a case for an object orbiting the Sun 32,000 times farther away than Earth. It would, however, be extremely faint and slow moving, and so would have escaped detection by present and previous searches for distant planets..

Long-period comets are believed to originate in a vast 'reservoir' of potential comets, known as the Oort cloud, surrounding the solar system at distances between about 10,000 and 50,000 astronomical units from the Sun. (One astronomical unit is approximately the average distance between the Earth and the Sun.) They reach Earth's vicinity in the inner solar system when their usual, remote orbits are disturbed. Only when near to the Sun do these icy objects grow the coma and tails that give them the familiar form of a comet. Dr Murray notes that the comets reaching the inner solar system include a group coming from directions in space that are strung out along an arc across the sky. He argues that this could mark the wake of some large body moving through space in the outer part of the Oort cloud, giving gravitational kicks to comets as it goes.

The object would have to be at least as massive as Jupiter to create a gravitational disturbance large enough to give rise to the observed effect, but currently favoured theories of how the solar system formed cannot easily explain the presence of a large planet so far from the Sun. If it were ten times more massive than Jupiter, it would be more akin to a brown dwarf (the coolest kind of stellar object) than a planet, brighter, and more likely to have been detected already.

So Dr Murray speculates that such an object, if it exists, will be planetary in nature and will have been captured into its present orbit since the solar system formed, even though the probability of such an event seems low on the basis of current knowledge.

Though a large, distant planet is a fascinating possibility and the evidence is suggestive, Dr Murray nevertheless stresses that he is not ruling out other possible explanations for the observed clustering of the comet orbits.