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.
Oort Cloud
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 favored 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.
