-
- 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.
|
