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Puzzle Of Cometary Orbits Hints At Large Undiscovered Object


ROYAL ASTRONOMICAL SOCIETY PRESS NOTICE

Date: 7 October 1999                                     
For immediate release

Ref. PN 99/32
Issued by: Dr Jacqueline Mitton
RAS Press Officer
Phone: Cambridge  ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892
E-mail: jmitton@dial.pipex.com

RAS Web: http://www.ras.org.uk/ras/

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CONTACT FOR THIS RELEASE

Dr John B. Murray (j.b.murray@open.ac.uk)
Phone: 01908 652118

Dept. of Earth Sciences, The Open University,
Milton Keynes MK7 6AA

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PUZZLE OF COMETARY ORBITS HINTS AT LARGE UNDISCOVERED OBJECT 

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.

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