Re: Nemesis - a.k.a. Dark Star

["Terry Colvin" <colvint@fhu.disa.mil>]

Forwarded from the SPACE TECH list:
Date:    14/1/1996 3:11 PM

     Date: Sat, 13 Jan 1996 01:39:40 GMT
     From: Kevin Mosman <saskxm@unx.sas.com> 
     Subject: Nemesis - a.k.a. Dark Star

     In article <9511298202.AA820296902@fhu.DISA.MIL>, "Terry Colvin" 
     <colvint@fhu.DISA.MIL> writes:
     For a dinosaur list, there sure are frequent items to provoke astronomers. 
     The latest scoop on "Nemesis" (yes, the quotations imply my assessment) 
     seems to be as follows:
     
     Whitmire and Jackson (1984 Nature 308, 713) followed the Raup and 
     Sepkoski periodic-extinction claims by asking what sort of 
     astrophysical clock would have the asserted 26-million-year period 
     and be able to induce mass extinction (i.e. drive up the impact 
     rate). A disturbance in the Oort cloud would certainly fill the bill 
     (the Oort cloud being the postulated reservoir for long-period 
     comets, needed to maintain the presence of any such in the inner 
     solar system against their trapping into short-period orbits and 
     subsequent destruction by successive solar passages - the Oort
     cloud has yet to be directly observed, but objects are known to 
     populate its inner counterpart the Kuiper belt). They suggest that 
     a solar companion in a 26-million year, elongated orbit could 
     trigger such influxes of comets. The required mass, given that we 
     have no companion identifiable as a normal star, would make it a 
     "brown dwarf" - formed in the same way as ordinary stars, but with 
     insufficient mass to sustain core hydrogen fusion. Such an object 
     would be very difficult (and correspondingly interesting) to 
     detect. A 26-million year period implies a semimajor axis to the 
     orbit of Nemesis given by Kepler's third law as (26,000,000)^2/3 = 
     88000 AU, or 1.4 light-years; to see a strong periodic signal in 
     gravitational disturbance requires that the orbit be highly 
     eccentric, so the maximum distance from the Sun would be close to 
     twice this (round numbers, halfway to the next stellar system 
     though of course not necessarily in that direction). The same issue 
     of Nature also contains preliminary dynamical analysis of the
     idea, and discussion of crater ages and periodicities. 

     ..snip... [removal of some reasons why Nemisis is considered unlikely]

     this is not my field by any means, but I would like to ask whether it 
     might be possible that a 26 million year cycle could perhaps result 
     from multiple bodies orbiting in the Oort in some harmonic orbits 
     such that only every 26 million years do enough of these [perhaps 
     relatively largish] bodies become co-radial (in conjuction?) and 
     therefore by virtue of their combined gravitational effects cause 
     nearby smaller bodies to be deflected inwards.

     would such a scenario be ruled out due to it requiring either too many 
     bodies to be stable or perhaps too large of bodies to be as yet 
     undiscovered?

     --
     Kevin L