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Date: 8 Jan 86 05:11:13 GMT
From: ucdavis!lll-crg!mordor!  (Jordan Kare)
Subject: Re: Re:  Tenth planet

>>      Perhaps the "tenth planet" is not actually a planet, but perhaps a weak
>> stellar companion to Sol, already known as "Nemesis".
>	Named (or so claimed by the author of the paper) such because if such
>a companion is not found, the publication will prove to be HIS nemesis.

	Nemesis can't be the "tenth planet" affecting Neptune's 
orbit, as it would be too far away (and would move both Neptune 
and the rest of the solar system including the sun by the same amount).
	There is a theory that Planet X (either "ex" or
"ten" depending on how much you like Roman numerals :-) exists, and
has a peculiar orbit that precesses in such a way as to disturb
the cometary cloud and cause comet showers (and extinctions)
every thirty or so million years.  This is an alternative to the
Nemesis theory, but it doesn't work, on various (rather complicated) 
celestial mechanics grounds, leaving Nemesis as the leading hypothesis.  
	Also, "Nemesis" was not the first choice for the name
of the companion star.  Since (if it exists) it was responsible
for killing off the dinosaurs, first choice was to name it after the
most famous mythological dragon killer and call it "George".
		-- A Muller Group Nemesis Hunter

Date: 8 Jan 86 05:29:56 GMT
From: ucdavis!lll-crg!mordor!  (Jordan Kare)
Subject: Re:  Tenth planet

Sorry I didn't catch this before my preceeding posting
>>      Perhaps the "tenth planet" is not actually a planet, but perhaps a weak
>> stellar companion to Sol, already known as "Nemesis".
>Alas, Nemesis has met its fate :-).
>(Quick digression:  the theory alluded to here is the notion that periodic
>extinctions are caused by near approaches of a companion star, which is in a
>*very* long-period orbit [30MY or so], stirring up the Oort cloud and causing
>a rain of comets into the inner Solar System.)
>The problem with making the Sun a binary star is that Nemesis has to be a
>godawful long way out to have such a long orbital period, and it appears
>that such an orbit simply is not very stable over geological time scales.
>It is not consistent with extinctions at clockwork-regular intervals, at
>the very least.
>				Henry Spencer @ U of Toronto Zoology
>				{allegra,ihnp4,linus,decvax}!utzoo!henry

This is not correct.  Extensive simulations (Notably by Piet Hut
of the Princeton Institute for Advanced Studies) have shown that
the mean lifetime of Nemesis's orbit is roughly a billion years.
The orbit is disturbed by passing stars, dust clouds, etc. but
can easily remain stable enough to explain the few data we have,
which only cover the last 250 million years (a mere moment... :-))
If Nemesis was formed with the solar system, then it probably started
in a closer orbit and has been perturbed out to its present
distance; in another couple of billion years it might be gone
(so we need to find it quick :-)).
The evidence for periodic extinctions and periodic cratering, which
the Nemesis theory was created to explain, is subject to dispute.
(The evidence for catastrophic impacts associated with extinctions
is very strong; only the periodicity is speculative).
But IF periodic extinctions do occur, the Nemesis theory DOES 
explain them, and is still the ONLY theory which does so successfully.
Despite what you may read in the New York Times editorial pages, 
Nemesis lives on....
				Jordin Kare
				Formerly of UC Berkeley/LBL Astrophysics
				Home of Nemesis and much, much more....

Date: 19 May 89 20:49:45 GMT
From:  (Jordan Kare)
Subject: Re: Sun's invisible partner NEMESIS

In article <13140@ut-emx.UUCP> nather@ut-emx.UUCP (Ed Nather) writes:
>In article <>, (Daniel Evans) writes:
>> 	A couple of years back, I had read some accounts about a tiny (non-
>> identical) "twin" of our sun, which swings by every few million years or so.
>> Someone referred to it as "Nemesis".
>> 	How much evidence is there supporting this theory?
>>  What are the details?  
>There aren't any.
>> How did they decide what its path is? 
>They were trying to explain "periodic" mass extinctions in the fossil record
>by invoking comets to smash the earth and create a "gravitational winter" by
>ruining the earth's ecology, on a regular basis.  The "periodicity" was, at
>best, suspect, but why not try?  They invoked the mythical "Oort cloud" of
>comets that supposedly (and invisibly) surrounds the solar system, and placed
>the mythical "Nemesis" in an orbit that would perturb them periodically, thus 
>raining destruction on our unsuspecting planet and its inhabitants.

Oh, come now!  I take it, Ed, that you will not consider Nemesis to
exist unless/until it is directly observed.  Does this mean you won't
accept the existence of electrons until someone takes a photograph of one?

The Nemesis hypothesis postulates that the Sun is part of a binary
star system. At perihelion (closest approach to the Sun), the 
companion passes through
the quite non-mythical Oort cloud (a large and only roughly-defined
region surrounding the Solar System where, based on numerous
observations of first-pass and long-period comets, most comets
appear to originate).  The companion is big enough and slow-moving
enough to "scramble" the orbits of many comets, some of which
fall into the inner solar system (most are ejected into deep space).
This creates a "comet shower" lasting of order 1 million years,
during which the Earth is much more  (10x) likely to be hit by 
a comet than usual.  

	The existence of Nemesis was proposed to account for a
26 million year periodicity in the fossil extinction record,
combined with strong evidence that at least one extinction
(the Cretaceous-Tertiary, 65 Myears ago) was caused by some sort
of large-body impact).
The periodicity (with some changes in best-fit timing) remains statistically
significant after several years of (often very skeptical) analysis;
the most recent results involve tracking the extinction rate for
some 10,000 genuses (originally only a few hundred families were 
tracked).  The theory made several predictions, notably that
impact craters would also be found to be concentrated at 
regular time intervals, and that multiple impacts would occur at
each extinction, spread over of order 1 Myear.
(since, if at least one impact
occurs on each "pass", the "average" number of impacts must be 
greater than one).

	The known orbital period gives the size of the orbit:
the major axis of the orbit is 2.7 light years.  The requirement
to go through the Oort cloud gives a 
"most probable" ellipticity of about 0.7, as I recall.  
Passing stars "frequently" (every few Myear) come closer than
the companion to the sun, but they have relatively high
velocity (they're not bound to the Sun) and do not strongly
perturb either the companion or the comet cloud.  Over
about 1 billion years, though, they would be expected to 
disrupt such a weakly bound pair; this means that either the
companion was only captured ~1 Gyear ago or (more likely) it
used to be closer in and has been kicked out to its current
distance in the last ~1 Gyear.

>> What kind of star is it?  
>It is a mythical star.
>> Is it visible through a telescope?

	The companion star has a mass between approximately
.01 and .1 solar masses.  Smaller and it would require a very unlikely
orbit to produce the postulated effects; larger and it would be
too conspicuous an object to have been overlooked so far.  This 
makes it a "brown dwarf", probably too small to have significant
fusion going on, and too cold (surface temperature below 2000 degrees)
to produce significant visible light, though it should be
a fairly bright infrared source.  It might, however, be bright
enough to look like a very dim red star; as such it would
be (at first look) indistinguishable from numerous red dwarfs and
particularly from much more distant red giant stars.  UC Berkeley
is still running a search that will inspect all catalogued faint red stars in
the northern hemisphere looking for "parallax" -- evidence that
the star is close, because it moves against the background.
However, a failure of this search will not rule out a companion, 
just prove that it is not a) bright in the visible and b) in the
northern hemisphere.  Note that there is no a priori reason for the companion
to lie in the plane of the solar system; it could be anywhere in the sky.

The Nemesis hypothesis is indeed "just" a hypothesis, in that there
is no direct confirmation.  The two predictions above, however, have
certainly not been disproved, and there is some evidence in their
favor -- some periodicity in the handful of well-dated craters, and
some evidence for multiple impacts at the major extinctions, including
evidence for _both_ ocean and land impacts at the CT boundary.
Currently no other theory accounts as well for periodicity of either
extinctions or craters, or for multiple impacts.

>>  Was this just a trendy theory that got tossed out after a while?

 No, absolutely not.  It is a speculative theory, but one which
has some support and no contradictory evidence.

>> 	My wife's sixth-grade students want to know...
>Teach them the difference between theory and observation, and you'll do
>them a life-long favor.

	Agreed.  But make sure they neither accept theories as fact
_nor_ dismiss them as falsehoods.

>Ed Nather
>Astronomy Dept, U of Texas @ Austin

Jordin Kare
Special Projects Group, LLNL
-formerly at- U. C. Berkeley/ Lawrence Berkeley Labs.

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