by David M. Raup
"The Sun has a Dark Sister. Long ago, before even great
grandmother's time, the two suns danced together in the sky. But the
Dark One was jealous that her sister was so much brighter, and in
her rage she cursed us for not loving her, and loosed comets upon
the world. A terrible winter came, and darkness fell and bitter
cold, and almost every living thing perished. After many seasons,
the Bright Sister returned to her children, and it was warm and
light once more, and life was renewed. But the Dark Sister is not
dead. She is only hiding. One day she will return."
One view holds that the Sun has a companion star (the red dot-
click image right) on a
highly eccentric orbit that enters the Oort Cloud around its
perihelion, and therefore periodically sends comets cascading in
toward the planets.
- "Comet" by Carl Sagan
CF: Nemesis: (Greek Mythology) The goddess of retributive justice or
It is said "History repeats itself." We are now in a transitional
period from Industrial Civilization to Information Civilization.
Cycle or wave rather than simple extrapolation of the present line
or trend gives us fertile imagination of changes.
By this Nemesis story, we can imagine the longest, 26 million year
cycle of change!! May the civilization which can imagine such a
longest cycle continue on at least duration of about half of this
Cuvier vs. Lyell
Death of species
Iridium Anomaly, etc.
Two Opinion Polls
Nemesis is born
1. Nemesis story:
Nemesis is one of several names given to our Sun's small companion
star. This little star is now about two light years away and moving
away. But in another few million years, it will return and head back
toward the Earth. The inward trip will take another dozen or so
million years, and before the orbit is complete, Nemesis will pass
through the Oort Cloud, an envelope of billions of comets that go
around the Sun beyond the outer planets.
As Nemesis passed through the Oort Cloud, its own gravitational
force will deflect some of the comet orbits in random ways. Some
will be sent in toward us.
As a result, one or more of the errant comets will collide with
Earth. And we know from the geological records of Earth history that
such collisions can be devastating. One incident killed the
dinosaurs and another got the last of trilobites. Many of the major
biological crises of our past, the mass extinctions were evidently
caused by large-body impact.
And because the Nemesis orbit has a period of 26 million year.
The Nemesis story is now familiar, but there are a few inescapable
First, nobody has ever seen Nemesis and there is no direct
observational evidence that our Sun has a companion star.
The Oort Cloud of comets has never been seen.
The demise of the dinosaurs by comet impact is debatable.
The 26 million year periodicity in mass extinctions may or may not
Are we living on a safe planet or should we have chosen a better
Dinosaurs' mass extinction 65 million years ago:
which provided the space for our mammalian ancestors to evolve and
diversify. Humans are probably here now because (among many other
factors) of the death of the dinosaurs.
Chances of comets or asteroids hitting the Earth:
In 1908, an extremely small
comet fragment exploded over Tunguska in
Siberia and knocked down about 6,000 sq.miles of forest.
As to the dinosaur's extinction 65 million years ago, the impacting
body has been estimated at about 6 miles in diameter. Our atmosphere
was so choked with fine debris and water vapor that the entire earth
becomes dark. This would prevent photosynthesis and cause the demise
of active animals dependent on plants fro food.
3/4 of stars in our galaxy are double or multiple stars. It has been
natural to concentrate the SETI search on single stars. But the
possibility of a small second star raised the possibility that the
evolution of complex life may thrive on (or even require) the
adversity of this kind of double-star system.
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2. Cuvier vs. Lyell:
Much of the debate over the Nemesis theory stems from the
implications of "catastrophism." What is a catastrophe? It is
something sudden and something not predicted in advance. It is big
in comparison to what is normal or expected. This is also an element
For the geologist, there has long been a question of whether
catastrophic events accomplish more change in the long run than the
sum of everyday calmer, background processes. (Catastrophism vs.
Meteorite is the term given to any rock found on Earth that is of
extraterrestrial origin, regardless whether they are comets or
asteroids. Comets undoubtedly hit the Earth and make craters, but
none has ever been positively identified (except perhaps at
Tunguska). The composition of comets is believed as "dirty
snowballs" - mainly ice studded with rock fragments. But we have not
had specimens to analyze.
At present, about 100 impact craters have been authenticated. They
range in age up to about two billion years and in size up to 140 km
Geologic time scale from the past 600 million years of Earth
history. The sequence of eras (Paleozoic, Mesozoic, Cenozoic) and
periods (Cambrian, Ordovician, Silurian, and so on) is base
primarily on fossils. Followings are several of the major extinction
65 myr: K-T (Cretaceous/Tertiary boundary)
220 myr: Late Triassic (Triassic/Jurassic boundary)
250 myr: Late Permian (Permian/Triassic boundary)
365 myr: Late Devonian (Devonian/Carboniferous boundary)
440 myr: Late Ordovician (Ordovician/Silurian boundary)
600 myr: Base of Cambrian (Precambrian/Cambrian boundary)
Schindewolf, German paleontologist:
proposed that the Permian mass extinction had been cause by a nearby
exploding star, a supernova. If the supernova were only ten light
years away, the visible and infrared radiation would produce a heat
wave lasting many weeks and attendant climatic effect. Also, the
atmosphere would be bombarded by high doses of X-rays and
There are historical records of sighting of very distant supernovae:
in Europe in 1604 and 1572 and in China in 1054. It has been
estimated that a supernova explosion within 100 light years may
occur on average every 750 million years. McLaren, Canadian geologist, suggested that the Devonian extinction
was an indirect result of an enormous meteorite impact.
found at the Cretaceous-Tertiary boundary.
Also in 1984, in Australia was found at the top of Devonian
Harold Urey, US chemist, Nobel laureate:
Oxygen isotope ratios in fossils to deduce temperature of the
geologic past. He looked at the date of extinction events and the
ages of tektites. Tektites are small glassy blobs that are found
occasionally in soils and rocks produced by meteorite impact. He
interpreted the most boundaries in the geologic time scale are
placed at significant extinction points.
E.J. Opik, Irish astronomer:
suggested that the damage of comet impact would be limited to
"lethal area" around the impact point. Because many plants and
animals are naturally restricted to a single area or region. This
provinciality might make it easier to produce a mass extinction of
the type we see in the fossil record.
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3. Death of species:
How long species last?:
The averages fall within a surprisingly narrow range: from about
1-10 million years.
40 million species are alive today.
Species living today are only less than 1% of the total number that
Ubiquity of extinction ware recognized early in 19th century.
(Chronology of Earth history)
Extinction are not uniformly distributed in geologic time. Some
intervals, which we now call "mass extinctions."
Origin of species:
There are two kinds of species origination.
One is the "origin of species" that Darwin talked about: simple
change in a single evolutionary line over time by natural selection.
If this change is substantial enough, the descendant is a new
The other is when a lineage of a species branches or buds to form
another, coexisting species. The branching process of speciation has
been the subject of an enormous amount of research in evolutionary
biology in recent years.
The number of species extinctions has probably been about the same
as the number of speciation events. Two processes - origination and
extinction - that are as different as birth and death but that have
remained in reasonable balance.
Platitude or tautologies:
"Species go extinct when the size of the breeding population
approaches zero", or "Species die out because they can no longer
But the dinosaurs had been doing very well for 140 million years and
then, over a fairly short time, they died out completely. The
mammals had been coexisting with dinosaurs for upwards of the 140
Dinosaurs did not rule the Earth any more than lions today rule the
Earth. At their acme, dinosaurs had as few as 50 species living at
any one time. There are about 5,000 species of mammals living today
and about an equal number of reptiles. (In Mesozoic more reptiles
and fewer mammals) Dinosaurs were a minor part of the biology of
Earth was in a cooling phase during the late Cretaceous, and
dinosaurs were generally confined to the warmer regions. Also the
number of species of dinosaurs did decline: as few as 25 coexisting
near the end.
Marine sedimentary sequence is more complete in fossilization, which
indicate severe extinctions near the end of Cretaceous. Out of 790
families of marine animals 120 (15% ) were extinct by the end of
Cretaceous. The figure for the taxonomic level of genus is
We tend to think of mammals as survivors, but may mammalian groups
were hard hit and lost most of their species. The marsupial mammals
suffered profound losses and nearly died out.
In any event, the best available estimates are that between 60-80%
of marine species died out. This is not quite as high as 96%
estimate for the Permian mass extinction.
On the survivors' side, reef corals themselves got through the
crisis pretty well, as did most deep-sea animals. Future research to
identify the winners and losers, so that we have a better chance of
learning what environmental stresses were responsible for the
Pleistocene extinction event: (7,000 -10,000 BC) Mammoths,
mastodons, horses, camels, sloths, sabertooths, and other large
animals once thrived in North America died out in a rather short
interval. The kill rate was about 70%. But the killing was almost
completely restricted to large, terrestrial mammals. The timing is
about right for the migration of early man from Asia to North
America, also hunting sites have been found. This may have been the
first man-made extinction.
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4. Iridium Anomaly, etc.:
Iridium is normally almost absent from the Earth's crust, but
relatively common in meteorites.
The article in Science, June 1980:
by Luis and Walter Alvarez, etc.
A large asteroid hit the Earth 65 million years ago and force of
impact sent up into the atmosphere about 60 times the asteroid's
volume in pulverized rock and fragments of the asteroid itself (with
is iridium). The atmosphere became so clogged with dust that
sunlight was blocked and photosynthesis was inhibited. This, in
turn, broke down food chains and led to the demise of animals
dependent on plants for food. The size of the impacting asteroid was
estimated from the amount of iridium. The diameter of the body was
10 plus or minus 4 km.
Karl Turekian of Yale in 1981 presented a paper:
Osmium is another
of the platinum-group elements that is commonly present in
meteorites but extremely rare in ordinary rocks of the earth crust.
Furthermore, the ratios of the isotopes of osmium differ
substantially between the crust and meteorites.
The paper reported osmium isotope ratios much closer to those of
meteorites than the crust, and concluded with a strong statement of
support for impact at
K-T boundary. In fact, minor difference in
osmium isotope ratios among the several sample suggest that there
might have been more than one impact.
Two separate minerals called stishovite and coesite, both form of
quarts, are often associated with the shock metamorphism. Bruce Bohor at USGS reported finding shocked quartz at K-T boundary sites
both in Europe and North America. This was impressive, because the
shocked quartz is a tried-and-true indicator of impact.
Smaller glassy particles called microtektites have been found in
some sedimentary rocks, and these are interpreted to be byproducts
of meteorite impacts. They found tiny spherules as altered
microtektites. There remains much argument over whether the
spherules were originally microtektites.
More Iridium-Anomaly sites:
The K-T anomaly had been found around the world and in virtually all
kinds of sedimentary environments, from the deep sea to swamp
deposits on the continents.
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5. Two Opinion Polls:
Scientific questions ought not to be settled by popular vote, but
the opinions of groups have an enormous influence on the course of
In summer of 1984, quasi-scientific survey of about 500 geologists,
paleontologists, and geophysicists in Europe and North America:
21%: convinced mass extinction caused by meteorite impact
40%: There we a K-T impact but did not cause the extinctions
27%: There was no K-T impact
12%: There was neither mass extinction nor K-T impact.
In October of 1985, 118 out of 300 attendant of the Society of
On the question of extinction:
4%: accepted the impact as the major cause of extinction. (The
dinosaurs had been in decline for a long time before the meteorite
43% accepted the impact but did not caused the extinction,
27%: felt there was no mass extinction
Times article in fall of 1985 by Robert T. Bakker, a dinosaur
expert, Univ. of Colorado Museum:
"The arrogance of those people is simply unbelievable. They know
next to nothing about how real animals evolve, live and become
extinct.... The real reasons for the dinosaur extinctions have to do
with temperature and sea level changes, the spread of diseases by
migration and other complex events. But the catastrophe people don't
seem to think such things matter...
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6. Periodic Extinction:
In 1977, Alfred G. Fischer and Michael A. Arthur published a paper titled "Secular Variations in the Pelagic realm", claiming that the
major extinctions of the past 250 million years were evenly spaced,
coming every 32 million years. This was anathema!
We all knew that the history of the Earth was too complex to be
amenable to such a simplistic description. What would keep the
system on time?
In spring of 1983, John Sepkoski, Jr, a paleontologist and the
author published that the extinctions seemed to be regularly spaced
Suppose a hypothetical experiment:
Draw a card from an ordinary deck of cards every morning for 250
days. If the card is a black ace, we put an X on a calendar for that
day. The card is replaced and the deck shuffled for the next day. At
the end of the 250 days, there will be a scattering of X's, but how
will they be spaced?
On the average, the X's should occur every 26 days (the chances are
2/52) How these experimental distributions really look like?
The results are completely typical of points arrayed on a line at
random. Rather than a waiting time of about 26 days between black
aces, we find that most gaps are smaller. A few long gaps are
balance against a lot of short ones to produce the average of 26.
Periodic extinction events for the past 120 million years (myr):
below red dots show the most probable positions in time of the 8
statistically significant extinctions. The horizontal black bars
show the worst case uncertainty. Each of the events has been
assigned a "cycle number" following the hypothesis that the events
are exactly 26 myr apart.
The blue straight line defines a perfect
fit to the 26 myr periodicity. Two events, numbered 5 and 7 are
missing from the record: either they did not occur or they have not
been found. The most recent 4 events are the best dated and fit the
hypothesis. The K-T events is the third one.
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7. Nemesis is born:
The Sun's motion in the galaxy:
The Galaxy is disk-shaped. As it rotates in space, our Sun and its
planets move slowly up and down the galactic plane. Solar system to
complete a full cycle of movement between 62-67 million years. The
Sun crossed the galactic plane twice in each complete oscillation
every 31-33 million years.
Our sun is currently very close to the galactic plane, yet the most
recent extinction was 11-12 million years ago. It suggests that we
are now about midway between two events, which should put us near
the maximum distance from the galactic plane!
The Companion Star:
A small solar companion on a highly eccentric orbit - an orbit that
carries the companion through the Oort Cloud once per revolution
about the Sun. Accidental disturbance of comet orbits in the Oort
Cloud then produces a comet shower on earth and the comet impacts,
causing mass extinction. The companion star must be very small less
than a tenth of the Sun, and positioned now about two light years
If there is a companion star, why have we not seen it? The companion
would be by far the closed star to the Earth, about half the
distance to the next closest, Proxima Centauri.
In Jan. 1985, D.P. Whitmire and J.J. Matese suggested that the comet
showers could also be produced by an unseen tenth planet, Planet-X,
lying beyond the orbit of Pluto.
Periodic extinction under fire:
The reaction among paleontologists
was largely negative
They (the authors) did not use a
standard definition of mass extinction
The fossil record is too
incompletely known for broad and valid statistical analysis
The taxonomy of most fossil groups
is too messy to allow use of catalogs of families and their time
The uncertainty in geologic dating
undermines any attempt to track the history of life with enough
precision to find such cycles
The appearance of such periodicity
may just be a result of the uncertainties in classification and
dating of fossils
They used a culled sample of only 567 families
In the analysis, family extinctions were assigned to stratigraphic
intervals, averaging a little more than six million years each.
This explains the apparent regularity in the spacing of events
Each extinction was caused by
different and independent forces
Extinctions are complex events
controlled by many independent factors. A search for simple
causes is futile
Long-term changes in sea level are
the major cause
Long-term changes in climate are the major cause.
Several of the criticism related to the uncertain nature of the
If there is uncertainty in the observation data, any
conclusions based on them will be uncertain. This is true in some
things, but not with the kind of statistical testing.
One rather curious objection to the Nemesis idea is that a wobble in
the orbit would cause an average of 10% variation in the length of
the period. It is too perfect to be explained by Nemesis.
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From Chicago Sun-Times in 1985 wrote:
"The discovery by University of Chicago paleontologists
and J. John Sepkoski Jr. at first sounds like a vaudeville joke: The
bad new is: The end is coming. The good news is; It's 13 million
years away.... We always thought of the dinosaur as dumb and
deserving to be extinct. Not true. Dinosaurs, it turns out, were
merely victims of circumstances.
Certain life-forms are subject to mass extinction, and this happens
roughly every 26 million years. A new life-form then arises. This
caused quite a stir among other scientists to figure out why...."
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