Nemesis!

Highly recommended, as a readable introduction to the process of working through from scientific hypothesis to theory, is Richard Muller's Nemesis. (By remarkable coincidence it is available in this library, or it will be, once I turn it in.)

The Nemesis theory is a fun one.  Having a look at the graph below will help explain it (from Rampino MR, The galactic theory of mass extinctions: An update. Celestial Mechanics & Dynamical Astronomy 69 (1-2): 49-58):



What we're seeing here is the number of living species (represented by the rather brutal 'species kill percentage') over time.  We see that there are frequent "death spikes", where large numbers of species perish all at once.  It turns out these even happen with some regularity, something like every 26-30 million years, depending on how much you trust the age measurements of the fossils.  You can see that the death spikes are often accompanied (especially in more recent times, when they're easier to find) by signs of comet, asteroid, or meteor impacts--leaving behind craters and iridium and kicking up dust ("ejecta").

The fact that the impacts are happening fairly regularly isn't really the subject of debate any more (Muller's book takes you through some of the early discoveries of these impacts).  But the cause and exact frequency of the impacts is another matter.  Why are we getting bombarded with such regularity?  It can't be coincidence.

Muller and his colleagues originally came up with the idea of Nemesis, a theoretical companion star to the sun.  When it approaches every 26 million years or so, it wrenches a bunch of comets out of their stable, distant lives in the outer solar system, and sends them hurtling towards the sun, some of which will eventually impact Earth.  The book takes you through the development of the hypthosis as he rules out several potential alternate theories, debates evidence with other teams of scientists, and fights to prevent the mass media from completely getting it wrong or embarrassing him.  It's a great window into the world of scientific discourse.  How are theories developed?  How are they disproven?  How do teams of experts in different fields effectively work together?  What happens when another reputable team has a different theory?  How do "nutty" theories gain respect, and how does any theory stand up to scrutiny?

The book was published in 1988, so it's somewhat dated, but the theory itself is still around and depending on what you read, appreciating about the same level of moderate acceptance.  It's left with Muller preparing to search for the Nemesis star, and nearly twenty years later it has still never been definitively identified--making it easy to write off the theory.  Detractors also usually cite a lack of evidence for the periodicity of the extinctions (despite the graph seen above, the dating isn't entirely accepted), or insufficient proof that Nemesis could have a stable orbit with respect to the sun.  The most commonly-studied alternate theory these days is that the solar system actually oscillates up and down with respect to the galactic plane while it orbits around the galactic center, and when it crosses this plane (every 26 million years), the extra mass of dust and gas nearby is enough to perturb some comets.  Muller actually considers and dismisses this theory within his book.  So, the matter is hardly settled.

Anyway, while it's obviously not helpful to the theory that no candidate star has been found, it's not hopeless either--such a star would be very small and currently is at its furthest from our solar system.  So the good news is that if we believe the theory, the "putative death star" (as geologist Eugene Shoemaker referred to it) is a good 10-15 million years away from a return trip.

Posted at 05:18PM Oct 09, 2006 by WILSON, JOSHUA in Book Reviews  |