The Science of Shooting Stars

by Jim Al-Khalili

As every parent and teacher knows, when it comes to science the topics guaranteed to grab a child’s attention (or an adult’s) are space and dinosaurs. So, after many years of being treated to a bountiful procession of earnest popular science books whose titles always seem to include a subset of the words “creation,” “fabric,” “reality,” “universe,” “elegance” and “beauty,” one called “Dark Matter and the Dinosaurs” was bound to grab my attention. Yet despite the title, this is no children’s book, or even a clever, if blatant, marketing ploy. This is solid, if somewhat speculative, science. As unlikely as it may sound, Lisa Randall’s book hints at a potential connection between the sudden extinction of the dinosaurs 66 million years ago and that mysterious cosmic substance known as dark matter that continues to confound and thrill physicists in equal measure.

Ms. Randall, a Harvard professor, is one of the world’s leading theoretical physicists, and her book — there is no other way of putting this — is a cracking read, combining storytelling of the highest order with a trove of information on subjects as diverse as astrophysics, evolutionary biology, geology and particle physics. What’s remarkable is that it all fits together.

The book begins a little ploddingly. Certainly Ms. Randall is not in a hurry to justify her title or advertise her main thesis. What hooked me, though, was her fascinating disentangling of space terminology: meteoroids (lumps of rock in space that occasionally hit Earth), meteors (the visible streaks of light, or “shooting stars,” that meteoroids create as they burn up in the atmosphere) and meteorites (the surviving fragments of rock that land on Earth if the meteoroid doesn’t completely burn up). OK, got that? Good. Let’s not confuse them anymore. Similarly, larger objects in space that are nevertheless smaller than planets are either giant rocks in the inner solar system (asteroids) or mini worlds of ice and dust in the outer solar system (comets).

Having become captivated, I revised my impression of the book’s pace from “plodding” to a far more generous “unhurried,” for Ms. Randall leads us on an epic journey that begins with asking what wiped out the dinosaurs. We have all heard, of course, that it may have been the stupendously powerful impact of some extraterrestrial body (think asteroid or comet). But what many will not have appreciated is that this event is really no longer in any doubt. Ms. Randall, showing a thorough level of research into a wide range of disciplines, takes the reader by the hand and patiently unpacks the evidence. One of the characters she discusses is the geologist Walter Alvarez, who first proposed the impact hypothesis. His book “T. Rex and the Crater of Doom” did lead me to wonder whether his title had given her the courage to choose her own.

Once we have been persuaded that the extinction of the dinosaurs, and probably other major extinction events in Earth’s history, could have been caused by a comet impact, Ms. Randall moves on to a more interesting question: Are mass extinctions periodic and, if so, why? They certainly seem to be, though she acknowledges that the mass extinctions discernible in the fossil record could have been triggered by any number of causes: volcanic eruptions, tectonic plate movements, climate change brought about by solar cycles or changes in the Earth’s magnetic field, or — the thesis of this book — extraterrestrial impacts.

The whole subject of extinctions is messy and complicated, since it involves biology and geology, and rather than insist on one cause, Ms. Randall cleverly turns to a different question: whether extraterrestrial impacts themselves are cyclic. There is, she points out, persuasive evidence from studying impact craters that large bodies have smashed into the Earth on a regular basis every 20 million to 30 million years or so. This periodicity might, she suggests, come from the way the solar system wobbles up and down as it orbits round the center of our galaxy. The puzzle is why this wobble would lead to an increase in the chances that the Earth will be bombarded by comets.

As you can begin to tell, the sequence of cause and effect starts to get convoluted. But the bottom line, and the topic that Ms. Randall and her colleagues at Harvard are working on, is that it is all down to dark matter, the elusive, invisible “stuff” that is needed if we are to understand how galaxies are able to remain intact and indeed how they formed in the first place. For without the additional gravitational glue provided by dark matter, we would be unable to explain the structure of our universe. Like the impact that wiped out the dinosaurs, the existence of dark matter is not really in doubt any longer — the evidence, from the way stars move within galaxies or the way light is bent round regions of dark matter, is too strong.

But while physicists debate what dark matter might be made of, and how we might go about finding out, Ms. Randall and her team suggest something new: that dark matter may, for reasons I will leave you to find out from the book, form a thin, dense disk in the plane of the galaxy and that the solar system may pass through this disk every — yes, you’ve guessed it — 20 million to 30 million years. The dark matter applies a gentle tug on a junkyard of debris floating around the outer reaches of the solar system, known as the Oort cloud (where, crucially, comets come from), knocking the icy missiles from their orbits and sending a few hurtling earthward.

The whole chain of events may sound tenuous, to say the least, but remember that Walter Alvarez’s suggestion that a comet impact wiped out the dinosaurs 66 million years ago was just as speculative — wacky, even — as recently as the 1980s. Nevertheless, the last third of the book is spent speculating on the nature of dark matter and how likely it is that Ms. Randall’s hypothesis could be correct. The jury is still out and will be for some time, but I do hope she turns out to be right — the book title is too good.

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