Beware! Comets ☄

Comets, let's face it, have rather a bad name. Traditionally, they're been seen as bad omens. Harbingers of doom, fortelling disaster. So ingrained into our consciousness is this line of thought, that the very word "disaster" stems from words meaning "bad star" (from dis and astro)! Comets have been featured in movies (ranging from the kitsch to the reasonably well received) and authors have written about them for some time. But are they really killers? Maybe comets are just misunderstood...

In all fairness, it's widely accepted that there was a giant impact around 65 million years ago, and it's widely (albeit not universally) believed that this event killed off the dinosaurs in the Cretaceous–Tertiary extinction. While it's been hypothesised that other impacts may have been responsible for certain other mass extinctions in Earth's history, Nathan Kaib and Thomas Quinn of Washington University are evidently not so sure.

The two have run some numerical simulations to investigate long period comets. Long period comets are those which come from the very edge of the solar system. Loosely bound by gravity, the Sun is thought to be surrounded by the Oort cloud -- a shell of icy objects. A nearby star passing too close can perturb these cosmic snowballs, causing them to fall towards the Sun and flare into brilliant comets as they start to warm up. If these errant oort cloud objects aren't flung into interstellar space, they can fall into huge elliptical orbits, orbiting the Sun on timescales of thousands to millions of years.



Although it was long believed that the outer reaches of the oort cloud were the main source of such long period comets, Kaib and Quinn's simulations suggest that the actual source is the inner oort cloud. There's a big difference there. A couple of light years of difference, in fact (the outer oort cloud is believed to stretch as far as three light years away from the Sun!). A close encounter with another star can still cause a shower of comets from here -- though even without such a gravitational nudge, it was still found that most long period comets come from the inner and not the outer oort cloud.

The upshot of their simulations, very simply, it's unlikely that more than two or three of these objects could have struck Earth over the past 500 million years, and that's assuming the maximum possible number of oort cloud objects. The (minor) Eocene-Oligocene extinction about 40 million years ago has been suggested before to be the result of a comet shower. If Kaib and Quinn are right, it must have been the most intense cometary shower to have occurred since the Cambrian period! This reduces the chance that extinctions are caused by comet showers. To use Kaib's own words, "...comet showers are probably not likely causes of mass extinction events."

As Jupiter's recent bruise will attest, the giant planets are very helpful in shielding Earth from cometary impacts, deflecting comets or simply bearing the brunt of their impacts so that we don't have to. Interestingly, this means that whatever giant impact occurred to cause the infamous Cretaceous–Tertiary extinction was a purely random event. A fluke. Frankly, I'm not entirely sure how comfortable I am with that...


Image: Comet McNaught, reproduced with kind permission from Chris Picking of Starry Night Skies Photography. His pictures are great. You should go and look at some more!

Reference: Reassessing the Source of Long-Period Comets - Kaib & Quinn (2009)