Darker than Nemesis: Was it dark matter that killed the dinosaurs?

Nature tries to see what was behind the comet that killed the dinosaurs – and other mass extinctions that seem to happen every 35 million years. One guess: Our solar system passes through a disk of dark matter that knocks meteors and comets into our planet:

Meteorites regularly pepper Earth’s surface. Thirty years ago, physicists suggested that this bombardment intensifies cyclically, pointing to some underlying cosmic cause. One proposed explanation is that the Sun has an as-yet-undetected companion star, dubbed ‘Nemesis’ or ‘Death Star’, that regularly swings by, sending comets from the remote Oort cloud flying into the inner Solar System.

In the latest paper, theoretical physicists Lisa Randall and Matthew Reece, of Harvard University in Cambridge, Massachusetts, reignite another proposal, which puts the supposed periodicity down to the way the Sun — and the Solar System with it — move inside the Milky Way. As the Sun follows the swirling motion of the Galaxy’s arms, circling around the galactic centre, it also moves up and down, periodically crossing the plane that cuts the Galaxy into a top and a bottom half like the two bread slices in a sandwich. The authors suggest that as the Sun oscillates up and down, it crosses a denser layer of dark matter — like the ham in the middle — causing a gravitational push and pull that disturbs comets in the Oort cloud.

Previous models could not account for a gravitational force strong enough to cause the effect. But Randall and Reece show that a thin disk of dark matter at the centre of the Galaxy could do exactly that, causing comet storms with a periodicity of about 35 million years. This would match some weak statistical evidence found in recent surveys of impact craters. Their paper is due to appear in Physical Review Letters.

They compared their model, using a 35-million year cycle, with the record of craters more than 20 kilometres wide and created in the past 250 million years. Compared to random comet bombardments, their model had a likelihood ratio of 3, meaning it agreed with the observed crater dates three times better than a random rate.

So, we’re surrounded by dark matter above and below us… and we wobble into it every so often. And nearly everything on Earth dies. Feeling lucky?