Its name has always made it sound ominous – and now dark matter could have a menacing role in Earth’s history. A recent explanation for the identity of the mysterious stuff leads to a scenario in which it could be to blame for the extinctions of dinosaurs, or at least send a few extra comets shooting our way.

Although the sequence of events connecting dark matter to dinosaurs, or even comets, is still pretty tenuous, it is intriguing because it brings together two big open questions: the identity of dark matter and whether there is a pattern to comet strikes on Earth.

It is also almost poetic to think that dark matter, which gets its name from its mysterious nature, could have helped to destroy ancient life on Earth.

Astronomers reason dark matter must exist because of its gravitational pull, which shows up as an inexplicable tug on the motion of galaxies, but they don’t actually know what it is.


Last year, Lisa Randall and Matthew Reece of Harvard University and their colleagues came up with a model that suggested there are thin, unseen discs of dark matter lurking within galaxies, or at a slight angle to them.

Dense disc

As the solar system orbits the centre of our galaxy, the Milky Way, it bobs up and down on a roughly 70 million year cycle. This means it would pass through such a dark matter disc every 35 million years. Randall and Reece noted that this cycle was reminiscent of previous analyses of comet impacts on Earth, which revealed that they appear to spike every 35 million years.

Although a patchy crater record makes it hard to see a firm pattern, the pair wondered if there was a connection between cometary strikes and the solar system’s passage through the dark matter disc.

First, they showed that when this happens, the disc would exert a stronger gravitational pull on the solar system. Such a pull could disrupt the Oort cloud, a distant collection of frozen material thought to surround the solar system and produce some comets, including comet ISON, which last year caused a stir as it flew towards the sun. “The disc is more dense, so the effect of the gravitational tidal force is bigger,” says Randall.

Second, they looked at craters on Earth, larger than 20 kilometres, that were created in the last 250 million years. Comparing their ages with the 35-million-year cycle of the bobbing solar system showed that in some cases, a spike in the frequency of comet strikes closely overlapped passing through the disc, in others it didn’t. Overall, though, analysis suggested that the dark matter idea would be more likely to produce the observed patterns in crater frequency than an even rate of cratering.

Gaia to the rescue

So did dark matter kill the dinosaurs? The Chicxulub crater that has been linked to the extinction of the dinosaurs 66 million years ago does not completely coincide with the solar system passing through the disc – but Randall says there are enough uncertainties that there still could be a link. “It comes reasonably close, it’s not a perfect fit but it’s one possibility,” she says.

A further complicating factor is that craters are formed by comet and asteroid impacts, but only comets originate in the Oort cloud and so are far enough out to be diverted by dark matter. The team hopes to distinguish between the two types of impacts in future to hone their analysis.

Luigi Foschini of the Brera Astronomical Observatory in Milan, Italy, says the theory is worth further scrutiny: “I think it is always worth searching for as many hypotheses as possible.” But he is also wary because neither the pattern in comet strike frequency, nor the disc theory of dark matter is confirmed.

Soon we could know more, about the disc, at least. The recently launched Gaia telescope will map the motion of a billion stars in the Milky Way. Because dark matter should affect their motion, examining stars in the region where the disc should be will help reveal whether there is more dark matter there.

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