The conventional wisdom is that a cosmic impact caused the extinction of the dinosaurs 65 million years ago — but now two scientists are adding an unconventional twist to the theory: Maybe, they say, dark matter helped.
The hypothesis, set forth in a research paper by Harvard theoretical physicists Lisa Randall and Matthew Reece, blends two of the science world's biggest mysteries: What is behind our planet's waves of mass extinctions? And what is the nature of dark matter, the mysterious stuff that so far has been detected only by virtue of its gravitational effects?
Physicists still haven't figured out what dark matter is, but Randall and Reece suggest that there's a thin, dense disk of the stuff running along the midplane of our Milky Way galaxy. Our solar system traces an up-and-down, wavy motion through that plane as it travels around the galaxy, like a cork bobbing in a lake.
When the solar system passes through the disk of dark matter, that could exert enough of a gravitational influence to disrupt the orbits of comets on the solar system's edge. Such comets could become more likely to head into the inner solar system, periodically raising the chances of collision with Earth.
Some astronomers see a rising and falling pattern in the rate of cratering on Earth, tracing a cycle that lasts 25 million to 35 million years. Could extinction-level "storms" of comets account for those periodic rises in cratering? "We conclude that if a dark disk exists, it could play a significant role in explaining the observed pattern of craters, and perhaps even mass extinctions," Randall and Reece say in their paper.
The big 'ifs'
There's more than one big "if" to Randall and Reece's scenario. First of all, the evidence for periodicity in Earth's cratering rate is sketchy. Even the authors admit that the pattern is "not clearly established," and a different researcher concluded in 2011 that any such perceived pattern is merely a statistical artifact.
It's also not clear what role comets may have played in Earth's die-offs. The prevailing view is that the dinosaurs' demise was sparked by a giant asteroid, not a comet. Catastrophic climate change has been cited as the likeliest suspect in other mass extinctions.
Moreover, the existence of the galactic dark matter disk is not yet proven. Over the past few years, readings from the European Space Agency's Planck satellite and NASA's Fermi Gamma-Ray Telescope suggest that a particular kind of glow emanating from the Milky Way's center may indeed be caused by dark matter collisions. This week, a different team of astrophysicists shared a new analysis of gamma-ray data that lent further support to such claims. But more evidence is still needed.
Planet X? Forget about it!
More evidence could be on the way, at least as it pertains to dark matter. A detailed analysis of gamma-ray data for the dwarf galaxies orbiting the Milky Way could confirm (or rule out) the signature of dark matter annihilation. Meanwhile, Randall and Reece say ESA's recently launched Gaia satellite could produce better data about the potential for cometary perturbations.
"More precise measurements of the Milky Way's properties will thereby provide a sharper statistical test of the comet shower hypothesis," they write.
Even though the dark matter doomsday scenario is still highly speculative, it has gained more credence than other proposed explanations for periodic comet showers.
One such explanation, known as the "Nemesis hypothesis" or the "Death Star hypothesis," proposes that the sun has an as-yet undetected companion star. A similar hypothesis suggests there's an unseen "Planet X" on the edge of the solar system. Supposedly, the gravitational effect of Nemesis, or Planet X, or Nibiru, would periodically make comets go haywire.
All those concepts were dealt a potentially fatal blow on Friday, when NASA said data from its Wide-field Infrared Survey Explorer revealed thousands of previously undetected stars and brown dwarfs in our celestial neighborhood ... but no evidence of a Planet X.
"The outer solar system probably does not contain a large gas giant planet, or a small companion star," Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Penn State University said in a NASA news release. A paper on the findings appears in the Astrophysical Journal.
Update for 3:20 a.m. ET March 9: The scenario for the dark matter disk and mass extinctions has been met with respectful if not enthusiastic comment from other astrophysicists. On Nature's website, Mario Livio of the Space Telescope Science Institute is quoted as saying the idea is "very interesting" even if the evidence is far from compelling. And Luigi Foschini of the Brera Astronomical Observatory told New Scientist that "it is always worth searching for as many hypotheses as possible."