There’s one sure way to spice up science headlines: add dinosaurs—and when you combine dinosaurs with space and exotic cosmology, you can’t go wrong.
Thirty years ago, a scientific hypothesis brought space and extinction together. Scientists noticed what appeared to be a regular repeating pattern of massive extinction events in Earth’s history, falling roughly every 26 million years. What could explain such a pattern? One possibility is that the cause lies in the ecosystems themselves: something about biological communities causes them to fail when they reach certain milestones. Another possibility is that the cause is external, resulting either from the geological processes of the Earth, or perhaps from something extraterrestrial.
Perhaps the most famous (though not the most severe) extinction event in history is the one that occurred at the end of the Cretaceous period and brought about the end of the dinosaurs; the evidence is now conclusive that a major meteor, asteroid, or comet impact near the present Yucatan played a decisive role in this extinction event. But if an impact event caused at least one major extinction event, could impacts be a more general cause of extinctions? Other evidence seemed to show, in addition to periodicity in extinctions, a similar periodicity of around 30 million years or so in the rate of impacts on the Earth’s surface. We thus have two pieces of evidence that fit together: extraterrestrial impacts and extinctions both occurring at similar intervals, the former being an obvious potential cause of the latter.
But what could cause impacts from space to occur in a regular pattern?
Periodicity is not uncommon in astronomy, and there’s an obvious source: orbits. Objects that travel in orbits around other objects naturally tend to display regular, periodic behavior. This line of thinking led to the proposal that it was in fact orbital motion that led to the regular peppering of Earth with extraterrestrial objects.
Far out beyond the orbit of the most distant planets lies the Oort cloud, a vast spherical region surrounding our Sun that is populated by icy bodies that, when they fall near the Sun, form comets. When the early solar system formed, the massive planets carved out a relatively clear bubble inside of which the Earth and the familiar planets orbit. The remaining debris ended up in one of three places: either sucked into the Sun or the forming planets, shepherded into the asteroid or Kuiper belts, or flung into the deep space of the Oort cloud. Every now and again, one of these Oort cloud objects comes falling into the inner solar system, appearing in our skies as a comet, but generally these bodies orbit slowly in the distant reaches, uninclined to make their way inwards.
Unless, that is, a gravitational tug upsets them, casting them in and out in showers of cosmic debris. This sort of gravitational tug can be given, for example, by a passing star—and it was just this realization that led, in 1984, to the suggestion that our Sun was not alone.
Instead, it was proposed that the Sun (like most stars in the galaxy) had a companion, a dim and distant dwarf (dubbed “Nemesis”) that followed in a wide orbit. Every thirty million years or so, Nemesis swung close enough in to the Oort cloud to scatter comets about, sending some of them inward on paths that would inevitably lead at least a few to impact the Earth. Thus, we find a pattern to impacts and, subsequently, extinctions on Earth.
In the years that followed, however, the Nemesis hypothesis foundered on the fact that no trace of the hypothetical star was found by any observations. The latest evidence supports the non-existence of Nemesis. A thorough examination of data from the WISE (Wide-field Infrared Survey Explorer) spacecraft, published just recently, found no bodies of significance out to 10,000 AU, and no signs of larger bodies at farther distances have been forthcoming. Nemesis was proposed as speculation, and a quarter century of observation has led to no observational support. The Sun, as far as we can tell, is alone, and no large planets or dwarf stars lurk beyond Neptune that could explain disturbances of the outer solar system that could lead to periodic comet showers.
But that doesn’t leave us bereft of hypotheses. The newest idea to explain periodic comet showers has to do not with large bodies disturbing the Oort cloud with gravity, but rather with the motion of the entire solar system itself. What’s more, the newest iteration of this latest hypothesis invokes one of the hot topics of modern astrophysics, dark matter.
Just as the planets of the solar system orbit the Sun, the Sun itself orbits the center of the Milky Way galaxy, making a giant circuit that takes hundreds of millions of years to complete. But unlike the fairly flat orbits of the planets, the Sun’s orbit is wavy, coasting up and down through the plane of the galaxy. The reason for these waves is that the galaxy’s disk contains a significant amount of mass: as the Sun drifts upwards, the galaxy’s mass pulls it back down, until it plunges through the disk and emerges on the other side, drifting away until gravity pulls it back, repeating the process over again.
As it turns out, the period of this motion above and below the galactic plane appears to be about seventy million years, meaning that the solar system passes through the galactic plane about every—you guessed it—thirty-five million years or so, notably close to the purported period of increased impacts and extinctions. That period alone is enough to be suspicious; it may very well be that the ordinary gravitational disturbance of passing through the plane alone is enough to upset the distant solar system. The newest examination of the theory, however, invokes the latest speculative physics. Dark matter is a proposed form of matter that makes up the bulk of the universe and that interacts with normal matter in ordinarily negligible ways; there is no known means of detecting dark matter directly, but it is invoked to explain a number of observations that cannot be explained by normal matter alone.
Suppose, then, a new paper asks, that not only does the galactic disk contain a high concentration of normal matter, but it might contain disks of dark matter as well, and as the solar system plunges through these disks on its rising and falling track around the galaxy, it is this dense dark matter that causes icy comets to be scattered and sent to collide with the Earth.
Perhaps. Or perhaps motion through the galaxy alone is enough, even without dark matter, to account for the solar system’s disturbance. Or, it could be that we’re chasing phantoms—the statistical detection of impact periodicity and extinction patterns is weak: maybe there really is no periodicity to be explained. As always in science, time will give us more to consider.
Image: Nature/C. Carreau-ESA