At present, our Solar System is in its twentieth orbit of the Milky Way near the inner edge of a spiral feature known as the Orion Arm or, less poetically, the Local Arm. The ghostly arms are not permanent features of a disc galaxy like the Milky Way. Rather, they are concentrations of gas and dust where stars form, produced by disturbances within the Milky Way, or on occasions by a jolt from outside, such as a supernova or the passage of the Solar System through one of the dusty gas clouds that congregate in spiral arms.
Life-threatening asteroids and comets are more frequent when a planetary system is crossing one of the galaxy’s spiral arms, where gas clouds stack up in the equivalent of a hydrostatic jump.
In an extreme case, the heat and light from the Sun might be dimmed by intervening material, spawning an Ice Age. There are doubtless other hazards associated with the passage through what amounts to a galactic traffic jam. The last time the Solar System passed through a spiral arm was roughly 230 million years ago, or its 19th orbit of the Milky Way, at the start of its present circuit.
The spiral region it passed through then was not the Orion Arm, because the arms themselves have been moving and changing as time passes; but the Sun was roughly in the same part of its orbit that it is in now in the Paleozoic Era which began 541 million years ago with the Cambrian explosion, the extraordinary diversification of marine animals, the emergence of life on to land, and the evolution of reptiles and ended about 252 million years ago with the end-Permian extinction, one of the greatest catastrophes ever to hit life on Earth –a series of disasters that wiped out 95 per cent of all marine life on Earth- including the longest ice age in the history of animal life: the 100-million-year Late Paleozoic Ice Age.
It was the Palaeozoic death of so many species that opened the way for the survivors to evolve into new life forms, notably the dinosaurs, and to flourish. Such an extreme extinction event demands an extreme explanation, and although at such a distance in time we can never be certain, observes astrophysicist John Gribbin, author of Alone in the Universe, the circumstantial evidence points to the encounter with a spiral arm, even if we cannot identify the smoking gun itself.
It’s the luck of the cosmic draw, that our Solar System doesn’t encounter spiral arms more often – but why? The reason Gribbin suggests “is partly because of the distance we are from the center of the Galaxy, which places us in a gap between arms, and partly because the Sun’s orbit around the Milky Way is, unusually, very nearly circular. The Sun has stayed in this gap between the arms for a long time because, although the whole Solar System orbits the Galaxy once every 250 million years or so, the spiral pattern takes twice as long to move around the Galaxy. By the time the Sun has completed one orbit, the pattern has moved on by half an orbit, so it takes correspondingly longer to catch up, giving a long time for evolution to do its work before being interrupted.”
“Even in a circular orbit,” Gribbin observes, “a planetary system closer to the center of the Milky Way would encounter spiral arms more often, because the arms wind up towards the galactic center. Planetary systems farther out from the middle of the Galaxy than ourselves might encounter spiral arms even less often than we do; but there are good reasons to think that there may be few, if any, planetary systems out there.”
After approximately 12 billion years, and fewer than 60 orbits around the Milky Way, our sun will reach the end of its fiery existence. We know that the Sun and its planets follow an orbital path around the Milky Way galaxy that completes a circuit every 230 million years or so through a landscape of undulating concentrations of mass and complex gravitational fields, orbiting and drifting in a three-dimensional ballet.
The result is that our solar system, says astrophysicist Caleb Scharf at Columbia University and author of The Copernicus Complex, “like billions of others, must inevitably encounter patches of interstellar space containing the thicker molecular gases and microscopic dust grains of nebulae. It takes tens of thousands to hundreds of thousands of years to pass through one of these regions. This may happen only once every few hundred million years, but if modern human civilization had kicked off during such an episode, we would have barely seen more than the nearest stars— certainly not the rest of our galaxy or the cosmos beyond.”
But could our planetary circumstances have been that different and still produce a radio-creating, space-faring species? Would more changeable orbits in a planetary system or passage through interstellar clouds, also thwart the emergence of life in some way? “It’s a possibility,” suggests Scharf, “that the planetary requirements for forming sentient life like us will necessarily always present the senses and minds of such creatures with a specific cosmic tableau, a common window onto the universe.”
A darker, more radical view of our galactic landscape than Scharf’s is that of Harvard astrophysicist Lisa Randall who asks in her study “Dark Matter and the Dinosaurs: The Astounding Inteconnectedness of the Universe”: “Has Earth’s journey through the Milky Way Galaxy triggered mass-extinction events? If the solar system, as it orbited the center of the galaxy, were to move through the Milky Way’s dark-matter disk, Randall theorizes that the gravitational effects from the dark matter might be enough to dislodge comets and other objects from what’s known as the Oort Cloud and send them hurtling toward Earth. Randall suggests that “those oscillations occur approximately every 32-35 million years, a figure that is on par with evidence collected from impact craters suggesting that increases in meteor strikes occur over similar periods.”
“Those objects are only weakly gravitationally bound,” said Randall. “With enough of a trigger, it’s possible to dislodge objects from their current orbit. While some will go out of the solar system, others may come into the inner solar system, which increases the likelihood that they may hit the Earth.”
Though dark matter is believed to be non-interacting, Randall and Matthew Reece, Harvard assistant professor of physics, suggested that a hypothetical type of dark matter could form a disk of material that runs through the center of the Milky Way.
“We have some genuinely new ideas,” Randall said. “I’ll say from the start that we don’t know if they’re going to turn out to be right, but what’s interesting is that this opens the door to a whole class of ideas that haven’t been tested before, and potentially have a great deal of interesting impacts.”
Our Solar System orbits around the Galactic center, taking approximately 250 million years to make a complete revolution, weaving and bobbing, crossing the plane of the Milky Way approximately every 32 million years, which coincides with the periodicity of the impact variations.
This bobbing motion, which extends about 250 light years above and below the plane, is determined by the concentration of gas and stars in the disk of our Galaxy. This ordinary “baryonic” matter is concentrated within about 1000 light years of the plane. Because the density drops off in the vertical direction, there is a gravitational gradient, or tide, that may perturb the orbits of comets in the Oort cloud, causing some comets to fly into the inner Solar System and periodically raise the chances of collision with Earth. However, the problem with this idea is that the estimated galactic tide is too weak to cause many waves in the Oort cloud.
In their study, Randall and Reece focus on this second hypothesis and suggest that the galactic tide could be made stronger with a thin disk of dark matter. Dark disks are a possible outcome of dark matter physics, as the authors and their colleagues recently showed. Here, the researchers consider a specific model, in which our Galaxy hosts a dark disk with a thickness of 30 light years and a surface density of around 1 solar mass per square light year (the surface density of ordinary baryonic matter is roughly 5 times that, but it’s less concentrated near the plane).
So, did a thin disk of dark matter trigger extinction events like the one that snuffed out the dinosaurs? The evidence is still far from compelling. First, the periodicity in Earth’s cratering rate is not clearly established, because a patchy crater record makes it difficult to see a firm pattern. It is also unclear what role comets may have played in the mass extinctions.
The prevailing view is that the Chicxulub crater, which has been linked to the dinosaur extinction 66 million years ago, was created by a giant asteroid, instead of a comet. Randall and Reece were careful in acknowledging at the outset that “statistical evidence is not overwhelming” and listing various limitations for using a patchy crater record. But the geological data is unlikely to improve in the near future, unfortunately.
On the other hand, advances in astronomical data are expected with the European Space Agency’s Gaia space mission, which was launched last year and is currently studying the Milky Way in unprecedented detail. Gaia will observe millions of stars and measure their precise distances and velocities. These measurements should enable astronomers to map out the surface-density of the dense galactic disk as a function of height. Close to the plane, astronomers could then directly see whether there is a “disk within the disk” that has much more mass than we could account for with the ordinary baryonic matter. Evidence of such a dark disk would allow better predictive modeling of the effects on comets and on the life of our planet.
Over the next several years, Randall said, the Gaia satellite will perform a precise survey of the position and velocity of as many as a billion stars, giving scientists far greater insights into the shape of the galaxy and into the potential presence of a disk of dark matter.
The image at the top of the page above is composite of the dark matter disk (red contours) and the Atlas Image mosaic of the Milky Way obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology. (J. Read & O. Agertz)
The Daily Galaxy, Max Goldberg, via Harvard and Daisuke Nagai, Department of Physics, Yale University, Dark Matter and the Dinosaurs, Caleb Scharf Copernicus Complex PDF and John Gribbin Alone in the Universe
Image credit: End Permian Great Dying Shutterstock