This week's guided tour through the Cosmos by Neil deGrasse Tyson is "The Immortals." As our sun travels through the Milky Way, it is not only accompanied by its planets but by trillions of distant comets, too. An increased likelihood of life-threatening comet impacts could occur when the Sun passes through a possible dark matter disk in the Galaxy. Our Solar System orbits around the Milky Way’s center, completing a revolution every 250 million years or so.
Our Solar-System's Scary Orbit Through Milky Way's Dark Matter Disk
Scientists have uncovered possible evidence of this galactic bumpiness in an apparent periodic fluctuation in the rate of large crater-forming impacts—the kind that likely killed off the dinosaurs. The frequency of impact fluctuations closely matches the rate at which the Sun passes through the plane of the galactic disk. However, it hasn’t been clear what element in the disk could be influencing comet trajectories.
Two theoretical physicists have put forward a hypothesis that inserts dark matter as the missing piece between Solar System motion and possibly life-threatening comet impacts. In a paper published in Physical Review Letters, Lisa Randall and Matthew Reece from Harvard University suggest that some of the mysterious invisible matter, which makes up 85% of all matter in the Universe, could exist in a thin disk that disturbs the path of certain comets so that they are more likely to collide with our planet.
Comet impact events appear to have played a significant role in shaping Earth’s history, creating craters and possibly causing mass extinctions. Many of these comets come from the Oort cloud, a spherical envelope of icy bodies in the outer edge of the Solar System extending from just outside the orbit of Neptune to halfway to the next nearest star. Because the Oort cloud is so distant from the Sun, it is highly susceptible to perturbations from gravitational forces coming from other bodies. Indeed, there have been some indications that the frequency of impacts (from both comets and asteroids) on Earth oscillates on a timescale of about 25 to 35 million years, which suggests a connection between the dynamics at the outer edge of the Solar System and the comet shower strikes on Earth.
Two hypotheses have been proposed to explain the possible periodicity in comet impacts. One idea involves the gravitational pull of an as-yet-undiscovered distant companion star (called Nemesis) or planet (called planet X) that periodically disturbs comets in the Oort cloud and causes a large increase in the number of comets visiting the inner Solar System and thus in the frequency of the impact events on Earth. Neither Nemesis nor planet X was detected with NASA’s Wide-field Infrared Survey Explorer (WISE) space telescope, effectively ruling out the theory that an object in our Sun’s neighborhood can explain the impact fluctuations.
An alternative hypothesis involves a gravitational influence of the dense galactic disk on the Solar System . Our Sun orbits around the Galactic center, taking approximately 250 million years to make a complete revolution. However, this trajectory is not a perfect circle. The Solar System weaves up and down, crossing the plane of the Milky Way approximately every 32 million years, which coincides with the presumed 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 new 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).
Although one has to stretch the observational constraints to make room, their thin disk of dark matter is consistent with astronomical data on our Galaxy. Focusing their analysis on large (>20km) craters created in the last 250 million years, Randall and Reece argue that their dark disk scenario can produce the observed pattern in crater frequency with a fair amount of statistical uncertainty.
Randall and Reece’s dark disk model is not made of an ordinary type of dark matter. The most likely candidate of dark matter—known as weakly interacting massive particles (WIMPs)—is expected to form a spherical halo around the Milky Way, instead of being concentrated in the disk. This WIMP dark matter scenario has been remarkably successful in explaining the large-scale distribution of matter in the Universe.
But, there is a long-standing problem on small-scales—the theory generally predicts overly dense cores in the centers of galaxies and clusters of galaxies, and it predicts a larger number of dwarf galaxy satellites around the Milky Way than are observed. While some of these problems could be resolved by better understanding the physics of baryonic matter (as it relates, for example, to star formation and gas dynamics), it remains unclear whether a baryonic solution can work in the smallest mass galaxies (with very little stars and gas) where discrepancies are observed.
Alternatively, this small-scale conflict could be evidence of more complex physics in the dark matter sector itself. One solution is to invoke strong electromagnetic-like interactions among dark matter particles, which could lead to the emission of “dark photons”. These self-interactions can redistribute momentum through elastic scattering, thereby altering the predicted distribution of dark matter in the innermost regions of galaxies and clusters of galaxies as well as the number of dwarf galaxies in the Milky Way.
Although self-interacting dark matter could resolve the tension between theory and observations at small-scales, large-scale measurements of galaxies and clusters of galaxies only allow a small fraction (less than 5%) of the dark matter to be self-interacting. Recently, Randall, Reece, and their collaborators showed that if a portion of the dark matter is self-interacting, then these particles will collapse into a dark galactic disk that overlaps with the ordinary baryonic disk .
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.
"Are We Entering the Danger Zone?"
Is there a genocidal countdown built into the motion of our solar system? Recent work at Cardiff University suggests that our system's orbit through the Milky Way encounters regular speedbumps - and by "speedbumps" we mean "potentially extinction-causing asteroids".
Professor William Napier and Dr Janaki Wickramasinghe completed computer simulations of the motion of the Sun in our outer spiral-arm location in the Milky Way that revealed a regular oscillation through the central galactic plane, where the surrounding dust clouds are the densest. The solar system is a non-trivial object, so its gravitational effects set off a far-reaching planetoid-pinball machine which often ends with comets being hurled into the intruding system.
The sun is about 26,000 light-years from the center of the Milky Way Galaxy, which is about 80,000 to 120,000 light-years across (and less than 7,000 light-years thick). We are located on on one of its spiral arms, out towards the edge. It takes the sun -and our solar system- roughly 200-250 million years to orbit once around the Milky Way. In this orbit, we are traveling at a velocity of about 155 miles/sec (250 km/sec).
Many of the ricocheted rocks collide with planets on their way through our system, including Earth. Impact craters recorded worldwide show correlations with the ~37 million year-cycle of these journeys through the galactic plane - including the vast impact craters thought to have put an end to the dinosaurs two cycles ago.
Almost exactly two cycles ago, in fact. The figures show that we're very close to another danger zone, when the odds of asteroid impact on Earth go up by a factor of ten. Ten times a tiny chance might not seem like much, but when "Risk of Extinction" is on the table that single order of magnitude can look much more imposing.
You have to remember that ten times a very small number is still a very small number - and Earth has been struck by thousands of asteroids without any exciting extinction events. A rock doesn't just have to hit us, it has to be large enough to survive the truly fearsome forces that cause most to burn up on re-entry.
Professors Medvedev and Melott of the University of Kansas have a different theory based on the same regular motion. As the Sun ventures out "above" the galactic plane, it becomes increasingly exposed to the cosmic ray generating shock front that the Milky Way creates as it ploughs through space. As we get closer to this point of maximum exposure, leaving the shielding of the thick galactic disk behind, the Kansas researchers hold that the increasing radiation destroys many higher species, forcing another evolutionary epoch. This theory also matches in time with the dinosaur extinction.
Either way, don't go letting your VISA bill run up just yet. "Very close" in astronomical terms is very, very different to "close" in homo sapien time.
The characteristic spiral arms of the Milky Way regions where stars and gas are a little closer together -- waves of higher density than elsewhere in our galaxy's disc. Their additional gravity is normally too weak to alter a star's path by much, but if the star's orbital speed happens to match the speed at which the spiral arm is itself rotating, then the extra force has more time to take effect.
Simulations completed by Rok Roskar of the University of Zurich, Switzerland, show that a lucky star can ride the wave for 10,000 light years or more. Our sun is an example, with some measurements implying that the sun is richer in heavy elements than the average star in our neighbourhood, suggesting it was born in the busy central zone of the galaxy, where stellar winds and exploding stars enrich the cosmic brew more than in the galactic suburbs. The gravitational buffeting the solar system received then might also explain why Sedna, a large iceball in the extremities of the solar system, travels on a puzzling, enormously elongated orbit (arxiv.org/abs/1108.1570).
The cosmic panorama at top of page is courtesy of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) project and the Spitzer Space Telescope. The galactic plane itself runs through the middle of the false-color view. Spitzer's infrared cameras see through much of the galaxy's obscuring dust revealing many new star clusters as well as star forming regions (bright white splotches) and hot interstellar hydrogen gas (greenish wisps). The pervasive red clouds are emission from dust and organic molecules, pocked with holes and bubbles blown by energetic outflows from massive stars. Intensely dark patches are regions of dust too dense for even Spitzer's infrared vision to penetrate.
Is Earth's Biodiversity Linked to the Solar-System's Milky Way Orbit?
1n 1999, Astronomers focusing on a star at the center of the Milky Way, measured precisely how long it takes the sun to complete one orbit (a galactic year) of our home galaxy: 226 million years. The last time the sun was at that exact spot of its galactic orbit, dinosaurs ruled the world. The Solar System is thought to have completed about 20–25 orbits during its lifetime or 0.0008 orbit since the origin of humans.
Using a radio telescope system that measures celestial distances 500 times more accurately than the Hubble Space Telescope, astronomers plotted the motion of the Milky Way and found that the sun and its family of planets were orbiting the galaxy at about 135 miles per second. That means it takes the solar system about 226 million years to orbit the Milky Way and puts the most precise value ever determined on one of the fundamental motions of the Earth and its sun.
The sun circles the Milky Way at a speed of about 486,000 miles per hour. And every object in the universe is moving apart from the other objects as the universe expands at a constantly accelerating rate.
The sun is one of about 100 billion stars in the Milky Way, one of billions of ordinary galaxies in the universe. The Milky Way is a spiral galaxy, with curving arms of stars pinwheeling out from a center.The solar system is about halfway out on one of these arms and is about 26,000 light years from the center. A light year is about 6 trillion miles.
For their solar system measurement, the astronomers focused on Sagittarius A, a star discovered over two decades ago to mark the Milky Way's center. Over a 10-day period, they measured the apparent shift in position of the star against the background of stars far beyond. The apparent motion of Sagittarius A is very, very small, just one-600,000th of what could be detected with the human eye, the astronomers said.
The measurement adds supports to the idea that the Milky Way's center contains a supermassive black hole- an object, much smaller than our own solar system, contains a black hole about 2.6 million times more massive than the sun.
Earlier this year, a team of researchers at the University of Kansas came up with an out-of-this-world explanation for the phenomenon of mass extinctions on Earth that hinges upon the fact that stars move through space and sometimes rush headlong through galaxies, or approach closely enough to cause a brief cosmic tryst.
Researchers at the University of California, Berkeley found that marine fossil records show that biodiversity increases and decreases based on a 62-million-year cycle. At least two of the Earth's great mass extinctions-the Permian extinction 250 million years ago and the Ordovician extinction about 450 million years ago-correspond with peaks of this cycle, which can't be explained by evolutionary theory.
Our own star moves toward and away from the Milky Way's center, and also up and down through the galactic plane. One complete up-and-down cycle takes 64 million years- suspiciously close to the Earth's biodiversity cycle.
Butterfly-3 Once the researchers independently confirmed the biodiversity cycle, they then proposed a novel mechanism whereby which the Sun's galactic travels is causing it.
It’s no secret that the Milky Way is being gravitationally pulled toward a massive cluster of galaxies, called the Virgo Cluster, which is located about 50 million light years away. Adrian Melott and his colleague Mikhail Medvedev, speculate that as the Milky Way rushes towards the Virgo Cluster, it generates a so-called bow shock in front of it that is similar to the shock wave created by a supersonic jet.
"Our solar system has a shock wave around it, and it produces a good quantity of the cosmic rays that hit the Earth. Why shouldn't the galaxy have a shock wave, too?" Melott asks.
The galactic bow shock is only present on the north side of the Milky Way's galactic plane, because that is the side facing the Virgo Cluster as it moves through space, and it would cause superheated gas and cosmic rays to stream behind it, the researchers say. Normally, our galaxy's magnetic field shields our solar system from this "galactic wind." But every 64 million years, the solar system's cyclical travels take it above the galactic plane.
"When we emerge out of the disk, we have less protection, so we become exposed to many more cosmic rays," Melott has said.
The boost in cosmic-ray exposure may have a direct effect on Earth's organisms, according to paleontologist Bruce Lieberman. The radiation would lead to higher rates of genetic mutations in organisms or interfere with their ability to repair DNA damage. In this way, the process could lead to new species while killing off others.
Cosmic rays are also associated with increased cloud cover, which could cool the planet by blocking out more of the Sun's rays. They also interact with molecules in the atmosphere to create nitrogen oxide, a gas that eats away at our planet's ozone layer, which protects us from the Sun's harmful ultraviolet rays.
Richard Muller, one of the UC Berkeley physicists who co-discovered the cycle, said Melott and his colleagues have come up with a plausible galactic explanation for the biodiversity cycle.
If future studies confirm the galaxy-biodiversity link, it would force scientists to broaden their ideas about what can influence life on Earth. "Maybe it's not just the climate and the tectonic events on Earth," Lieberman said. "Maybe we have to start thinking more about the extraterrestrial environment as well."
The Daily Galaxy via Daisuke Nagai, Department of Physics, Yale University, American Physical Society, cardiff.ac.uk and newscientist.com
Image credits: APS/Alan Stonebraker, E. Mercer (Boston Univ.), et al., SSC, JPL-Caltech, NASA, and the GLIMPSE Team