Our Solar System is currently located in its twenty-eighth orbit of Milky Way at the inner edge, or the Local Arm, of a spiral feature known by the Orion Arm. Ghostly arms are not permanent features in a disc galaxy such as the Milky Way. Instead, they are clusters of gas or dust that form stars. They can be created by disturbances within the Milky Way or occasionally by a jolt to the Solar System through one or more of the dusty cloud of dusty gases.
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.
Evidence points to the Encounter With a Spiral Arm
In extreme cases, the heat and light of the Sun could be dimmed by interfering material, leading to an Ice Age. There are many other dangers associated with passing through this galactic traffic jam. The Solar System’s last passage through a spiral arm occurred approximately 230,000,000 years ago. It also happened at the beginning its current 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.
The Palaeozoic death so many species of life opened up new life forms for the survivors, particularly the dinosaurs. This extreme extinction event requires an extreme explanation. Although we cannot be certain at this distance in time, John Gribbin (author of You are the only one in the universe,The circumstantial evidence supports the existence of a spiral arm even though we are unable to identify the smoking gun.
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. Because the entire Solar System orbits the Galaxy approximately once every 250 million year, but the spiral pattern takes twice the time to move around it, the Sun has remained in the gap between the arms. 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.”
Each 230 million years, a landscape of complex gravitational fields and undulating mass concentrations, orbiting and drifting in three-dimensional ballets.
Our sun will die after approximately 12 billion year and 60 orbits around The Milky Way. We know that the Sun and its satellites orbit the Milky Way galaxy on an orbital path that passes through a landscape with complex gravitational fields and undulating mass concentrations. They orbit and drift in a three-dimensional ballet.
Astrophysicists conclude that this means that the solar system is complete. Caleb ScharfColumbia University. 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. These regions can take tens to hundreds of thousands of to thousands of years to traverse. 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.”
Could our planetary conditions have been so different that we could still create a radio-creating, space-faring species despite their changing orbits? Could more complex orbits within a planetary system, or passage through interstellar space clouds, hinder the emergence and development of life? “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 Perilous Journey” –Our Solar System Has Completed 20 Orbits of the Milky Way
The Milky Way’s Dark-Matter Disk?
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.”
Although dark matter is thought to be non-interacting, Randall, Matthew Reece (Harvard assistant professor of Physics), suggested that a hypothetical form of dark matter could form the middle of the Milky Way as a disk.
“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 the Galactic Center and crosses the planes of the Milky Way about every 32 million years. This coincides with the frequency of the impact variations.
This is due to the gas and star concentrations in the Galaxy’s disk. This ordinary “baryonic” matter is concentrated within about 1000 light years of the plane. Because density decreases in vertical directions, there is a gravitational tide that can perturb comet orbits in Oort cloud. This may cause some comets fly into the inner Solar System, increasing the chance of colliding with Earth. But this idea is flawed because the galactic tide estimated to be too weak for the Oort cloud to produce many waves.
Second Hypothesis: Galactic Tide is strengthened by Dark-Matter Disk
Randall and Reece concentrate on the second hypothesis in their study and suggest that a thin disc of dark matter could make the galactic tide stronger. As the authors and their collaborators recently demonstrated, dark disks could be a result of dark matter physics. 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).
What caused extinction events similar to the one that decimated the dinosaurs by a thin disc of dark matter? Even so, the evidence is far from convincing. 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 not clear what role camets played in mass extinctions.
According to most people, the Chicxulub Crater, which is linked to the extinction of dinosaurs 66 million years back, was not created by a comet but a giant asteroid. 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. Unfortunately, the geological data will not improve in the near future.
The Chicxulub Impact -Did an ‘Impossible’ Magnitude-12 Earthquake Change Our World?
The Gaia Mission: Unprecedented Details
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 be able to measure the exact distances and velocity of millions stars. Astronomers should be able to use these measurements to determine the density of the dense galactic disc’s surface as a function its 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. The existence of such a dark disk could allow for better prediction of the effects of comets on our planet’s life.
Randall stated that the Gaia satellite will be performing a precise survey over the velocity and position of up to a billion stars over the next few years. This will give scientists greater insight into the structure of the galaxy as well as the possibility of dark matter.
The Last Word
Astrophysicist (and dailygalaxy.com Editor) Jackie Faherty says: “I think New York Times reporter, Maria Popova, put it best when she said that Lisa Randall’s work is creative computational cosmology. This category includes the Milky Way dark matter disk theory. It’s highly speculative, but intriguing. However, recent work using the European Space Agency’s Gata DR1 catalog — the most precise astrometric catalog ever produced — disfavor the presence of a thin dark matter disk. Gaia allows us the opportunity to study the motions millions of stars. If the thin dark matter disk was present in any significant abundance, the stars mapped by Gaia would have revealed its influence.”
Rutgers University Physics Professor Eric GawiserFaherty also agrees. Send an email to The Daily Galaxy, he writes: “RecentThese are the results Gaia’s spacecraft shows that the Milky Way doesn’t have any significant amounts of dark matter. It is unlikely that the 20th orbit of our Milky Way galaxy will see a large amount of dark matter. But our universe is full of surprises!”
Avi ShporerResearch Scientist, MIT Kavli Institute for Astrophysics and Space Research via Jacqueline Faherty, Eric Gawiser, Harvard Daisuke Nakai, Department of Physics at Yale University, Dark Matter and the Dinosaurs Caleb Scharf Copernicus Complex PDF John Gribbin Alone in the Universe
The composite image at the top of this page is the dark matter disk (red contours), and Atlas Image mosaic of Milky Way, taken as part of Two Micron All Sky Survey (TWOMASS), a joint effort of the University of Massachusetts and Infrared Processing and Analysis Center/California Institute of Technology. (J. Read & O. Agertz). Image End Permian great Dying, Shutterstock Licence.
Avi Shporer Research Scientist, MIT Kavli Institute for Astrophysics and Space Research. A Google ScholarAvi was once a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL). His motto, not surprisingly, is a quote from Carl Sagan: “Somewhere, something incredible is waiting to be known.”
