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article imageGalactic dark matter may account for Earth being hit by comets

By Robert Myles     May 4, 2014 in Science
Cambridge - Earth coming under bombardment by comets could be a regular astronomical event, suggest two Harvard University researchers in a recently published research paper.
The bad news is that some evidence from terrestrial crater impacts may point to a regular pattern of Earth taking a hammering from asteroids and comets.
But on a brighter note, the end is not quite nigh. The researchers suggest that such potential extinction level events only occur every 35 million years or so.
In their paper, titled “Dark Matter as a Trigger for Periodic Comet Impacts” and published in the journal Physical Review, researchers Lisa Randall and Matthew Reece advance the hypothesis that a disk of dark matter, the mysterious substance that many scientists believe accounts for the Universe’s “missing” mass, concealed in the plane of our galaxy, the Milky Way, could be the force behind asteroids and comets taking a trajectory inbound to Earth, albeit with a regularity measured in eons.
In the far reaches of our solar system, astronomers theorize, lies the Oort Cloud, a swirling mass of cosmic debris, the leftovers from when massive gravitational forces were at work, molding and shaping our solar system to something resembling what we see today.
Lying far beyond the orbit of Pluto, at an estimated 50,000 astronomical units — that’s 50,000 times the distance between Earth and the Sun — the Oort Cloud is home to a huge collection of comets. From time to time, the latest theory goes, gravitational forces operate to pluck distant comets from the Oort Cloud, setting them on a trajectory toward the inner solar system, i.e., our cosmic neighborhood, and a possible collision with Earth.
But what causes this to happen is open to conjecture. The Oort Cloud lies one-quarter of the way toward the next nearest star to Earth, Proxima Centauri. So, on the face of it, there are no nearby astronomical bodies that could exert a gravitational influence causing comets, whiling away the millennia in the Oort Cloud, to take a detour toward Earth.
In the past, some astronomers have speculated that an, as yet undiscovered, massive planet, or even a hidden companion star to the Sun might have some bearing on events in the Oort Cloud.
Such speculation cannot be entirely ruled out. Just recently scientists using NASA’s WISE survey and Spitzer Space Telescope discovered the previously unknown brown dwarf object now designated WISE J085510.83-071442.5, lying just 7.2 light years from Earth.
That made WISE J085510.83-071442.5 the fourth closest star system to Earth. The discovery of such a massive brown dwarf, relatively close to Earth, illustrated that, just as astronomers thought they had nearby star systems pretty much mapped out, new, more powerful telescopes may require a redrawing of galactic maps.
The two Harvard researchers, though, make the suggestion for the first time that the explanation for such an apparent pattern of asteroid and comet bombardment may be a disk of dark matter present in the Milky Way; the same theoretical dark matter disk would explain why our galaxy doesn’t spin itself apart.
The Harvard pair acknowledge certain obstacles stand in the way of their theory.
Firstly, and most basically, the periodicity of crater creation on Earth hasn’t been firmly established, nor is there any certainty what types of space objects colliding with the planet caused different craters.
The second obstacle concerns the nature and existence of Dark Matter. This “unknown,” at least, may be resolved as more powerful telescopes become operational.
The conventional view of Dark Matter holds that it consists of weakly interacting massive particles. The clue’s in the name — such particles wouldn’t have the gravitational wherewithal to cause comets and asteroids to eject from the Oort Cloud.
Instead, Randall and Reece suggest some Dark Matter could comprise what they term as “strong electromagnetic-like interactions among dark matter particles" which by their nature would exert a greater gravitational pull.
Here’s how the theory goes. Our solar system orbits around the center of our galaxy, in what’s sometimes called a cosmic year, once every 225 to 250 million years. But the solar system’s orbit around the galactic center “wobbles” by oscillating up and down during that transit, crossing the galactic plane once every 32 million years (see main illustration above).
Assuming that a dark matter disk was concentrated along the galactic plane, then it might cause tidal disruption of objects in the Oort Cloud, stirring up asteroids and comets much like a hornets’ nest being disturbed. So when our solar system transited that galactic plane every 32 million years, it would be like someone walking into a swarm of hornets, but with the sting being that some of these “rumbled up” asteroids and comets would be on a collision course with Earth.
As more powerful space telescopes, like the European Space Agency’s (ESA) Gaia, “billion star surveyor,” and the ESA's Euclid mission start returning data over the next few years that will give us our best view of the Milky Way galaxy to date. Then, this latest theory seeking to explain why Earth has periodically suffered bombardment by comets will stand or fall.
More about Comets, Asteroids, extinction level events, Oort cloud, Dark matter
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