The solar system's up-and-down motion across our galaxy's disc periodically exposes it to higher doses of dangerous cosmic rays, new calculations suggest. The effect could explain a mysterious dip in the Earth's biodiversity every 62 million years.
The solar system moves through the Milky Way rather like a child on a merry-go-round. It completes a circuit of the galaxy once every 225 million years or so but as it goes it bobs up and down through the dense galactic disc.
Previous research had suggested this motion might affect Earth's climate as the solar system passes through the giant hydrogen clouds concentrated in the galaxy's spiral arms. Some researchers have said these clouds could be dense enough to sprinkle the Earth's atmosphere with dust, blocking out sunlight and cooling the planet.
Others have suggested the gravitational pull of the clouds may dislodge comets from their spherical halo surrounding the solar system and send them crashing into the Earth, causing major extinctions.
Still other researchers have pointed out that the clouds could compress the solar wind, which shields the solar system from energetic cosmic rays from the galaxy. These cosmic rays - charged particles accelerated to high energies by supernova explosions - could then leak into the Earth's atmosphere. There they could spur the formation of clouds - cooling the planet - and destroy the ozone layer, killing off species by allowing harmful ultraviolet light to reach the Earth's surface.
However, the solar system takes a few hundred thousand years to pass through one of these giant clouds, and the fossil record does not show regular dips in biodiversity on this timescale, say Mikhail Medvedev and Adrian Melott of the University of Kansas in Lawrence, US. Instead, Medvedev cites a 2005 Nature study showing the number of species has dropped about every 62 million years for at least the past 542 million years.
This timescale coincides with the 64 million years it takes for the solar system to move vertically through the disc of the galaxy and back again, he says. Medvedev presented research linking the two effects at an astrobiology conference in Washington DC in the US this week.
Most of Earth's biggest extinctions occurred when the solar system was at its most northerly point in its cycle, which stretches about 230 light years above the galactic plane. Medvedev says that more cosmic rays enter the Earth's atmosphere at that point, killing off species.
He says the effect is similar to the compression of the solar system's protective solar wind when it passes through a giant hydrogen cloud. The Milky Way's stars produce a wind of charged particles whose magnetic fields deflect incoming cosmic rays from beyond the galaxy.
But the entire Milky Way is moving due north at 200 kilometres per second towards a giant grouping of galaxies called the Virgo Cluster. This movement compresses the galactic wind on the galaxy's north side, allowing in higher levels of potentially life-harming extragalactic cosmic rays, says Medvedev.
"When the Sun is moving up through the galactic plane, the cosmic ray flux is increasing, and when it goes down through the plane, it's decreasing," he told the conference. He said the periods of high extragalactic cosmic ray influxes match observed lows in biodiversity so well that the alignment has just a one in 10 million chance of being a coincidence.
"I think it's very convincing," says Paul Davies, an astrobiologist at Macquarie University in Sydney, Australia. He says the research could shed light on "habitable" zones where life could most easily take hold.
These zones are found in the solar system where liquid water can persist, and in the Milky Way between the galactic centre - where radiation levels are dangerously high - and the galaxy's outskirts, which have a dearth of the heavy elements necessary for life.
"Then the question is: is there an intergalactic habitable zone?" he asks. He says the possibility that there are "sides of the galaxy where the cosmic ray flux could be high" suggests there is such a region between galaxies where the flux is low.
By Ron Gauvin
Sat May 10 19:17:07 BST 2008
Will the Solar System move through the Galaxy's Horizontal Plane in the near future. As many suggest. And, they are suggesting it will increase the gravitational forces as well. And, that this is what is causing Our Planets Atmosphere to act the way it has. Creating more intense storm systems. And, more volcanic & quake activity as of late. This all makes sense to me if we are in fact moving up or down to this dangerous area of the Galaxy. Sincerely, Ron GauvinBy Rich
Thu Jun 19 01:12:40 BST 2008
Ron, like you I am trying to find this very same answer. However, in my search I came across a very interesting report by Richard Hoagland & Dave Wilcock. It gives evidence that the earth isn't just experencing changes but our entire solar system is going through changes.By Ron Gauvin
Tue May 13 16:57:48 BST 2008
Are we within months of crossing the Galaxtic Disc? Are we going up to it or down? Atmosphere is acting like we are getting very close to the zone! I feel the Galaxies pull is effecting the intensity of our atmosphere and storms presently. Sincerely.All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
If you are having a technical problem posting a comment, please contact technical support.
14:09 04 July 2008
