June 01, 2010
In 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.
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."