A pair of astrophysicists concluded that complex life in the universe is apt to be more rare than previously believed due to blasts of cosmic radiation that would wipe out any planetary organisms more complex than microbes.
According to Science magazine, the conditions that enable life to thrive and flourish on Earth are even more rare that scientists previously thought.
Exploding stars are constantly emitting blasts of lethal gamma radiation. Throughout the estimated 100 billion extant galaxies, the scientists say, only 10 percent possess the correct combinations of life-supporting elements and shielding from cosmic gamma rays.
"It's kind of surprising that we can have life only in 10% of galaxies and only after 5 billion years," says Washburn University physicist Brian Thomas, who was not a part of this study. But "my overall impression is that they are probably right."
The two astrophysicists -- Tsvi Piran of the Hebrew University of Jerusalem and Raul Jimenez of the University of Barcelona in Spain -- published their findings Thursday in a paper published by the journal Physical Review Letters.
These interplanetary and interstellar bursts of gamma radiation were discovered in 1967 by satellites scanning the Earth for secret nuclear tests. The bursts come in two types, short and long. Short bursts that only last a second or two and are believed to be the result of neutron stars or black holes collide in space.
The longer bursts of gamma radiation can last for ten or more seconds and occur when stars collapse, burn out or explode. These waves of radiation are less common, but much more intense, releasing 100 times as much energy as short bursts.
According to Piran and Jimenez, the long bursts are potentially much more lethal to life. The radiation itself would not kill organisms on a planet, but it would set about a series of chemical and physical reactions that would strip the planet of its ozone layer, allowing deadly ultraviolet (UV) light from the planet's sun to rain down unimpeded.
There is a 50 percent chance, say the scientists, that Earth was hit by one of these long bursts of gamma rays in the past billion years. Some researchers suggest that a gamma ray blast may have caused the Ordovician extinction, a planet-wide cataclysm which took place an estimated 450 million years ago, wiping out 80 percent of Earth's species.
Gamma radiation blasts are much more common at the dense centers of galaxies and less likely at the edges. Earth's solar system inhabits a remote edge of the Milky Way galaxy.
However, most galaxies are smaller and less complex than the Milky Way, the researchers said, and more vulnerable to gamma blasts. A full 90 percent of galaxies, they say, are downright uninhabitable because of them.
Brian Thomas believes that the statistics generated by Piran and Jimenez may be unduly grim. He said that the radiation exposures outlined by the team would be dangerous, but not universally lethal to every organism on a planet.
Piran agrees, telling Science, "It's almost certain that bacteria and lower forms of life could survive such an event," but for more complex and intelligent life forms, "it would be like hitting a reset button. You'd have to start over from scratch."