Gamma-ray bursts may repeatedly wipe out life
Chances of sentient beings on other planets may be lower than thought
CHANCE OF LIFE Because sterilizing gamma ray bursts occur more frequently near centers of galaxies, more-distant planets in the Milky Way have a greater likelihood of evolving life forms, scientists have calculated.
Courtesy of R. Jimenez, adapted by E. Otwell
Deadly invisible jets of high-energy radiation may short-circuit life throughout the universe. A study reported in the Dec. 5 Physical Review Letters concludes that these gamma-ray bursts occur frequently enough in about 90 percent of galaxies to sterilize planets, including Earthlike worlds that would otherwise be ideal for life. Earth itself has been zapped, the study suggests, perhaps contributing to one or more of the planet’s mass extinctions.
Some scientists say the study doesn’t properly account for the resilience of life, particularly if that life is protected by an ocean or an ice shell. Nonetheless, the paper’s sobering conclusions may temper recent optimism about the prospects for extraterrestrial life, particularly regarding the discovery of Earth-sized planets orbiting other stars.
Gamma-ray bursts, or GRBs, are jets of the most energetic form of electromagnetic radiation that are emitted by exploding massive stars and the collisions of certain pairs of stars. NASA’s Fermi Gamma-ray Space Telescope detects about one burst per day. Though a burst lasts at most a few minutes, it outshines all the other gamma-ray sources in the universe combined. “We see so many because they’re so bright you can see them throughout the whole universe,” says Neil Gehrels, an astrophysicist at the NASA Goddard Space Flight Center in Greenbelt, Md.
Gehrels was one of several scientists who, about 20 years ago, started thinking about the bursts’ impacts on Earth and other planets like it. Using computer simulations designed to study the effect of solar flares on Earth’s atmosphere, researchers discovered that a quick zap of gamma radiation from a nearby burst would break up molecules in the atmosphere and nearly destroy the ozone layer. Ultraviolet radiation from the sun could then damage the DNA of organisms on the planet’s surface and exterminate much of the world’s plankton, the bottom of the ocean food chain. “It’s a potential major whammy for life on the planet,” says Adrian Melott, an astrobiophysicist at the University of Kansas in Lawrence.
Raul Jimenez, a theoretical physicist at the University of Barcelona, decided to study whether GRBs delivered whammies often enough to disturb life throughout the cosmos. Along with theoretical physicist Tsvi Piran at the Hebrew University of Jerusalem, Jimenez culled recent data on the frequency of bursts in various types of galaxies and reviewed previous estimates of the minimum energy required to deliver a lethal blow. “We thought GRBs would have no influence whatsoever,” Jimenez says.
To their surprise, Jimenez and Piran found that Earth has almost certainly experienced a lethal burst. And there’s a roughly fifty-fifty chance that a lethal burst struck within the last 500 million years, a period that has included at least five mass extinctions.
Yet according to the researchers, Earth is well placed in the outer suburbs of the Milky Way, about 28,000 light-years away from the galactic center. GRBs occur far more often in that dense region of stars than they do in the outskirts of the galaxy. If Earth’s distance to the galactic center were halved, the planet would be sterile, the researchers conclude. “We are far enough away not to be completely wiped out,” Jimenez says, “but not far enough for GRBs to not have any effect at all.”
The researchers found that most planets in the universe aren’t as lucky. Gamma-ray bursts occur most frequently in regions that are packed with stars or that are composed almost entirely of hydrogen and helium. Only about 10 percent of galaxies offer any real estate that is distant enough from these conditions. “The universe may be a more sterile environment than we think,” Jimenez says. “The density of intelligent life may be very low.”
And as bad as the conditions for life are now, the researchers say they were even worse billions of years ago, when the universe was more densely packed with matter and stars hadn’t had as much time to forge elements heavier than hydrogen and helium.
Melott praises the paper but emphasizes that its conclusions are relevant only to life on an Earthlike planet. “If a planet has a really, really thick atmosphere, or if there’s life under ice, then those types of places would be nearly immune to the effects of a GRB,” he says. For example, many scientists suspect that life could thrive in oceans buried under sheets of ice on worlds such as Jupiter’s moon Europa and Saturn’s moon Enceladus (SN: 5/17/14, p. 20). And until about 500 million years ago, Earth’s life may have been immune to gamma rays because it was all underwater, says Margaret Turnbull, an astrobiologist at the Global Science Institute in Antigo, Wis. “One could legitimately ask, for the vast majority of Earth’s history, ‘Who cares?’ ”
Turnbull says the study doesn’t give life enough credit. Since life sprung up on Earth, “continents have moved, entire mountains have exploded, ice ages have come and gone, asteroids have slammed into the ground,” she says. She points out that oxygen was toxic to most life on Earth when some microbes first started churning out the gas through photosynthesis. Despite all these catastrophes, complex life prevailed. “I submit that GRBs cannot be the limiting factor for the appearance of life throughout the galaxy,” she says.
Nonetheless, Gehrels says that it’s worth pursuing the connection between GRBs and life, especially at a time when astronomers are extrapolating that the Milky Way alone harbors billions of potentially habitable planets (SN: 11/30/13, p. 13). Gamma-ray burst research also may influence projects such as the SETI Institute’s Galactic Center Survey, which employs radio telescopes to sweep the packed stellar neighborhood near the Milky Way’s core in search of transmissions from intelligent life-forms.
Melott says the bursts may be most useful for understanding mass extinctions throughout Earth’s history. A decade ago he was part of a team that hypothesized that the late Ordovician mass extinction, which wiped out more than 80 percent of Earth’s species about 440 million years ago, was at least partially caused by a GRB. That will be tricky to prove, though: Unlike the asteroid that wiped out the dinosaurs, a gamma-ray burst probably has no definitive signature in the geologic record.
