The universe may be a lonelier place than previously thought. According to a pair of astrophysicists, just one in ten galaxies in the observable universe can host complex life like that on Earth. Everywhere else, gamma ray bursts, or star explosions, would regularly wipe out any life forms more complex than microbes. According to the experts, the detonations also kept the universe lifeless for billions of years following the big bang.
“It’s kind of surprising that we can have life only in 10% of galaxies and only after 5 billion years,” says Brian Thomas, a physicist at Washburn University in Topeka who was not involved in the work. But “my overall impression is that they are probably right” within the uncertainties in a key parameter of the analysis.
Scientists have long mused over whether a gamma ray burst could harm Earth. Satellites meant to detect nuclear weapons testing discovered the bursts in 1967, and they now appear at a rate of around one per day. They are classified into two categories. Short gamma ray bursts last for a second or two and are most likely caused by the collision of two neutron stars or black holes. Long gamma ray bursts last tens of seconds and are caused by massive stars burning out, collapsing, and exploding. They are more rare than short ones, but they have 100 times the energy. In gamma rays, which are incredibly energetic photons, a long burst can outshine the rest of the universe.
That seconds-long flash of radiation itself wouldn’t blast away life on a nearby planet. Rather, if the explosion was close enough, the gamma rays would trigger a chain reaction of chemical processes that would destroy the ozone layer in a planet’s atmosphere. Without the protective atmosphere, deadly UV radiation from a planet’s sun would rain down for months or years, causing a mass extinction.
How likely is that to happen? In a work in publication at Physical Review Letters, Tsvi Piran, a theoretical astrophysicist at the Hebrew University of Jerusalem, and Raul Jimenez, a theoretical astrophysicist at the University of Barcelona in Spain, investigate that apocalyptic scenario.
Astrophysicists once thought that gamma ray bursts would be most common in galaxies where stars are quickly formed from gas clouds. However, new data reveals that the picture is more complicated: Long bursts are most common in star-forming areas with low amounts of elements heavier than hydrogen “metallicity,” as astronomers call it.
Piran and Jimenez estimate the rates for long and short bursts across the galaxy using the average metallicity and the general distribution of stars in our Milky Way galaxy. They discover that the more energetic extended bursts are the true killers, and that the chances of Earth being exposed to a lethal blast in the last billion years are roughly 50%. According to Piran, some astrophysicists believe a gamma ray burst caused the Ordovician extinction, a global calamity that wiped off 80 percent of Earth’s species some 450 million years ago.
The researchers then calculate how badly a planet would be fried in various locations of the galaxy. They discover that planets within around 6500 light-years of the galactic center have a greater than 95 percent chance of having been hit by a lethal gamma ray burst in the last billion years due to the sheer amount of stars in the center of the galaxy. Generally, they conclude, life is possible only in the outer regions of large galaxies. (Our solar system is around 27,000 light-years away from the center.)
Things are even bleaker in other galaxies, the researchers report. Most galaxies are small and metallic in comparison to the Milky Way. As a result, they argue, 90% of them should contain too many lengthy gamma ray bursts to sustain life. Furthermore, for around 5 billion years after the big bang, all galaxies were like that, making lengthy gamma ray bursts impossible anywhere.
But are 90% of the galaxies barren? That may be going too far, according to Thomas. The radiation exposures Piran and Jimenez talk about would do great damage, but they likely wouldn’t snuff out every microbe, he contends. “Completely wiping out life?” he says. “Maybe not.” But Piran says the real issue is the existence of life with the potential for intelligence. “It’s almost certain that bacteria and lower forms of life could survive such an event,” he acknowledges. “But [for more complex life] it would be like hitting a reset button. You’d have to start over from scratch.”
According to Piran, the findings could have practical consequences for the search for life on distant worlds. For decades, scientists at the SETI Institute in Mountain View, California, have employed radio telescopes to look for signals from intelligent life on distant worlds. But SETI researchers are looking mostly toward the center of the Milky Way, where the stars are more abundant, Piran says. That’s precisely where gamma ray bursts may make intelligent life impossible, he says: “We are saying maybe you should look in the exact opposite direction.”