It may be the biggest question in science: does life exist elsewhere in the universe?
For those who hope the answer is “yes”, the recent harvest of exoplanets by NASA’s Kepler Space Telescope has been hugely encouraging. As of early 2015, in the small slice of sky under its exacting gaze, Kepler has found at least a thousand extrasolar planets, a handful of which might be rocky Earth-sized planets in the so-called ‘habitable’ zone where liquid surface water may exist. Extrapolating Kepler’s results, astronomers estimate our Milky Way galaxy alone might hold some 10 billion Earth-like planets. With that much real estate, complex or even intelligent life must have formed on at least some of these planets, right?
Well, maybe not. In a sobering paper published late in 2014 in the prestigious Physical Review Letters, astrophysicists Tsvi Piran and Raul Jimenez argue that most planets in the universe have been wracked by frequent galactic-scale environmental catastrophes that could destroy nascent life more complex than a single-celled organism.
The sources of these catastrophes are so-called long gamma ray bursts (LGRBs). These enormously violent events occur upon the collapse of a massive star which runs out of fuel, collapses, and sprays out more energy and radiation than a supernova along a highly directional beam. Only the most massive stars collapse and create a LGRB, but the universe contains a lot of stars, so in the universe as a whole, these events happen frequently. Using space-based satellites, astronomers notice some sort of GRB somewhere in the sky about once a day. They flash only briefly, but they are so bright we can easily detect them across billions of light years if we happen to lie in the path of the gamma rays.
A long gamma ray burst lasts for just a few seconds, but it ejects so much energy it would wreak havoc upon the biosphere of any nearby planet in the direction of its beam. The surface of a close-by planet might be fried by the gamma rays themselves. But even at a distance of a few thousand light years, a LGRB would destroy the protective ozone layer in the atmosphere of an Earth-like planet for several weeks or even months. Without ozone, ultraviolet light from a planet’s star would irradiate biological life (as we currently understand it) and completely degrade the biosphere. Any complex surface life forms would be doomed. Simple life might survive, but its evolution to more complex forms would be set back by millions or billions of years.
The likelihood of a planet suffering a blast from a LGRB increases with the density of nearby stars, since a small fraction of these stars will detonate as LGRBs. So regions of a galaxy where stars are packed close might be especially inhospitable. Using estimates of the distribution of stars in galaxies, the intensity and frequency of long GRBs, and our current understanding of stellar lifecycles, Piran and Jimenez calculated the probability of a planet in a spiral galaxy like the Milky Way suffering from a LGRB.
The result? In any 500 million year period, a planet within about 13,000 light years of the center of the Milky Way has a 95% chance of getting blasted by a lethal LGRB.
Further from the galactic center, where stars are further apart on average, the news is a little better. About 30,000 light years from the center of the Milky Way, the chance of a planet suffering a lethal LGRB is about 50% in a 500 million year period. Our Earth is about 25,000 light years from the galactic center, so even in our relatively benign neighborhood, there’s a roughly 50-50 chance our planet suffered the effects of a LGRB sometime in the last 500 million years. Some scientists speculate such an event may have caused one of the five known mass extinctions on Earth, perhaps the Cambrian–Ordovician extinction about 488 million year ago. No one knows for sure.
(At present, no stars capable of generating a LGRB presently lie close enough to Earth to do us harm, so we are in no danger from these events in the foreseeable future).
How about other galaxies? In most cases, the news is worse. In our own relatively sparse galaxy cluster, the so-called Local Group, galaxies are hundreds of thousands or millions of light years apart. So planets are in no danger from LGRBs from nearby galaxies such as the Large Magellanic Cloud or the Andromeda Galaxy. But most galaxy clusters are much denser than our Local Group, so the environment might be more dangerous. The study by Piran and Jimenez shows only galaxy clusters with 1/10th the density of the average galaxy cluster have an environment safe for Earth-like life to evolve over the long term.
Billions of years ago, the results are even more woeful. Galaxies were smaller then, stars were closer together, and massive stars in the earlier universe were more likely to detonate as LGRBs. After running the numbers, Piran and Jimenez found that until about 5 billion years ago, most planets in the universe had a high probability of suffering a GRB. So complex biological life (again, as we currently understand it) was extremely unlikely to form on any planet in the first 8.8 billion years of the universe.
At the end of their paper, the two speculate that the lethal effects of GRBs may at least partly explain the famous Fermi Paradox. This argument, first posed by the physicist Enrico Fermi, wonders why with such a large number of stars in the universe, and with the great age of the universe allowing enough time for life to evolve and slowly propagate across a galaxy, there should be many civilizations within range of contact of Earth. So where are they? According to this study, maybe they were fried by gamma rays before they got off the ground.
But the study by Piran and Jimenez contains many estimates and approximations. The most important is the assumption that all life in the universe has an Earth-like fragility to the direct and indirect effects of gamma rays on a planet’s atmosphere. We have no idea if this is true.
Astronomer and SETI advocate Seth Shostak tries to look on the bright side. Even with the inner part of many galaxies, and with entire dense clusters of galaxies off limits to complex life, there still may be plenty of stars, perhaps the majority of stars in a relatively safe galaxy like the Milky Way, in the safe zone for life to evolve. “The idea that certain regions of many galaxies might be sterile — or at least limited to life that requires a microscope to see — is interesting and sobering”, say Shostak. “But pessimism about habitats for life has a bad track record. We once thought that in our own solar system, only Earth had the right conditions for life. Today, we know of a half-dozen worlds within the reach of our rockets that could support some biology.”
His advice is to “… do the experiment. The theoreticians may point to the sky and say ‘it’s dead, Jim.’ But unless you look, you’ll never know for sure.”