According to many science fiction movies, life thrives throughout the universe. Star Wars recounts long-past battles in “a galaxy far, far away.” In Contact, Ellie Arroway travels through wormholes to Vega where she encounters aliens who have billions of years of history. More recently, Interstellar depicted the search for planets in remote galaxies that could serve as a new home (after we manage to destroy Earth’s capacity to support human life). However, research into powerful gamma-ray bursts (GRBs) puts a damper on the unbridled optimism that complex life pervades the universe.
Although scientists don’t understand GRBs in detail, they know a lot about their observable characteristics. GRBs are bright bursts of gamma rays (light more energetic than x-rays) that last from fractions of a second up to hours and emit hundreds of times more energy than a supernova. Because of the enormous amount of energy involved in a GRB, astronomers have investigated the probability of one in the vicinity of Earth and the damage a close GRB would cause.
If Earth were close enough, radiation from a GRB would ionize our planet’s upper atmosphere and destroy its ozone shield. Consequently, the increased flux of ultraviolet light would exterminate virtually all life on the surface including marine plankton, thus depriving other marine life of a main nutrient. Some scientists have argued that a GRB explains the Ordovician extinction event roughly 450 million years ago. And research gives a 50 percent chance of a close GRB occurring in the past 500 million years. The probability increases to near certainty over Earth’s entire history! Calculations also show that any planet located close to the center of our galaxy would receive lethal doses of radiation from GRBs. Only at distances greater than 32,000 light-years does the GRB exposure probability drop below 50 percent.1
Earth sees a small exposure to GRB radiation because our planet resides in a low-density region in the outskirts of a high-metallicity galaxy (astronomers consider elements other than hydrogen and helium as metals). Low-metallicity galaxies host more GRBs. The fact that the cluster containing our galaxy has only two large members (the Milky Way and Andromeda) means that the cluster includes a smaller number of low-metallicity satellite galaxies—further reducing Earth’s exposure to lethal GRBs.
Studies of GRB rates throughout the universe also yield interesting results. Only 10 percent of large galaxies in the universe fit the criteria for hosting a planet with sufficiently low exposure to GRBs. In order to limit the number of GRBs close to a planet hosting life, a number of conditions must be met:
- The universe must have a cosmological constant.
- The universe must be at least 5 billion years old.
- The planet’s galaxy must reside in a small cluster with few low-metallicity satellites.
- The planet’s star must reside in the outskirts of a large member of the galaxy cluster.
Earth meets all these conditions. Such a confluence of finely tuned characteristics indicates that Earth was intended to support human life. It also means that we are unlikely to find similar life elsewhere in the universe—unless God chose to create on a distant planet, too.
- Tsvi Piran and Raul Jiminez, “Possible Role of Gamma Ray Bursts on Life Extinction in the Universe,” Physical Review Letters 113 (December 5, 2014): 231102.