As a copious source of gamma-rays, a nearby Galactic Gamma-Ray Burst (GRB) can be a threat to life. Using recent determinations of the rate of GRBs, their luminosity function and properties of their host galaxies, we estimate the probability that a life-threatening (lethal) GRB would take place. Amongst the different kinds of GRBs, long ones are most dangerous. There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 Gyr close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place during the last 500 Myr causing one of the major mass extinction events. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life, we explore the potential effects of GRBs to life elsewhere in the Galaxy and the Universe. We find that the probability of a lethal GRB is much larger in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, this probability drops below 50%. When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies and life can exist in only ~ 10% of galaxies. Remarkably, a cosmological constant is essential for such systems to exist. Furthermore, because of both the higher GRB rate and galaxies being smaller, life as it exists on Earth could not take place at z>0.5. Early life forms must have been much more resilient to radiation.
Very interesting, thank you! I especially like the insight that for evolution to go on uninterrupted for 5 billion years, you don’t just need a particular type of planet (not hot, not cold, in a stable orbit) in a particular region of the galaxy (on the outskirts), but this planet also needs to be inside a particular type of galaxy (big, old, not dense) that happens to be in a particular type of intergalactic environment (not dense, lacking low metallicity dwarf galaxy neighbors). This helps with the sharper version of the Fermi paradox that assumes the possibility of intergalactic travel.
I’m not a physicist, but as far as I understand the paper, their assumption of what constitutes a “lethal” amount of Gamma Ray Burst damage to a planet seems kind of arbitrary. Their description indicates that it’d kill everything on the surface and everything underwater that feeds on plankton. But I see nothing to indicate that, say, life on hydrothermal vents, or bacteria living deep underground (which exist on our planet at least two miles down) couldn’t survive what the authors call “lethal”. So abiogenesis would not need to happen again in those cases, nor would evolution of very basic metabolic structures that evolution would again build upon. Even a small bunch of tiny replicators that survived with, say, three billion years of previous evolution under their belt might re-colonize the planet much more quickly and diversely than newly arisen ones could.
Meaning that as a layman, I don’t see how we’d distinguish between a past where Earth was hit by a “lethal” GRB 2 billion years ago (when there were just eukaryotes, procaryotes and cyanobacteria), and one were it wasn’t.
Actually, I’m not a layman, and I have some ideas.
The Proterozoic (2 billion years ago) is a time period that geologists affectionately call the ‘boring billion’. In those rock strata, we very often find biogenic stromatolites, crumpled accretionary structures produced by the accumulation of mineral waste products in microbial metabolism. In the wild, they look like lumpy rocks on coastlines and in lakes, with a thin biofilm on top. Think of them as the microbial forests through which the early eukaryotes would have foraged and hunted. These ecosystems are also exclusively shallow-water, since they require sunlight and water in copious supply.
As such, they would be wiped out by a ‘moderate’ gamma ray burst, since they don’t have the protection of deep oceans. In other words, there would be a specific moment at which accretion halted for every biogenic stromatolite at the same time. This would be followed, in geologic history, by a shortish period in which newly lithified sediments lacked a biological influence, as life clawed its way back from the deep oceans. Even if the biosphere that followed was indistinguishable from the previous incarnation (which itself seems unlikely), we’d be able to see an interruption.
We haven’t yet found evidence of such a hiatus in the Precambrian. It’s a big history, so it’s always possible that the evidence will come in later- but it’s worth pointing out that we have found interruptions of comparable magnitude, from different sources.
Meaning that as a layman, I don’t see how we’d distinguish between a past where Earth was hit by a “lethal” GRB 2 billion years ago (when there were just eukaryotes, procaryotes and cyanobacteria), and one were it wasn’t.
Indeed, according to Wikipedia at least, we don’t know whether the Ordovician–Silurian extinction event was caused by a GRB or not.
BTW, this recently showed up on arXiv:
(“At z > 0.5” approximately means ‘more than 5 billion years ago’.)
(I only have read the abstract so far.)
Very interesting, thank you! I especially like the insight that for evolution to go on uninterrupted for 5 billion years, you don’t just need a particular type of planet (not hot, not cold, in a stable orbit) in a particular region of the galaxy (on the outskirts), but this planet also needs to be inside a particular type of galaxy (big, old, not dense) that happens to be in a particular type of intergalactic environment (not dense, lacking low metallicity dwarf galaxy neighbors). This helps with the sharper version of the Fermi paradox that assumes the possibility of intergalactic travel.
I’m not a physicist, but as far as I understand the paper, their assumption of what constitutes a “lethal” amount of Gamma Ray Burst damage to a planet seems kind of arbitrary. Their description indicates that it’d kill everything on the surface and everything underwater that feeds on plankton. But I see nothing to indicate that, say, life on hydrothermal vents, or bacteria living deep underground (which exist on our planet at least two miles down) couldn’t survive what the authors call “lethal”. So abiogenesis would not need to happen again in those cases, nor would evolution of very basic metabolic structures that evolution would again build upon. Even a small bunch of tiny replicators that survived with, say, three billion years of previous evolution under their belt might re-colonize the planet much more quickly and diversely than newly arisen ones could.
Meaning that as a layman, I don’t see how we’d distinguish between a past where Earth was hit by a “lethal” GRB 2 billion years ago (when there were just eukaryotes, procaryotes and cyanobacteria), and one were it wasn’t.
Actually, I’m not a layman, and I have some ideas.
The Proterozoic (2 billion years ago) is a time period that geologists affectionately call the ‘boring billion’. In those rock strata, we very often find biogenic stromatolites, crumpled accretionary structures produced by the accumulation of mineral waste products in microbial metabolism. In the wild, they look like lumpy rocks on coastlines and in lakes, with a thin biofilm on top. Think of them as the microbial forests through which the early eukaryotes would have foraged and hunted. These ecosystems are also exclusively shallow-water, since they require sunlight and water in copious supply.
As such, they would be wiped out by a ‘moderate’ gamma ray burst, since they don’t have the protection of deep oceans. In other words, there would be a specific moment at which accretion halted for every biogenic stromatolite at the same time. This would be followed, in geologic history, by a shortish period in which newly lithified sediments lacked a biological influence, as life clawed its way back from the deep oceans. Even if the biosphere that followed was indistinguishable from the previous incarnation (which itself seems unlikely), we’d be able to see an interruption.
We haven’t yet found evidence of such a hiatus in the Precambrian. It’s a big history, so it’s always possible that the evidence will come in later- but it’s worth pointing out that we have found interruptions of comparable magnitude, from different sources.
Indeed, according to Wikipedia at least, we don’t know whether the Ordovician–Silurian extinction event was caused by a GRB or not.