I don’t think filters have to be sequential—some could be alternatives to each other, and they might interact. Consider the following.
Each supernova sterilizes everything for several lightyears around them. This galaxy has three supernovas per century, and it used to have more. Earth has gone unsterilized for 3.6 billion years, i.e. each of the last (very roughly) 100 million supernovas was far enough away to not kill it.
That’s easy to do for a planet somewhere on the outer rim, but the ones out there seem to lack heavy elements. If single-celled, mullti-celled, even intelligent life was easy given a couple billion years of evolution, you still couldn’t go to space on a periodic table that didn’t contain any metals.
So planets in areas with lots of supernova activity (i.e. high density of stars) could simply never have enough time between sterilizations to achieve spacefaring civilization, while planets in areas with low density of stars/supernovas haven’t accumulated enough heavy elements to build industry and spaceships. Neither effect prohibits everything, but together they’re a great filter.
There could be other combinations of prohibitive factors, where passing one makes passing the other more difficult. Maybe you need to be a carnivore in order to evolve theory of mind, but you also need to be a herbivore in order to evolve agriculture and exponential food surplus. Or maybe you need tectonic plates to avoid stratification of elements, but you also need a very stable orbit around your star, and those two conditions usually rule our each other. I don’t know. It just seems that a practically linear model of sequential filters, where filters basically don’t interact with each other, is entirely too simplistic to merit confidence.
In a few years, we’ll have a much clearer picture of the chemical makeup of the closest few hundred exoplanets, and that’ll cut down the number of possible explanations of Fermi’s Paradox to a maybe sort of manageable size. Until then, this discussion is unlikely to lead anywhere.
Let’s say 3 supernovas per century and each sterilizing 10 light years in radius.
That produces an average sterilization volume of about ten cubic light years per year. Total volume of the galactic thin disc is on the order of 2*10^13 cubic light years. That produces a half life of sterilization on the order of trillions of years, though you can bring it down to billions if you increase the supernova rate by a factor of a thousand or increase sterilization radius out to 100+ light years.
We can probably discount the galactic core for any purposes though—I’ve seen fun papers proposing evidence that it undergoes periodic starbursts every few tens of millions of years and the galactic supernova rate then briefly goes up to something like one per year with most of them in the core.
Thanks, but it appears we’re both wrong. Here is a nice intro article that gives proper numbers on this very subject and concludes supernovae aren’t a life-forbidding problem even in the galactic center.
But high density of stars might lead to planetary orbit perturbations which could be. It appears the galaxy is a bit complicated.
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.
A much more plausible filter, along the same lines, is earth not ever going outside a certain range of temperatures, over four billions years or so, as Sun shone brighter.
There could be many filters along the same lines, such as never happening evolution of a very successful but simple organism that eats everything complex, prompting a restart.
Given our own existence, we can perhaps rule out theories which give very low probability of emergence of life in the whole universe, but the probability of emergence of life on a given habitable planet may still be incredibly low (if some molecules have to randomly combine in a certain specific way).
I don’t agree that metals and heavy elements are necessary for industry and spaceships: you can do quite a lot with light elements, particularly carbon (for example plastics, carbon fiber, etc.). Also, biology makes all of its structure through lighter elements.
That being said, I think you’re very much on the money with the general idea: I also thought something similar while reading the artifcle (that the filters are likely multivariate and interdependent), but not in as well thought out a way.
We can do quite a lot with light elements now, after we spent millennia figuring out metals. We still use a lot of metal equipment and catalysts in the manufacturing of polymers and carbon fiber. I’m sure there are processes for making them without metals, but getting civilization going in the first place would be much harder without elements heavier than iron.
I don’t think filters have to be sequential—some could be alternatives to each other, and they might interact. Consider the following.
Each supernova sterilizes everything for several lightyears around them. This galaxy has three supernovas per century, and it used to have more. Earth has gone unsterilized for 3.6 billion years, i.e. each of the last (very roughly) 100 million supernovas was far enough away to not kill it.
That’s easy to do for a planet somewhere on the outer rim, but the ones out there seem to lack heavy elements. If single-celled, mullti-celled, even intelligent life was easy given a couple billion years of evolution, you still couldn’t go to space on a periodic table that didn’t contain any metals.
So planets in areas with lots of supernova activity (i.e. high density of stars) could simply never have enough time between sterilizations to achieve spacefaring civilization, while planets in areas with low density of stars/supernovas haven’t accumulated enough heavy elements to build industry and spaceships. Neither effect prohibits everything, but together they’re a great filter.
There could be other combinations of prohibitive factors, where passing one makes passing the other more difficult. Maybe you need to be a carnivore in order to evolve theory of mind, but you also need to be a herbivore in order to evolve agriculture and exponential food surplus. Or maybe you need tectonic plates to avoid stratification of elements, but you also need a very stable orbit around your star, and those two conditions usually rule our each other. I don’t know. It just seems that a practically linear model of sequential filters, where filters basically don’t interact with each other, is entirely too simplistic to merit confidence.
In a few years, we’ll have a much clearer picture of the chemical makeup of the closest few hundred exoplanets, and that’ll cut down the number of possible explanations of Fermi’s Paradox to a maybe sort of manageable size. Until then, this discussion is unlikely to lead anywhere.
Really-quick-and-dirty calculation time!
Let’s say 3 supernovas per century and each sterilizing 10 light years in radius.
That produces an average sterilization volume of about ten cubic light years per year. Total volume of the galactic thin disc is on the order of 2*10^13 cubic light years. That produces a half life of sterilization on the order of trillions of years, though you can bring it down to billions if you increase the supernova rate by a factor of a thousand or increase sterilization radius out to 100+ light years.
We can probably discount the galactic core for any purposes though—I’ve seen fun papers proposing evidence that it undergoes periodic starbursts every few tens of millions of years and the galactic supernova rate then briefly goes up to something like one per year with most of them in the core.
Thanks, but it appears we’re both wrong. Here is a nice intro article that gives proper numbers on this very subject and concludes supernovae aren’t a life-forbidding problem even in the galactic center.
But high density of stars might lead to planetary orbit perturbations which could be. It appears the galaxy is a bit complicated.
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.
A much more plausible filter, along the same lines, is earth not ever going outside a certain range of temperatures, over four billions years or so, as Sun shone brighter.
There could be many filters along the same lines, such as never happening evolution of a very successful but simple organism that eats everything complex, prompting a restart.
Given our own existence, we can perhaps rule out theories which give very low probability of emergence of life in the whole universe, but the probability of emergence of life on a given habitable planet may still be incredibly low (if some molecules have to randomly combine in a certain specific way).
I don’t agree that metals and heavy elements are necessary for industry and spaceships: you can do quite a lot with light elements, particularly carbon (for example plastics, carbon fiber, etc.). Also, biology makes all of its structure through lighter elements.
That being said, I think you’re very much on the money with the general idea: I also thought something similar while reading the artifcle (that the filters are likely multivariate and interdependent), but not in as well thought out a way.
We can do quite a lot with light elements now, after we spent millennia figuring out metals. We still use a lot of metal equipment and catalysts in the manufacturing of polymers and carbon fiber. I’m sure there are processes for making them without metals, but getting civilization going in the first place would be much harder without elements heavier than iron.
(gorillas have theory of mind)[http://www.newscientist.com/article/dn18658-mindreading-gorillas-love-a-good-game.html#.VAm-UXVdX0o]
Although, maybe an ancestor of theirs was a omnivore?