Perhaps the good news is that having a planet in the habitable zone is only a part of the problem. There are plenty of other constraints.
Firstly, the star has to be the right size. A bit larger than our sun, and the star evolves off the main sequence more and more rapidly. It’s not sufficiently stable to last the several billions of years needed to get to our stage of life. It’s reckoned by some people that our Earth only has a billion years or so left as a home for multicellular life before our sun overheats it. (minus the human factor.)
This also shrinks the effective size of the habitable zone enormously—a suitable planet ought to be towards the outside of the zone at first so it doesn’t end up too close to the star later on when the evolving star heats up.
A smaller star is more stable, but gives a different problem—since the habitable zone is closer in to the star, the star exerts larger tides in the planet, which brake its rotation to a stop. This has already happened to Venus, for example.
Then the star has to have the right mix of elements. These elements can only come from the explosions of other stars—so the universe has to live a bit before planets are possible. Equally, once the planets have formed, you want the process of stellar formation not to carry on next to you—all these criteria essentially require the star to be formed in a spiral arm of a decent-sized galaxy like ours—not in the core, and not in the first round of stellar formation.
Earth contains elements from several kinds of stellar explosion, and needs them. This implies you might get planets with differing proportions of the chemical elements, which might not be so conducive to life.
The planet itself has to be the right size—too small, and it won’t retain an atmosphere, and also won’t retain geological activity over the needed timeframe. Too large, and it will retain hydrogen and/or helium, which results in a huge atmosphere and no life at all.
The planet needs a large moon—otherwise the axis of rotation of the planet tends to tumble chaotically, as Mars does, and Venus apparently did when it rotated faster. The moon stabilises the axial tilt, which could be stabilised at practically any tilt, not just Earth’s slight tilt which gives rise to reasonable seasons. The moon itself is a bit of a constraint in its own right—it would need to form in the late stage of planetary formation—computer simulations suggest this happens about 1 time in 12. Too small a moon might be driven out of orbit by the underlying planets rotation (our own moon’s been driven out a loong way as it is), and will cease to be any help at stabilising rotation. Too large a moon will again brake the planet’s rotation to a much lower level, resulting in long days and inhospitable temperatures. Apparently our moon will cease to protect the Earth from axial wobble in around another billion years. (if not for the human factor)
Many of these factors seem, with high probability, like genuine constraints (e.g. a second generation star). But I wonder whether others might be examples of anthropic generalizing from one example (e.g. availability of a specific ratio of elements). Presumably alien life would adapt to whatever conditions actually exist in their world.
Your post got me thinking on a completely different tangent: How much of the filter might be at a high tech level for most species but we managed to escape it based on what resources we actually lacked?
The most obvious example is the amount of U-235. If humans had arose 2 billion years ago there would be about six times as much U-235 on the planet (since U-235 has a half-like of around 700 million years), making it much easier to develop nuclear weapons. That could have a substantial negative impact on a species chance of not wiping themselves out.
I’m not sure how much this would matter in that fission weapons are a lot easier already to produce than fusion weapons. Moreover, this would also make it easier to use nuclear power for productive purposes. Even a Fermi style pile would be much, much easier to construct (Fermi’s original pile did not use enriched uranium). So the ability to use nuclear power in this way would help for simple electricity generation a lot as well as nuclear rockets (which would directly help beating the Great Filter). So overall this seems like a wash without a lot more data.
Is there some other isotope that could have a similar impact? Possibly something that now is very rare so we aren’t paying much attention to it? The other obvious candidates don’t seem to work. For example, tritium has a very short half life but natural processes produce more of it so that shouldn’t matter. Similarly, plutonium 239 has much too short a half-life so that any species that arose even after the first billion years wouldn’t see any substantial amounts of it. Maybe Plutonium-244? It is primordial, has a half-life around 80 million years, so would be around in larger quantities on a young planet. But I don’t know of any obvious fission chain for it, and the quantities produced would be very little, since it is not easily produced in supernovae.
Most accessible nuclear power on our planet isn’t stored as U-235 anyway. We can get a lot more by converting uranium into plutonium, which is heavily restricted because of how much easier it is to make weapons out of, or out of thorium, which is safer and more abundant but requires a different procedure to extract energy which has never been developed for commercial applications.
Each of the constraints you name, though plausible, don’t seem to be strong enough to act as a full filter. They appear to filter on the level of full percentage points rather than billionths of percentage points. Without finding dozens more filters of this kind, there would still be human-level life all over the place and hence (assuming no life in the universe) the Great Filter would be ahead of us.
A bunch of tiny filters is an unlikely scenario for explaining “The Great Silence”.
Duncan seems to be just talking about the Earth related constraints. It is possible to have those constraints as well as other constraints. I don’t see Duncan as directly claiming that the Earth filtration issues are the entire filter. His point seems to be more that being in the habitable zone is not the only filtration issue for a planet.
This is true. However, I also think the planetary filters are perhaps easier to evaluate than some of the others. How would I know whether the origin of life is something that’s really unlikely to happen on Earth in half a billion years, or something that will happen in trillions of places all over the planet in under half an hour, given the right conditions? I’m more tempted by the latter thought, but without knowing what the process actually is, or having any suitable examples other than Earth, it’s tough to know what the answer might be. We have no reason for thinking it’s difficult other than that we haven’t replicated it yet, though. That and the great filter question itself—if making intelligent life is easy, why isn’t it everywhere?
It is pretty easy to imagine lots of smallish filters as responsible. If I am doing my math correctly, then 9 filters that each weed out only 9 out of 10 solar systems would reduce the expected population of intelligent life in the Milky Way to just .1 systems. Since we know so much more about star formation I am a lot more comfortable saying “habital planet” will weed out 90% of systems, that is not nearly a big enough filter to account for what we see by itself but as you stack them filters of this size are quite substantial.
edit: er no that’s wrong, off by a few orders of magnitude. So yeah, gosh where is everyone?
Perhaps the good news is that having a planet in the habitable zone is only a part of the problem. There are plenty of other constraints.
Firstly, the star has to be the right size. A bit larger than our sun, and the star evolves off the main sequence more and more rapidly. It’s not sufficiently stable to last the several billions of years needed to get to our stage of life. It’s reckoned by some people that our Earth only has a billion years or so left as a home for multicellular life before our sun overheats it. (minus the human factor.)
This also shrinks the effective size of the habitable zone enormously—a suitable planet ought to be towards the outside of the zone at first so it doesn’t end up too close to the star later on when the evolving star heats up.
A smaller star is more stable, but gives a different problem—since the habitable zone is closer in to the star, the star exerts larger tides in the planet, which brake its rotation to a stop. This has already happened to Venus, for example.
Then the star has to have the right mix of elements. These elements can only come from the explosions of other stars—so the universe has to live a bit before planets are possible. Equally, once the planets have formed, you want the process of stellar formation not to carry on next to you—all these criteria essentially require the star to be formed in a spiral arm of a decent-sized galaxy like ours—not in the core, and not in the first round of stellar formation.
Earth contains elements from several kinds of stellar explosion, and needs them. This implies you might get planets with differing proportions of the chemical elements, which might not be so conducive to life.
The planet itself has to be the right size—too small, and it won’t retain an atmosphere, and also won’t retain geological activity over the needed timeframe. Too large, and it will retain hydrogen and/or helium, which results in a huge atmosphere and no life at all.
The planet needs a large moon—otherwise the axis of rotation of the planet tends to tumble chaotically, as Mars does, and Venus apparently did when it rotated faster. The moon stabilises the axial tilt, which could be stabilised at practically any tilt, not just Earth’s slight tilt which gives rise to reasonable seasons. The moon itself is a bit of a constraint in its own right—it would need to form in the late stage of planetary formation—computer simulations suggest this happens about 1 time in 12. Too small a moon might be driven out of orbit by the underlying planets rotation (our own moon’s been driven out a loong way as it is), and will cease to be any help at stabilising rotation. Too large a moon will again brake the planet’s rotation to a much lower level, resulting in long days and inhospitable temperatures. Apparently our moon will cease to protect the Earth from axial wobble in around another billion years. (if not for the human factor)
The orbit shouldn’t be too elliptical.
Earth is a bit special, it seems.
Many of these factors seem, with high probability, like genuine constraints (e.g. a second generation star). But I wonder whether others might be examples of anthropic generalizing from one example (e.g. availability of a specific ratio of elements). Presumably alien life would adapt to whatever conditions actually exist in their world.
Your post got me thinking on a completely different tangent: How much of the filter might be at a high tech level for most species but we managed to escape it based on what resources we actually lacked?
The most obvious example is the amount of U-235. If humans had arose 2 billion years ago there would be about six times as much U-235 on the planet (since U-235 has a half-like of around 700 million years), making it much easier to develop nuclear weapons. That could have a substantial negative impact on a species chance of not wiping themselves out.
I’m not sure how much this would matter in that fission weapons are a lot easier already to produce than fusion weapons. Moreover, this would also make it easier to use nuclear power for productive purposes. Even a Fermi style pile would be much, much easier to construct (Fermi’s original pile did not use enriched uranium). So the ability to use nuclear power in this way would help for simple electricity generation a lot as well as nuclear rockets (which would directly help beating the Great Filter). So overall this seems like a wash without a lot more data.
Is there some other isotope that could have a similar impact? Possibly something that now is very rare so we aren’t paying much attention to it? The other obvious candidates don’t seem to work. For example, tritium has a very short half life but natural processes produce more of it so that shouldn’t matter. Similarly, plutonium 239 has much too short a half-life so that any species that arose even after the first billion years wouldn’t see any substantial amounts of it. Maybe Plutonium-244? It is primordial, has a half-life around 80 million years, so would be around in larger quantities on a young planet. But I don’t know of any obvious fission chain for it, and the quantities produced would be very little, since it is not easily produced in supernovae.
Most accessible nuclear power on our planet isn’t stored as U-235 anyway. We can get a lot more by converting uranium into plutonium, which is heavily restricted because of how much easier it is to make weapons out of, or out of thorium, which is safer and more abundant but requires a different procedure to extract energy which has never been developed for commercial applications.
Each of the constraints you name, though plausible, don’t seem to be strong enough to act as a full filter. They appear to filter on the level of full percentage points rather than billionths of percentage points. Without finding dozens more filters of this kind, there would still be human-level life all over the place and hence (assuming no life in the universe) the Great Filter would be ahead of us.
A bunch of tiny filters is an unlikely scenario for explaining “The Great Silence”.
Duncan seems to be just talking about the Earth related constraints. It is possible to have those constraints as well as other constraints. I don’t see Duncan as directly claiming that the Earth filtration issues are the entire filter. His point seems to be more that being in the habitable zone is not the only filtration issue for a planet.
This is true. However, I also think the planetary filters are perhaps easier to evaluate than some of the others. How would I know whether the origin of life is something that’s really unlikely to happen on Earth in half a billion years, or something that will happen in trillions of places all over the planet in under half an hour, given the right conditions? I’m more tempted by the latter thought, but without knowing what the process actually is, or having any suitable examples other than Earth, it’s tough to know what the answer might be. We have no reason for thinking it’s difficult other than that we haven’t replicated it yet, though. That and the great filter question itself—if making intelligent life is easy, why isn’t it everywhere?
It is pretty easy to imagine lots of smallish filters as responsible. If I am doing my math correctly, then 9 filters that each weed out only 9 out of 10 solar systems would reduce the expected population of intelligent life in the Milky Way to just .1 systems. Since we know so much more about star formation I am a lot more comfortable saying “habital planet” will weed out 90% of systems, that is not nearly a big enough filter to account for what we see by itself but as you stack them filters of this size are quite substantial.
edit: er no that’s wrong, off by a few orders of magnitude. So yeah, gosh where is everyone?