This is the first in a series of posts I am putting together on a personal blog I just started two days ago as a collection of my musings on astrobiology ("The Great A'Tuin" - sorry, I couldn't help it), and will be reposting here.  Much has been written here about the Fermi paradox and the 'great filter'.   It seems to me that going back to a somewhat more basic level of astronomy and astrobiology is extremely informative to these questions, and so this is what I will be doing.  The bloggery is intended for a slightly more general audience than this site (hence much of the content of the introduction) but I think it will be of interest.  Many of the points I will be making are ones I have touched on in previous comments here, but hope to explore in more detail.

This post references my first two posts - an introduction, and a discussion of our apparent position in space and time in the universe.  The blog posts may be found at:

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All the upvotes! I am something of an astrobiologist myself, although my emphasis is on geobiology and planetary geology. My current day job is to map out Martian sedimentary rocks with an eye towards liquid water distribution and ancient habitability. My graduate thesis was closer to home, a study of Paleoarchean microbialites.

If you think your posts would benefit from a bit of collaboration, don't hesitate to ask. Otherwise, I'm eager to see what insights you have from a more astronomy-heavy and pure biology perspective.


Oh god please yes. I'm planning on talking about the limits of our observations in our own solar system and what is possible versus excluded both now and in the past, as well as origin of life research and the history of the scale and complexity of Earth's biosphere. Will PM you.

Furthermore, although smaller stars are much more common than larger stars (the Sun is actually larger than over 80% of stars in the universe) stars smaller than about 0.5 solar masses (and thus 0.08 solar luminosities) are usually ‘flare stars’ – possessing very strong convoluted magnetic fields and periodically putting out flares and X-ray bursts that would frequently strip away the ozone and possibly even the atmosphere of an earthlike planet.

I have been wanting better stats on this for a while. Basically, what percentage of the eventual sum of potential-for-life-weighted habitable windows (undisturbed by technology) comes from small red dwarfs that can exist far longer than our sun, offsetting long stellar lifetimes with the various (nasty-looking) problems? ETA: wikipedia article. And how robust is the evidence?


I don't know how exactly quantitative I can get beyond 'small stars are bad' - I don't think there's a lot of collation of the necessary data. I can try.

  • Stars bigger than ~1.2 solar masses (in the middle of spectral type F) would cook Earth-analogs before we had large amounts of multicellular life, though that might not be true for all theoretical lifebearing worlds.

  • is a fun source for known stars in the solar neighborhood as revealed by the Hipparcos satellite dataset. It might not be as rigorous as the datasets themselves but it's certainly easier to navigate. There are at LEAST 1400 stars within 50 light years of the Sun, only ~64 of which are solar type (spectral type G) and about ~46 of which are larger (F,A,B). There's a full 152 (or more! - out at 50 LYs some might be missed) K type stars that are smaller and longer lived than the sun in the same volume that are not smaller to the point of being M dwarf stars. Some of those stars might last up to 30 or 40 billion years, with any one planet possibly being habitable for a reasonable fraction of that time compared to Earth's up to 6 billion years. This does make me wonder if a large fraction of them are unlikely to spawn biopspheres for some unknown or underappreciated reason, or if we just got slightly 'lucky' in that we are in the upper quarter or so of stars that probably spawn complex biospheres in terms of mass, which actually isn't all that weird all things considered.

  • One might expect longer-lived K-type stars to have longer-lived biospheres and have a higher chance of spawning intelligent systems, making that 'upper quarter' thing a bit more odd. I have no answer to this. I can randomly speculate though: Maybe planets 'run down' in some way over time in these systems over timescales slightly longer than the Earth has in our solar system before the sun cooks it. Maybe the brightening sun is closely balanced by Earth's cooling geosphere putting out less and less CO2 and other such gases over time and these processes wouldn't balance for as long in a system with a slower-evolving star, such that solar-mass stars are more closely tuned to long-lived biospheres than we expect. Maybe star mass has unappreciated statistical ties to planet types. Or maybe we have just hit on one of the random weird things about our biosphere that given there are so many variables that go into each biosphere you will find a few of that aren't necessarily important but are nonetheless there. With a hundred uncorrelated variables, you'd expect us to be in the 95th percentile of several of them...

  • Maybe stars of masses that we see as only sometimes being flare stars is a case of individual stars occasionally flaring regularly and occasionally not over human timescales rather than different stars behaving differently over geological timescales - we've only been watching for a short time.

  • Stars smaller than ~0.5 solar masses (M type, red dwarfs) need a planet to be so close in to get an Earthly amount of radiation (< 0.3 AUs) that they probably tidally lock. There's endless arguments as to if that's a no-no or not for complex biospheres and what it means for atmospheres and hydrospheres. They're also often flare stars.

  • Red dwarf stars have a longer cooldown period from formation, which was done in our system pretty much during the planetary accretion stage, and might complicate habitability arguments.

...Conclusion: there's too many maybes here.

PS: I can't freaking wait for the GAIA mission dataset to come back, it'll be the greatest star map ever created by orders of magnitude...

ANOTHER EDIT: A lot of interesting discussion in the commentary of - looks like the limit of tidal locking for something with terrestrial illumination is closer to 0.7 solar masses.

YET ANOTHER EDIT: Another visualization of nearby star systems:

It appears that you are tacitly presuming that life typically co-evolves with a stellar system and that implicitly, life of earth complexity took a maximum of ~4.5b years to evolve(?). If this is case I'm curious what your thoughts on the recent paper by Sharov and Gordon might be. The paper applies statistical arguments to genomic complexity of earth organisms and argues that life as we know it on earth may in fact have taken as much as 9+/-2.5bn years to evolve (predating earth and the local star, potentially up to approximately the entire history of our galaxy and a significant percentage of the history of the stellar epoch)?

If life of complexity similar to that currently on Earth does take as much as 11.5 billion years to evolve, that does not leave a very large window for forerunner biologies and would tend to increase the probability that Earth life is in fact some of the most complex life to ever even have the opportunity to arise in our current light cone.

Is the position of their paper (basically: that life takes longer to evolve than we think) something you are already planning to address further in your article series? I'd be interested to hear what your interpretation might be.

The amount of line emission from a galaxy is thus a rough proxy for the rate of star formation – the greater the rate of star formation, the larger the number of large stars exciting interstellar gas into emission nebulae... Indeed, their preferred model to which they fit the trend converges towards a finite quantity of stars formed as you integrate total star formation into the future to infinity, with the total number of stars that will ever be born only being 5% larger than the number of stars that have been born at this time.

Is this a good proxy for total star formation, or only large star formation? Is it plausible that while no/few large stars are forming, many dwarfs are?


That depends on something called the "initial mass function" for a star forming region - the frequency distribution of masses produced. See for two estimated mass functions for our galaxy.

Until recently the consensus was that since the initial mass function was pretty similar throughout our own galaxy under very different environments, it should be similar in other places too. More recently there's been some controversial claims that 'Early type' (elliptical) galaxies may have a systematically different mass functionn than spirals that also varies by galaxy mass, see . Other research seems to contradict this, see . These papers become technical to a point at which I get lost pretty easily in reading them. If I am reading the paper referred to in the first link correctly though, their findings if true are consistent with one of two scenarios: either the mass functionin massive elliptical galaxies is biased towards the formation of large amounts of small stars, or it is biased towards the production of large amounts of large stars which are now dead and contributing excess compact mass in the form of dead star remnants. Both would be consistent with the data (which comes in the form of ratios of luminosity to galactic mass) but a mass function like that of most spirals would not be.

If the mass function stuff turns out true, I'm pretty sure it would distort the shape of the curve of star formation referenced in this post one way or another, but not change its ultimate form.

Interesting. Is what Greg Cochran said here and below reasonable?

"A lot of ice moons seem to have interior oceans, warmed by tidal flexing and possibly radioactivity. But they’re lousy candidates for life, because you need free energy; and there’s very little in the interior oceans of such system."

A primary candidate for free energy in icy moons is thermal venting at the bottom of the liquid oceans; they do have rocky cores, after all. If Jupiter's tidal forces can cause the volcanism on Io, then it's reasonable to assume that they can also cause the rocky interior of Europa to produce volcanoes that vent heat and interesting ions in to the liquid water.

There's also a surprising amount of electrolysis going on in the ice of Europa, because Jupiter has such a terrifying electrical field. I doubt that's enough to sustain an ecosystem, but it's enough for me to fantasize about giant upside-down forests of filter-feeders digging their roots upwards to get at the free oxygen.

The preliminary results we're seeing on Pluto should also adjust your expectations in favor of ice-moon habitability; there, we see active tectonics on a Kuiper Belt Object even without the tidal forcing of a nearby planet. It seems that a giant pile of silicates and water ice provide a great deal of dynamism all on their own.

How does the free energy on an ice moon compare to the amount the sun shoots down at earth?


Earth gets, on average, 340 watts per square meter of sunlight (more at the equator than the poles and more during the day than at night) for a total flux of 1.7 10^17 watts. Earth also has a geothermal heat flux (partially from primordial heat of formation, partially tidal heating, partially radioactive heating) of 4.7 10^13 watts for 0.087 watts per square meter (concentrated in hot spots of course). Our geothermal flux is thus about 1/4000 our solar flux. Only a fraction of the geothermal energy flux will be in a form available to living things, specifically that which causes geochemical gradients to form. Though geothermal effects can also bring deep substances into contact with surface substances and allow them to interact and produce more energy than is contained in the gradient - like serpentinization, by which Fe2+ in particular very deep rock types plus surface water become Fe3+ oxides and hydrogen gas for a net release of energy. I am unprepared to compare the energy of dredged up chemicals to the heat flux. I do know that living things on Earth can easily live off these fluxes at hot springs and vents.

Europa is estimated to receive 7 * 10^12 watts of tidal heating driving geothermalisms (a full seventh that of Earth for something only 0.008 times as massive and 1/16 as much surface area as Earth, though one or two other sources I found have estimates a factor of two or three higher). Its radioactive heating is negligible compared to that number. This gives it an average geothermal energy flux of 0.23 watts per square meter, about 1/1500 what we get from the sun, a fraction of which becomes geochemical gradients accessible to life. There will be more geothermal action going on in there than on Earth. Again these geothermal flows could also dig up already-reactive substances from deep below that could react with substances in the ice/ocean, increasing the available energy.

There is indeed a bunch of water cracking happening at its icy space-exposed surface via radiation, with hydrogen sputtering off into space and oxidized compounds winding up in the ice which is believed to circulate down on megayear timescales into the lower layers and potentially into the liquid layers. This would allow another energy source via the oxidation of minerals or hydrocarbons dissolved in the liquids, which life could insert itself into as a middleman.

All the forthcoming numbers I am using are from "Energy, Chemical Disequilibrium, and Geological Constraints on Europa" by Hand et al. About 4 watts per square meter of sunlight is absorbed by the surface (of about 13 watts of total average incident light), only a tiny fraction of which would cause water-cracking. It gets about 0.125 watts per square meter of particle irradiation. If we assume a ridiculously unrealistically high far-upper-bound of 0.25 watts per square meter of water-cracking which generates oxygen at 237 kilojoules per mole, and that oxidizes iron from, say, a metallic state to rust (about 550 kilojoules per mole of oxygen gas) you get something like half a watt per square meter of oxygen-based energy flow.

That is DEFINITELY a drastic overestimate though. The paper mentioned above goes into some analysis I am utterly unprepared to comment on and suggests that given the surface age of Europa and its energetic environment, up to about 5 * 10^9 moles of 'oxidants' are delivered to the interior of Europa per year. Let's be completely naive and just compare that to the total photosynthetic flux of the Earth, assuming it's all oxygen. It comes to something like one millionth the photosynthetic productivity of Earth.

Of course, since these matter flows via geothermalisms or crust downwelling are very uneven compared to incident sunlight there will be localized hotspots like our own geothermal vents where things could be much more interesting than the above numbers would seem to indicate.


They're great candidates for life, especially given mounting ideas on the origin of life possibly being tied up with geochemistry and geology.

They're lousy candidates for big biospheres that utterly transform the geochemistry such that you could see it far away like ours has, or for complex life, since the total energy flux available in potentially clement environments is miniscule compared to here. But Earth's biosphere had to start with something a lot like the energy sources you have in the icy moons, not photosynthesis.

Excellent post - the analysis of our temporal rank with regards to star formation are interesting and novel (for me). I look forward to your next post.

The gist of this is that the astronomical evidence appear to strongly support mediocrity, and thus a prior in which biological life is not super rare.

I'm especially interested in what kind of bets you'd place concerning future discovery for life elsewhere in the solar system such as Europa. I hope you cover Mars too at some point.



Given the current state of origin-of-life research that is starting to support tying early biochemistry to geochemistry rather than 'primordial soup' type situations and the apparent speed of life's origin on Earth, it's easy to imagine life arising multiple times in multiple places in our solar system. Can't say that it did or didn't though, we'd need way more data.

Given the differences between Earth and other places in our solar system, there's definitely not going to be any other big biospheres with huge amounts of biomass and complexity that utterly remake the geochemistry of a body; instead any other biospheres would necessarily be both tiny and very hard to see due to hanging onto small matter and energy fluxes in protected regions of their homes.

Given the sheer difficulty of exploring other bodies for these small things in protected niches, it's entirely possible that we could've missed several of these small biospheres (past or present) so far. As such I would not be terribly surprised at future evidence of these things. Mars is great because you've got an exposed surface that may have been very different in the past that you can look for traces on plus inklings that there could still be productive niches in places its not inconceivable you could look at, it's 'close' (all things considered), and the interesting spots aren't all under kilometers of ice. It's also still an entire planet that has barely been looked at up close at all so far and where it costs an instrument's weight in gold to send it there.

In the case of Mars, we have an improbable advantage, because there is already a huge industry and body of knowledge devoted to the discovery of organic-rich rock deposits in regions that are likely to preserve complex carbon forms. If there ever was an ecosystem on the surface of Mars, Exxon will help us find it.

(Although actually, Mars lacks active tectonic plates, so it's not quite the same problem. But many industry tricks and technologies will transfer seamlessly.)

There's also the UFAI-Fermi-paradox: If AI is a significant risk and if intelligent life is common why aren't we all paperclips already? AI doesn't work as a filter because it's the kind of disaster likely to keep spreading and we'd expect to see large parts of the sky going dark as the stars get turned into pictures of smiling faces or computronium.

There's also the anthropic principle. We probably wouldn't be alive to ask the question in a universe where the earth has been turned into a strip-mall for aliens.

Though combining the anthropic principle with the Drake equation gives us another possibility.

Compute the Drake equation for a cone 60 light years thick extending back through time outwards from earth. x billion planets with y civilisations with z probability of producing a UFAI. Which gives you a rough estimate for the chances of someone elses UFAI killing us all within your lifetime.

There's also the UFAI-Fermi-paradox:

This is just the regular Fermi paradox/Great Filter. If AI has any impact, it's that it may make space colonization easier. But what's important for that is that eventually industrial civilizations will develop AI (say in a million years). Whether the ancient aliens would be happy with the civilization that does the colonizing is irrelevant (i.e. UFAI/FAI) to the Filter.

You could also have the endotherm-Fermi-paradox, or the hexapodal-Fermi-paradox, or the Klingon-Great-Filter, but there is little to be gained by slicing up the Filter in that way.

I may not have been clear, by UFAI I didn't just mean an AI which just trashes the home planet of the civilization that creates it and then stops but rather one which then continues to convert the remainder of the universe into computronium to store the integer for it's fitness function or some similar doomsday scenario.

It doesn't matter how safe you are about AI if there's a million other civilizations in the universe and some non-trivial portion of them aren't being as careful as they should assuming it's a non-trivial risk.

Which either argues for AI-risk not being so risky or for an early filter.

Let's consider a few propositions:

  1. There is enough cumulative early filtration that very few civilizations develop, with less than 1 in expectation in a region like our past light-cone.
  2. Interstellar travel is impossible.
  3. Some civilizations have expanded but not engaged in mega-scale engineering that we could see or colonization that would have pre-empted our existence, and enforce their rules on dissenters.
  4. Civilizations very reliably wipe themselves out before they can colonize.
  5. Civilizations very reliably choose not to expand at all.

1-3 account for the Great Filter directly, and whether biological beings make AI they are happy with is irrelevant. For #4 and #5 what difference does it make whether biological beings make 'FAI' that helps them or 'UFAI' that kills them before going about its business? Either way the civilization (biological, machine, or both) could still wipe itself out or not (AIs could nuke each other out of existence too), and send out colonizers or not.

Unless there is some argument that 'UFAI' is much less likely to wipe out civilization (including itself), or much more likely to send out colonizers, how do the odds of alien 'FAI' vs 'UFAI' matter for explaining the Great Filter any more than whether aliens have scales or feathers? Either way they could produce visible signs or colonize Earth.

yes, 1 is equivalent to an early filter.

2 would be somewhat surprising since there's no physical law that disallows it.

3 comes close to theology and would imply low AI risk since such entities would probably not allow a potentially dangerous AI to exist within any area they control.

4 is sort of a re-phrasing of 1.

5 is possible but implies some strong reason many would all reliably choose the same options.

For #4 and #5 what difference does it make whether biological beings make 'FAI' that helps them or 'UFAI' that kills them before going about its business?

Do you mean that an alien FAI may look very much like an UFAI to us? if so I agree.

1 is early filter meaning before our current state, #4 would be around or after our current state.

Do you mean that an alien FAI may look very much like an UFAI to us? if so I agree.

Not in the sense of harming us. For the Fermi paradox visible benevolent aliens are as inconsistent with our observations as murderous Berserkers.

I'm trying to get you to explain why you think a belief that "AI is a significant risk" would change our credence in any of #1-5, compared to not believing that.

ah I see.

ok, combinations. For each 1 to 5 I'm assuming mutually exclusive because I don't want to mess around with too many scenarios.

For AI risk I'm assuming a paper clipper as a reasonable example of a doomsday AI scenario.

1-high : We'd expect nothing visible.

1-low : We'd expect nothing visible.

2-high : This comes down to "how impossible?" impossible for squishy meatbags or impossible for an AI with a primary goal that implies spreading. We'd still expect to see something weird as entire solar systems are engineered.

2-low :We'd expect nothing visible.

3-high :We'd expect nothing visible.

3-low :We'd expect nothing visible.

4-high : Implies something much more immediately deadly than AI risk which we should be devoting our resources to avoiding.

4-low : We'd expect nothing visible.

5-high : We'd still expect to see the universe being converted into paperclips by someone who screwed up.

5-low : We'd expect nothing visible.

Ok so fair point made, there's a couple more options implied.

a:early filter,

b:low AI risk,

c:wizards already in charge who enforce low AI risk.

d:AI risk being far less important than some other really horrible soon to come risk.