[shireboy]

This could mean in the Drake equation ne -number of planets capable of life- is very small. A planet has to be hit with a comet big enough to deliver a large amount of water but not so big or fast to destroy it. And be in the Goldilocks zone of the star. Also the mass of the planet would play a part - gravity of more massive ones would be more likely to capture a comet. But again, too massive and I could see that hampering life.

[JamesLeonis]

The Drake Equation is filled with assumptions, like life must appear on a planet in the Goldilocks zone of a star. The whole equation has only one datapoint to extrapolate from. Tweak the equation's parameters and it will predict universes that only have one civilization per galaxy or worse! We have no way of knowing what those parameters are because we haven't seen other examples.

A major reason we are interested in Europa is because it might have underground oceans. Hypothetically, through tidal forces with Jupiter, the moon's core is hot enough to create oceans under the ice crust. Combined with hydrothermal vents you have the possibility for deep sea life similar to our own deep oceans. The Drake Equation does not predict this possibility.

[mr_mitm]

The Goldilocks zone doesn't enter the Drake equation at all.

As a reminder, this is the equation: https://en.wikipedia.org/wiki/Drake_equation#Equation

It makes very few assumptions.

[buran77]

The equation itself makes no assumptions. But anyone trying to calculate something with it must.

The last five factors in the equation will be filled in by assumptions based entirely on one data point, life on Earth. From your link:

  ne = the average number of planets that can potentially support life per star that has planets.
  fl = the fraction of planets that could support life that actually develop life at some point.
  fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
  fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
  L = the length of time for which such civilizations release detectable signals into space.

Can you define any one of those without assumptions, in a scientifically proven way?

[DennisP]

One approach is to give each variable a probability distribution. The greater our uncertainty about possible values, the wider the bell curve.

Drexler and colleagues did that, and found "a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe (53%–99.6% and 39%–85% respectively). ’Where are they?’ — probably extremely far away, and quite possibly beyond the cosmological horizon and forever unreachable."

https://arxiv.org/abs/1806.02404

[mianos]

A probability distribution describes how likely different outcomes are. It requires multiple observations or an assumed model that can represent variability.

[graycat]

Likely are also making a probabilistic independence assumption.

[lazide]

Probability distributions based on a single data point (of where we know there is life) aren’t reliable distributions.

[DennisP]

Which is why they set very wide ranges on the things we know little about. Doing that is less unjustified than guessing specific values, as people have usually done.

It's nowhere near a precise estimate of the probability of life. What it mainly shows is that the Fermi "paradox" is no such thing. It can look that way if we guess specific parameter values, but if we fully account for our uncertainty on the various parameters, then the result is a decent chance that we are alone, given the knowledge we have so far.

[Tagbert]

No, it’s just a starting point. We do need more data.

[scarmig]

It does assume that life must be associatable with a planet. It's a plausible assumption, but you could also hypothetically have life develop on a star itself or its remnants, comets, clouds of interstellar gas. Maybe even something more exotic than that (dark matter? some weird correlated statistical properties of the quantum foam?)

[dotancohen]

About forty years ago I read a terrific book about life forms that live on a star. Maybe Starquake was it called? Did to the abundance of energy on the surface of a star, they live their lives a million times faster than humans. Thus for both them and the humans who discover them, communication is difficult. I think the humans push these life forms to develop civilization, which from the human's perspective had them go from primitive animals into sophisticated beings of technology past their own in something like a day.

[LeifCarrotson]

That's "Dragon's Egg" by Robert L. Forward, a classic Sci-fi story:

https://annas-archive.org/md5/4c381ac344506d10037fc8e7747098...

The cheela lived on the surface of a neutron star, and they lived faster because the nuclear physics that powered their metabolism are far faster than the chemical and mechanical physics that power our own.

[ElFitz]

Andy Weir’s Project Hail Mary had an interesting take on that.

[gcanyon]

I was bothered by the nearly a-scientific-ness of PHM. The story was nicely done in general, but it feels like he pretends to be hard science fiction when he's really Star Trek-level.

[lordnacho]

It actually adds excessive structure.

The underlying model is just:

N*f

How many planets are there, and what proportion of them have detectable life?

The f does not have to be structured as fl->fi->fc, although we can see why you'd assume that kind of structure. It's simple to calculate the PI(series) when the model is just a funnel. Like the Million Dollar Money Drop gameshow.

But you could imagine a more complex model of probabilities that branches and merges. There could be events on the bayesian tree that amplify downstream events. For instance, suppose there is some pathway that if reached will leave certain minerals that future civilizations could use. This has happened already on earth at least once: lignin bearing plants could not be easily digested for a long time, and that led to coal formation during the carboniferous period.

You could imagine many such potential trees, but we only have one iteration.

[howieburger]

Those variables come with embedded assumptions they are essential and meaningful to discovery of life and civilization elsewhere in the universe.

For all we know civilization exists inside our car battery. Why assume it only exists on planets.

It's not explicit in it's assumption but implicit assumption the equation is meaningful.

[PepperdineG]

>For all we know civilization exists inside our car battery.

Rick Sanchez uses a microverse battery

[mr_mitm]

Thanks, I read that part before I shared it. It's pretty clear to me, these are pretty well defined quantities, just hard to measure. What is unclear is perhaps the definition of life. But at no point does it assume a planet must be in the Goldilocks zone. So perhaps you want to point out those assumptions you are talking about to me, because I don't see them.

Edit: the parent post has been edited substantially after I replied.

[buran77]

> these are pretty well defined quantities, just hard to measure.

They are "defined" conceptually, in words, not in physical quantities. It assumes we can assign a known value to any of that when we don't and likely never will. It's like saying "Let X answer the unanswerable question. X is the answer".

> at no point does it assume a planet must be in the Goldilocks zone

You could say it implies it with fl.

> Edit: the parent post has been edited substantially after I replied.

Only for legibility.

[the_af]

How can you extrapolate those terms from a single planet with known life without making assumptions?

[mr_mitm]

I can't, but the equation itself doesn't to that. The assumptions are up to the reader to make. That's why I think that the equation isn't particularly useful.

[crazygringo]

I'm assuming they were referring to this term:

> n_e = the average number of planets that can potentially support life per star that has planets.

The fact that the planet is neither too hot nor too cold would seem to be a major component of this term:

https://en.wikipedia.org/wiki/Habitable_zone

[mr_mitm]

That's just your interpretation. Take the equation at its face value and it does allow for life originating around some deep sea vents, like JamesLeonis speculated.

[DennisP]

It does seem unlikely that such life forms would ever become spacefaring.

[rowanG077]

That's a separate term in the equation.

[acestus5]

yeah you are right the Drake equation does not assume Goldilocks zone.

[stouset]

It goes the other way around. The Goldilocks zone is a shorthand attempt at helping us guess how many planets out there are capable of supporting life.

[hotstickyballs]

The biggest assumption is that it assumes only a single path to intelligent life.

[corimaith]

Even if you only had a handful of civilizations, the sheer time that has passed and size of the universe should mean that life should still be alot more apparent.

With sublight velocities achievable today, I recall it would only take around a million years for a Von Newmann probe to cover the entire galaxy. Such a probe is quite conceivable, so why isn't there more evidence of such probes everywhere?

Another point I feel is that proliferation of life should be an self-reinforcing affair, for intelligent life even more so. A spacefaring nation may terraform or just seed planets, and these in time will replicate similar behaviors. At a certain point, a galaxy teeming with life should be very hard to reverse given all the activity. A life itself isn't necessarily evolved from biology, AI machine lifeforms should also well suited to proliferate, yet we don't see them anyways.

[littlestymaar]

> With sublight velocities achievable today, I recall it would only take around a million years for a Von Newmann probe to cover the entire galaxy. Such a probe is quite conceivable, so why isn't there more evidence of such probes everywhere?

What are the incentives to build and deploy such a thing though? We as a civilization fail to fund things that have a ROI of more than a few years, how are you going to fund something that pays off after a million year?

[antonvs]

Exactly. Some of the biggest explanatory factors for the Fermi paradox are likely to be economics and politics: interstellar travel is unreasonably expensive, unimaginably slow, and has negative ROI unless your time horizons are beyond anything that's ever been used on Earth.

Consider that in some countries on Earth, we can't even get consensus that obtaining energy directly from the Sun via solar panels is a good idea.

[littlestymaar]

Also, people vastly underestimate how hostile space is: colonizing Mount Everest, the Antarctic or the continental plateau under sea would be far easier than colonizing Mars. And Mars is the most hospitable extraterrestrial place we know of.

[corimaith]

I don't think we would colonizing Mars, free floating colonies akin to O'Neil Cylinders orbiting Earth would probably be the more logical option. And with increasing robotic automation capabilities, it's not improbable to see these being built in the future.

[mr_toad]

At some point replicative drift will set in. How many replications is two million years? How long before the probes evolve? How long before they speciate? How long before a species turns on itself?

[fooker]

> Such a probe is quite conceivable, so why isn't there more evidence of such probes everywhere?

Time, not space, is your answer here.

Two reasons -

(1) civilizations might not survive long enough to do this.

(2) 13 billion years is a long time. So you have the reciprocal of that as the chances to be in the right year to see such a probe. And with results from the new telescope we now have hints that the 13 billion number is bogus, the universe is likely far older.

[raverbashing]

Yup

The fundamental problem with the Drake equation is that it's frequentist, not Bayesian

Hence why you get too high sensitivity to parameters you have no way of having an estimate with a small margin of error

We "don't care" about how many civilisations are out there, we care to the point where we can interact with them.

As mentioned, it has several assumptions. "Rate of birth of sun like stars" means nothing. You can "always" have an exception for life that will throw the data off: "star too bright but with a hot Jupiter tidally locked in front of your moon, shielding it" etc

[antonvs]

> star too bright but with a hot Jupiter tidally locked in front of your moon, shielding it

It seems unlikely that such exceptions would amount to more than part of a reasonable margin of error.

[raverbashing]

It is very likely that Earth itself is the exception of the exception and is part of a "margin of error"

[adastra22]

FYI just about every outer solar system moon or planetoid has a liquid ocean somewhere underneath. Europa is neither exceptional or even that interesting anymore.

[bethekidyouwant]

Not really there’s always gonna be water comets in the frost zone.

[andrewflnr]

I don't see any reason to believe that giant impact is the only way to get life-supporting amounts of water. We know Mars had liquid water. We know Titan has lots of ice. We're pretty sure Venus at least had noticeable amounts of water. Did all of these come from Theia-type impacts? I don't think we have any evidence of that.

[II2II]

Multiple impacts is the standard hypothesis for a source of Earth's water. If I recall correctly, outgassing from volcanos is another source.

Keep in mind, the solar system formed from a relatively homogenous nebula. It was the formation of the sun that forced lighter elements to migrate outwards, and that only happens if the lighter elements aren't already part of a larger object. There isn't much of a difference between a 10 km chunk of ice and a 10 km chunk of iron gravitationally speaking. Bouancy doesn't play a role here, so density doesn't matter. Outgassing does matter, but that is a slow process for large object, like the Earth, or for smaller objects on Earth crossing orbits that don't get too close to the sun.

It's also worth considering that each planet's situation is unique. There is much more water ice on the moons of the outer solar system because there was more water at the time of formation and the lower temperatures mean the water that was there stayed there. As for Mars, even though it is colder than the Earth, it is much less massive. As such, its atmosphere bleeds away lighter molecules (never mind lighter elements).

[andrewflnr]

> Multiple impacts is the standard hypothesis for a source of Earth's water.

Right, which is why it's baffling to me that everyone in this thread seems to be losing their mind over this result, thinking it affects the Drake equation and rewrites solar system dynamics. The multiple impacts thing might not have actually happened to earth, but there's still no reason to believe it wouldn't work.

[sesm]

Yep, mega-impact is a classic example of an ad-hoc hypothesis. For example, Moon formation is much better explained by multi-impact hypothesis, which also requires less assumptions.

[andrewflnr]

Dang, I can't post anything in this thread without someone thinking I agree with them that science is BS. Giant impacts aren't actually surprising in an early solar system that hasn't hit steady state yet. If multiple impacts better explain all the evidence for Theia, including the weird patterns of isotopes and possible fragments deep in the mantle, that's news to me.

[nebula8804]

I recently became addicted to the SpaceSimsx and SimulaVerse. My takeaway is that so many just slight deviations can extinguish life on earth. I used to think of possibility of life in other worlds just in the lens of statistics. There are so many combinations and possibilities that it seems inevitable. But seeing just how perfectly aligned our solar system is makes me really reduce the probable number of chances of other habitable scenarios in my mind.

These channels helped me realize just how important all the planets in the solar system are to our continued existence. Its as if we have an entire family thats just perfect to make our existence possible. An entire family each one quietly doing their part without fanfare or credit.

What if we had 2 moons with half the mass?: Destruction [0]: https://www.youtube.com/shorts/v3xxaTkKGTQ

What if we moved Earth 5% further from the Sun?: Destruction [1]: https://www.youtube.com/shorts/g-na5x0Kldk

What if we dimmed the Sun by 1%: Destruction [2]: https://www.youtube.com/shorts/Cc3DRRJxhB8

What If We Delete the Biggest Planet from Our Solar System?: Destruction [3]: https://www.youtube.com/watch?v=kHJpIWoksKw

What If We Delete All Gas Giants Except Jupiter?: You guessed it...Destruction [4]: https://www.youtube.com/watch?v=Eg9fPNg00EE

[WalterBright]

> perfectly aligned

No matter what the circumstances, live will evolve to perfectly match the conditions it is under. There are many species so perfectly adapted to their ecological niche, they are in great danger of extinction. Like peacocks, who are stuck in a local optimum with no way out.

[javier2]

Well, just keep in mind that its not a "perfect alignment" as this seems to prescribe some kind of intent. It's more a set of requirements that allowed life to not be extinguished so easily. We could never exist in a place where this didn't happen like this, so obviously we have to be here and not some place ealse. So its more that out of the 1 places we have seen that was made like this, we at least know life happened there.

[snorbleck]

Could a tardigrade withstand these scenarios? If the answer to any of those is yes, could we then say that even with the parameters skewed a bit, there is a chance life exists elsewhere, and under completely different and extreme or (not "normal") conditions? Is the tardigrade here as a "clue" to tell us that life could form potentially anywhere?

[HarHarVeryFunny]

The thing is that even for a super low probability event, the size of the universe is so huge and such events must be happening all the time.

e.g. Say chance of a random planet ever being hit by a water-carrying comet is one in a billion, then with 100B - 1T planets in the milky way it'd happen here 100-1000 times. If chances are only one in a trillion, and we're the one in the milky way, then there are still another 100B - 1T galaxies out there and therefore a similar number of such events.

[tshaddox]

> The thing is that even for a super low probability event, the size of the universe is so huge and such events must be happening all the time.

But numbers can go arbitrarily low.

[echelon]

And you have to have multiple low probability events. These probabilities multiply.

We had a good start. A Jupiter to clear the debris, a Theia impact to create tides and contribute to tectonics, a magnetic core, a shielded atmosphere. We had water delivered to us. Maybe even panspermia.

Maybe cell walls and mitochondria are hard. Maybe multicellular is hard. Maybe life on land is hard. Building lungs, rebuilding eyes, having actual energetic gasses on land...

Maybe life is easy, but intelligence is hard. Maybe civilization is hard.

Maybe technology development can only happen on dry land, because aqueous chemistry is hard in water. Sorry mollusks and cetaceans: you'll probably never be able to do chemistry or materials science.

Maybe you need water and carbon and other chemistries aren't robust enough.

Maybe you need lots of fossil fuel deposits to develop industry. And that requires growth without bacteria and decomposers for millions of years, implying a certain order to evolution.

Maybe you need a certain sized gravity well to escape.

Maybe surviving the great filter is hard and still ahead of us. Maybe every species can build tech where a kid in their garage can extinct the entire species by 3d printing grey goo.

There's just so much we don't know about how life could happen. Let alone intelligent life. We don't even know where we're headed.

[HarHarVeryFunny]

> Maybe life is easy, but intelligence is hard

Intelligence has evolved three times independently on earth - dinosaurs/birds (raptors, covids), mammals, and cephalopods (Octopus)

> Maybe you need water and carbon

Maybe so, but Oxygen and Carbon are only behind (albeit far behind) Hydrogen and Helium as the most abundant elements in the universe

[macintux]

My pessimistic side says that the conditions for intelligent life are so implausible that we’re unique, and when we drain the planet dry of easily-accessible fossil fuels we’ve deprived any successor civilization of its opportunity to escape the planet.

Basically I fear we’re the universe’s only shot of appreciating and populating the galaxy (or beyond) and we’re on the brink of throwing that away.

[Shekelphile]

> when we drain the planet dry of easily-accessible fossil fuels we’ve deprived any successor civilization of its opportunity to escape the planet.

There will be no successor civilization to humans. Earth won't be able to support multicellular life in a few hundred million years due to the sun becoming gradually more luminous over time, resulting in higher surface temperatures that will eventually culminate in a runaway greenhouse happening, as it already has on Venus. Due to human-driven climate change effects this event will certainly happen much sooner (<100m years) as well, which is simply not enough time for another intelligent species to evolve after a large-scale extinction event.

Even if life evolving on earth was an incredibly rare event the chance of such circumstances not happening elsewhere even in our own galaxy is infinitely small - there are trillions of planets and 100b+ stars. On top of that there are 100s of billions of galaxies within the observable universe as well.

[delta_p_delta_x]

> Due to human-driven climate change effects this event will certainly happen much sooner (<100m years) as well

No, it will not. Human driven climate change is drastic, but the Earth has seen far worse events than our anthropogenic carbon emissions. For instance, the Chicxulub impactor at the end of the Cretaceous changed atmospheric conditions overnight, and to a much greater degree than whatever we have cooked up. It was the equivalent of detonating the world's entire nuclear arsenal about a million times over.

Sure, it finished off the dinosaurs. But 66 million years later, we, the descendants of tiny rodent-like mammals, are still here, as are the dinosaur's own descendants, the birds.

Additionally, during the Carboniferous about 300 Mya, both carbon dioxide and oxygen levels were considerably higher than they are today, and life actually thrived. I would say that with the increasing luminosity, there will be at least a decent period on Earth where life returns to that sort of diversity. We are actually still only in an interglacial of an ice age—this has effectively sterilised large tracts of our planet by covering them with ice sheets, or locking permafrost into the soil and making them unavailable for large trees.

Let me be very clear: our emissions—if unchecked—will make life very difficult for us as the rising seas and temperatures scatter millions of people out of coastal cities in the tropics further north and south and cause war, division, strife, and discord like we have never yet seen. But actually bring forward the planet's overall demise? Nearly impossible.

Let's not have the hubris to think we puny humans could remotely affect the planet's geological timeline. If we somehow all disappear simultaneously, most direct evidence that we ever lived will disappear with us–perhaps within a hundred thousand to a million years of erosion and weathering. Our emissions will similarly lurch to a halt and will reach equilibrium within a similar time span. That's all it takes to remove our direct creations from the geological record.

[HarHarVeryFunny]

> There will be no successor civilization to humans. Earth won't be able to support multicellular life in a few hundred million years due to the sun becoming gradually more luminous over time

Modern humans have only been around for < 1 millions years, and all the technology we have invented is incredibly recent. 200 years ago we had neither electric light or bicycles.

Over the course of 100s of millions of years, as the sun's increasing luminosity becomes an issue, I'd have to assume we could create some sort of atmospheric solar shield to reflect or absorb a lot of the energy. Of course you can only postpone the inevitable (red giant).

Assuming the evolutionary lineage of our species survives a few hundred more million years (which seems rather doubtful), then it's not going to be homo sapiens any more - we'll have evolved into successor species that may be barely recognizable. If you go BACK in time 100M years, our ancestor was some mouse-like animal.

[HarHarVeryFunny]

As long as we have air and water (i.e. as long as we're alive), then we can make propellants such as Methane or Liquid Hydrogen and LOX, Hydrazine & Dinitrogen Tetroxide (or Hydrogen Peroxide).

[macintux]

None of which are, I assume, as easy/efficient/effective to integrate into a new civilization's tech tree as coal & oil.

[Qem]

> ...when we drain the planet dry of easily-accessible fossil fuels we’ve deprived any successor civilization of its opportunity to escape the planet.

On the flip side, that could also be plausibly a blessing, avoiding them to fall into the same trap of becoming too powerful before they get wise. These comics illustrate it: https://www.badspacecomics.com/post/grounded

[leoedin]

Even on Earth, the only reason humans exist is because the “local maximum” of the dinosaurs was wiped out by a meteor. Perhaps comparably intelligent dinosaurs would have eventually evolved - but it’s not a given!

[3eb7988a1663]

Dinosaurs existed for some 200 million years with no detectable signs of technology development[0]. Presumably, the steady state did not produce a scenario in which the intelligence niche would develop without some other less catastrophic global change event.

[0] Unless that episode of Voyager was right on the mark https://memory-alpha.fandom.com/wiki/Distant_Origin_(episode...

[HarHarVeryFunny]

Intelligence evolved at least three times on earth - dinosaurs (leading to corvids, but a raptors already intelligent), mammals and cephalopods (e.g. octopus).

I suspect that any evolutionary environment will eventually create enough variety and instability that some generalists emerge, creating a reward for intelligence. The rise in intelligence from early water-bound life to later forms was likely all driven by more complex and diverse environments.

[Qem]

Maybe they didn't produce an intelligent species just because they had not the luck of living in the unprecended time in the history of Earth with both high atmospheric O2 and very low atmospheric CO2 we enjoyed for a while, before we started to burn fossil fuels by the gigaton. See https://www.qeios.com/read/IKNUZU

[TuringTest]

It took several environment-changing events to get our unique kind of intelligence; mammals had to thrive in place of saurs; and then, Africa needed to be split by the Rift and to create the dry savannah.

This forced some apes to climb down the trees and depend on a diet of scavenging for meat, which happened to both increase brain size AND require improved intellect to survive, forcing the evolution of our hypertrophied symbolic brain.

Had this not happened however, other intelligent species could have filled the niche. There's no shortage of other intelligent species in our planet, not just other mammals but octopus and some birds. And then you get hive intelligence, which could equally be forced to evolve into a high problem-solving organism.

[GWBullshit]

You're not wrong, but you're in the wrong place to talk to people about low-probability events and how they multiply. Most Hacker News can't into elementary-school-level probability equations and will instead take the ostrich approach; there was some behavioral scientist dude from Cambridge Analytica who wrote about this and the TL;DR is that most "adults" have infantile minds that prefer various safety blanket mechanisms that society is more than ready to offer them just to do anything to have an excuse to not face the truth of what basic math reveals to more likely than not be true.

[mr_toad]

> But numbers can go arbitrarily low.

Which begs the question, why 1, and not zero? I can buy zero, or a very large number. But 1 exactly? Nature doesn’t do that.

[tshaddox]

There’s not a huge difference between zero and one, other than whether someone’s around to comment about it on HN. And even a second wouldn’t really tell us more about the probabilities.

[soVeryTired]

Siméon Denis Poisson would like a word with you.

But in seriousness, I agree.

[rdtsc]

Just as easily as we can multiply planets times systems times galaxies times cluster groups we can multiply multiple small probabilities of each chemical being present at the right time and right type, temperature ranges, gravity ranges, etc

[mattmaroon]

If the numbers you propose turn out to be accurate then the odds of there being other life are near zero because even 1/1000 planets are not habitable likely.

[HarHarVeryFunny]

Huh? Even in the 1-in-a-trillion case, there's still maybe 1 trillion galaxies each with one planet that was struck by a water bearing comet, so even if only 1/1000 of those are otherwise habitable, that still leaves a billion habitable planets in the universe with water.

I doubt water (H2O) is actually that rare. The most common elements by far, both in our own galaxy and the universe as a whole, are Hydrogen and Helium, but the next two most common are Oxygen and Carbon.

[pavlov]

Do other galaxies matter here? A civilization would need to be incredibly powerful to be detectable from another galaxy.

[ben_w]

At the moment, rapid and massive expansion seems likely with tech only just on the horizon.

Enough AI and robotics for an autonomous factory may be a mirage (such mirages have (metaphorically) happened before), but it seems like it's on the horizon.

Even with relatively mundane growth assumptions, that can go from "species inventing writing" to "Dyson sphere completed, is now sending out seeds to every accessible galaxy" on significantly less than the timescale of light crossing a spiral galaxy's disk.

[kingkawn]

Cmon the number of hypothetical extrapolations based on no data in these statements is beyond superstition to something like delusion

[javier2]

I think its the fact that if we really wanted to, we could probably make it happen already today. On a scale of a couple hundred million years, its possible we could reach most of our own galaxy, which is a small slice of time in the life span of the Milky Way. So the question remains, why hasn't this already happened, or has it?

[ben_w]

If I put citations into everything I write, I'd be a Wikipedia article, and people would still criticise the conclusions without reading any of them.

But contrawise, I do have data, they're broadly categorised as "history", "biology", and "all the stuff cited by Stuart Armstrong that time".

[pfdietz]

Galactic colonization, carried to saturation, would detectably modify the appearance of a galaxy. So called "type 3 civilizations" would convert a significant fraction of starlight to lower grade heat, which would be radiated. Searches have been conducted for this signature, with the result that no more than 1 in 100,000 galaxies has such a civilization, and with the result being consistent with none.

[runarberg]

I am not an astrophysicist but I have a hunch any speculations of galactic colonization fails to entertain just how big space actually is. I feel like there is ample reason to suspect the probability of galactic (or even interstellar) colonization is exactly 0, and no civilization in the history of the entire universe will ever colonize an entire galaxy (and probably not even more than a handful of solar systems outside their home world; if any).

[pfdietz]

Your argument shows a lack of understanding of exponential growth.

Any given colony has to create only slightly more than 1 additional colony in order to drive exponential growth. There doesn't have to be any coordinated action by a central authority for it to happen. For it not to happen (if it is physically feasible), in contrast, every species has to refrain from doing it at all points in their history, almost without exception. And those that do the colonization will seed additional colonies with a mindset that led to colonization; such mindsets will be selected for for further expansion.

[runarberg]

Permanent exponential growth is very rare in nature, and even rarer in biological systems. What we observe as exponential growth is usually only a partial observation of a logistical curve or is missing a system collapse at the end of the curve.

We have no reason to believe alien (or even human) civilization will continue to grow and expand forever. Heck even the human population curve has started to slow down and is now revealing it self to be a logistical curve.

But regardless of this, space is very very very big. And there are a lot of extremely hostile worlds out there. Any civilization will experience biological limitation to which worlds they can (and will want to) colonize. Likewise they will experience both economical and physical limitations to how far they will send their machines. Lets say an alien species is lucky and has a habitable world inside their solar system which they will colonize. I think this is likely. They also spot another world in a nearby solar system which takes them 200 years to travel to, eager colonists travel in a generational ship, and 600 years later the colony is thriving. Now they run out of nearby habitable worlds. There is a world of questionable quality 500 years away and they are unable to persuade enough people to fill a generational ship. Also they learned the stories of the passengers in the generational ship, their lives kind of sucked, we have it much better on this world. So it is better to just stay here. This might happen after 1 or 100 successful colonizations, but I think space is so freaking large, it will happen to all civilizations. At some point they will run out of worlds to colonize, and they will never expand far outside of some local area near their home world.

[pavlov]

This is interesting speculation, but it adds one more completely unknown variable to the Drake equation.

What’s the probability that a radio-capable civilization becomes a galactic type 3 one? Looking at the only example we have, it appears very unlikely. It seems much more probable that we’ll destroy ourselves within the next centuries.

[HarHarVeryFunny]

I guess it depends what question are we trying to ask. It may well be that there is no other intelligent life close enough to us, or coexisting with us in time, that we will ever be aware of it, but yet the universe may still be teeming with intelligent life.

In either case it's a statistical question of how common is life, and intelligent life, but of course there's the human interest in potential contact with another intelligent life form.

[OgsyedIE]

Unless collisions like the article suggests are a statistical inevitability, that is.

[dotancohen]

Hydrogen is the most common element in the universe. So long as you have elemental oxygen, it will react with things and hydrogen is the thing it will react with the most. So having water is almost a given for any Star system. Additionally, protoplanet and cometary collisions are in fact statistically inevitable. The real question is if water can be delivered at a point after enough gravity has amassed to ensure the water stays there.

[jug]

Right, that's the sticky point? The likelihood of a planet in the Goldilocks zone to be too hot in the early stage of stabilizing its chemistry that it requires seeding with "post-formation" chemistry? Is that likelihood close to 100%, or maybe not even near and we were just set up for a funny cosmic event.

[frankohn]

I agree. In addition to the chemical elements like water, as mentioned in the article, the impact with Theia also enabled strong magmatic activity at the core of the planet, and that was a critical element as well to sustain life.

Probably the strong magnetic activity of the Earth's core was key to maintaining the atmosphere, but also, the magmatic heat contributed to keeping the planet at a good temperature to support life when a young Sun provided significantly less radiation.

All these elements may suggest that the collision is needed to satisfy the very strict requirements about where the planet is located and about the size and composition of the colliding planet. This makes the probability for life-sustaining planets in the Drake equation extremely low.

As an indirect proof of the tightness of the condition is the fact that the Earth in its history had periods of climate extremes hostile to life, like the Snowball Earth when the planet was completely covered by ice and snow, or at the opposite extreme, the very hot periods when the greenhouse effect was dominating the climate.

[gtowey]

If Mars contained significant amounts of water in its past as we suspect, that would mean we have n=2 just in our own solar system.

[GWBullshit]

Speaking of Drake equations, you should (1) see the other comment here with this account name (2) check out the top Pirate Bay rip of Dark City (which predated that other movie) and turn on the English subtitles and count the number of times the characters look at or make gestures pointing to certain alignments of the text in the subtitles and, if you're true "hackers", try to figure out the encrypted messages in the text alignments that the characters are looking at/pointing to at key moments – and then when/if you figure out what the encrypted messages mean, try to figure out how the director worked together backwards so that they could have a script that aligns a certain way using subtitles and then make the scenes so that the actors are looking/pointing to key spots at just the right time.

If you appreciate technical things, you'd be in for a treat.

[Animats]

Right. It's discouraging. We now know that many stars have planets, and some of them are even in the Goldilocks zone. But if it takes a planetary collision to get water... And only one planetary collision, because each one wipes out essentially all life.

Look at the rest of the solar system. Mars - almost no water. Luna - almost no water. Venus, maybe water[1], but as steam. Too close to the sun and too hot.

[1] https://phys.org/news/2025-10-venus-clouds-reanalyzed.html

[Kerrick]

I find it incredibly encouraging. I fear aliens existing in sufficient enough quantity to find us more than I fear Earth being the only host to intelligent life until we escape it.

[Animats]

Probably not the only host, but habitable planets are sparse. If Einstein was right and faster than light travel is impossible, the number of planets we can even talk to is not that large. There are only 94 stellar systems within twenty light years.

[WalterBright]

Especially in the context of this, I am baffled by people who go to great lengths to prevent life from Earth arriving on other solar system bodies. Such as all the efforts to sterilize the probes.

[dleeftink]

Historically to prevent pre-contamination and erroneous biosignature readings. This may ease once we have some solid pointers or confidence there may be none (caution is likely warranted due to the gravity of such a discovery).

[WalterBright]

But we also lose opportunity.

Consider Mars. Endless probes for 50 years going to Mars looking for life. No clue of life has ever been found. At what point do we face the fact that Mars is a dead rock?

What we should be doing is collecting samples of extremophiles from the Earth, and attaching a few packets of them to every probe going to Mars, and see what happens. Probably nothing will happen, but it's worth a try.

[quotemstr]

Planetary collisions happen all the time. All of Mercury, Venus, Earth, and Mars in our solar system had them. We can see their signatures in other solar systems too: see https://en.wikipedia.org/wiki/List_of_extrasolar_planetary_c...

Whatever the great filter is, it's not planetary-scale collisions during the accretion phase of solar system formation.

[mr_toad]

A couple of dozen collisions out of 6000+ known exoplanets. Not exactly common, but not freakishly rare either.

[quotemstr]

Those are just the ones we caught in the act. That we can see 6,000+ means that there are many more we don't see.

[shaky-carrousel]

Not so much, because both Venus and Mars also have water. So whatever gave Earth water is common enough to also give it to the other inner planets.

[kulahan]

Don’t forget this can only happen once, really. You need it to be such a rare event that it doesn’t keep sanitizing the planet with repeated impacts, but one really perfect strike will bring what you need and allow life to form.

The number of instances where this (something unreasonably unlikely) happened in our cosmological history is kinda surprisingly high. I’m absolutely convinced there’s no advanced life (and CERTAINLY no technological civilizations) outside of earth.

One other example: we gained most of our adaptability, curiosity, and problem solving skills as very tiny mammals while dinos ruled the earth. The only way we ever took over the planet was thanks to an asteroid wiping out all those huge creatures. Suddenly, high adaptability and intelligence and resilience was what mattered, and being big and strong suddenly was a massive disadvantage.

Our intelligence exploded largely because that extinction event removed almost all major predators, turning earth into a giant survival puzzle sandbox for mammals to grow in.

Edit: our brains only grew big because it was the best means of survival - they’re crazy expensive, so without this “sandbox puzzle” effect, we probably never would’ve grown them.

[mr_toad]

> Suddenly, high adaptability and intelligence and resilience was what mattered, and being big and strong suddenly was a massive disadvantage.

Maybe it was just being small, puny, and having a tendency to cower in burrows was what saved us. Our ancestors may not have been much smarter than squirrels, and squirrels aren’t very bright.

Hominids brains didn’t get big until long, long after the KT extinction. A Tigers brain is not that much smaller than that of an an Australopithecus.

[kulahan]

Correct - that’s what SAVED us. What allowed us to thrive and dominate the planet was what I mentioned.

It may be more correct to say that growing a larger brain (larger than a lizard’s, I mean) was only realistically possible because of the sudden loss of predators.

[tastyfreeze]

Earth has been struck by large comets many times killing the majority of life on the planet each time. In an early solar system it would be more frequent. Once a comet impacts there is one less comet out there. The solar system cleans up over time making impacts less likely over time.

[kulahan]

There isn’t that much of a difference in the number of comets in space across just 5 billion years.

[mkl]

There's not much difference between now and 3.5-3.8 billion years ago, but before that there were a lot more: https://en.wikipedia.org/wiki/Late_Heavy_Bombardment

[nntwozz]

This also points to panspermia.

"The hypothesis that life, in the form of “seeds” or spores, is distributed throughout the universe, traveling between planets, moons, and other bodies via space dust, asteroids, comets, and possibly even spacecraft."

I want to think that the water contained life and not the barren earth.

[jcims]

Bit of a nit but the Drake equation is intended 'to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy' (from the Wikipedia entry for it). Nice thing is that you can drop a few terms to get to origin of life.

[adverbly]

No kidding... This would probably resolve the Fermi paradox if proven true...

[29athrowaway]

Mars is technically in the goldilocks zone of the solar system... but water on Mars boils at 0C or 32F due to low atmospheric pressure, which really sucks.

[xbmcuser]

Your assumption we need water for life to exist is in my opinion wrong. We only know Earth so assume that is what is needed for life to exist.

[joshuahedlund]

There is a hard limit on the number of atomic elements, and an even smaller limit on the number of soluble compounds that facilitate chemical reactions, and water is demonstrably both the best and the most common in the universe.

So while it may be possible for life to exist without water, any alternatives should be reasonably expected to be even more rare than water-based life

[caymanjim]

There's a reason life is carbon-based, and it's not random. It's the only element that works, due to abundance; ability to form many bonds; bonds that are just durable enough but not too durable. There's plenty of sci-fi about silicon-based life, but that's infeasible fantasy. And no other elements have any hope. If you have carbon-based life, you need water as solvent and medium.

It's a pretty safe assumption that all life requires water.

[Animats]

> There's plenty of sci-fi about silicon-based life, but that's infeasible fantasy.

Right. Silicon dioxide is quartz.

Longer analysis.[1]

[1] https://www.the-ies.org/analysis/does-silicon-based-life-exi...

[marcosdumay]

> due to abundance; ability to form many bonds; bonds that are just durable enough but not too durable

Well, the thing is that all of those are environment-dependent.

We do have data on a somewhat diverse set of environments, and it's enough to confirm what we knew about the flexibility of carbon. But it's not enough to disprove the alternatives.

[modius2025]

1. xbmcuser’s point: They challenge the anthropocentric (Earth-centric) assumption — “we only know life as we know it.” Philosophically valid, but scientifically weak without proposing a viable alternative chemistry.

2. joshuahedlund’s reply: Grounds the argument in chemistry and probability.

There are only ~90 stable elements → a finite combinatorial chemistry space.

Among possible solvents, water is the most abundant and chemically versatile (dipolar, wide liquid range, high heat capacity, good at dissolving ions and organics). → So even if other solvents can work (like ammonia, methane, formamide), the odds heavily favor water-based life.

3. caymanjim’s addition: Brings in carbon’s unique valence behavior:

4 valence electrons → can form stable, complex chains and rings.

Bonds are strong but not too strong → dynamic yet stable biochemistry.

Silicon (next best candidate) forms brittle, static lattices and poorly soluble oxides → bad for metabolism. → Therefore: if life is carbon-based, water is the only sensible solvent.

[yosefk]

What's the chemistry of life without water? Do you refer to the promising Russian studies of life sustained by alcohol?

[warent]

You’re getting a lot of negative feedback for whatever reason, but you’re absolutely right.

I for one remember reading about possible silicon/methane based life, etc. Actually, here’s a whole wikipedia article on what you’re talking about.

https://en.m.wikipedia.org/wiki/Hypothetical_types_of_bioche...

Perhaps HN folks will lose your scent now and direct their snark there

[Mistletoe]

But when you dig down deep on theories like that it just doesn’t make sense from a chemistry or physics standpoint. Everyone saw that Star Trek episode about silicon based life and ran with it as being possible. It’s just a show.

https://en.wikipedia.org/wiki/The_Devil_in_the_Dark

https://lweb.cfa.harvard.edu/~ejchaisson/cosmic_evolution/do...

[mensetmanusman]

The Drake equation is whatever math leads to Earth being the only planet with intelligent matter out of infinity planets.

[renshijian]

A brilliant summary! You've deepened the question from "Does a habitable planet exist?" to "Does a planet successfully complete the complex dynamics of life's origin?" The habitable zone is merely the ticket, while the conditions you mention are the truly demanding filters. Considering this, the probability of our existence truly seems miraculous

[ctrlp]

The likelihood of those criteria might be vastly different in a younger universe than in this one, no?