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Comment by Aerroon | Hacker News Reader

This could be pretty bad news for finding complex life in space.

The stability of Earth's carbon cycle is thought to be (partly) a result of plate tectonics. Carbon gets trapped in rocks and volcanoes emit carbon again.

Venus doesn't seem to have plate tectonics. One consequence of this is thought to be that volcanoes are much more common on Venus. It's also thought that 300-700 million years ago Venus went through a resurfacing event where basically the surface of the planet was replaced (the floor is lava).

If the above is true and plate tectonics is a result of a planetary collision then life like ours should be even more rare than we thought.

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Extra conditions like this mean that the path we took would be rarer but doesn't necessarily make life any more rare. There could be plenty of other pathways to a stable carbon cycle. Or it might not even be a prerequisite, you could imagine a scenario where life gets a foothold then instigates the cycle itself.

We just won't know until we've found something. And for us talking now we probably just won't know.

Especially because they may all just be indicators of something else. I've heard magnetic fields, moons, and plate techtonics are all important, but that really could just be "the right kind of planetary collision not too long ago"

Sure, but then add the evolutionary path on top of that. We had hundreds of millions of years of dinosaurs and no evidence of intelligent life.

Then you've got to consider that Earth is basically approaching its twilight years. The planet is 4.5 billion years old, macroscopic multicellular life is 600 million to a billion years old. Humans have been around for 100,000 years. In 500 million to a billion years Earth will become uninhabitable again.

Oh and our sun is unusually stable.

Throw all of this together with an added "right type of huge planetary collision" and I feel like you're writing a lot more zeros into that probability.

Of course it's possible that life could take a different path, but there was a lot of opportunity on Earth and it took a long time to get there.

All that comes after the planet is suitable for emergence, which is the focus of the conversation. We can reinvent the drake equation all day if we want, but yeah, definitely, those are more factors to consider.

> There could be plenty of other pathways to a stable carbon cycle.

To be fair, carbon is the only base that we have (first-hand) experience with. There _could_ be other bases.

Anything else would be so exotic that it’s not worth comparing to Earth.

PBS Space Time: What If Alien Life Were Silicon-Based?https://youtu.be/469chceiiUQ

There are very few atoms that allow for the complex scaffolding for shapes. Many atoms are too large (and thus bond too weakly). Silicon is interesting, but it has some difficulties.

https://en.wikipedia.org/wiki/Silicon–oxygen_bond

> Silicon–oxygen single bonds are longer (1.6 vs 1.4 Å) but stronger (452 vs. about 360 kJ mol−1) than carbon–oxygen single bonds.

It is much easier to lock up oxygen and silicon in SiO than in CO compounds and in turn makes them less available for more complex structures of life.

My organic chemistry professor always liked to point out that while CO2 is a gas that is easily dealt with following metabolism, SiO2 - silica/quartz - is most decidedly not a gas. Add that to the list of challenges for silicon-based lifeforms. Not to say that it isn't possible, but it does constrain the solution space somewhat.

Wouldn't large body collisions be a fairly common occurrence in young planetary systems though?

Of course any additional condition makes life rarer, I'm just thinking this one might not make it rarer by as many orders of magnitude as it might look like at first glance.

And yet - where's all the plate techtonics in our solar system? Only 1 of four inner rocky planets seems to experience it.

Out of what, 500-1000 billions of planets just in milky way? I dont think folks do realize how big those numbers are. And there is no reason to ignore rest of the universe if we talk about probabilities and statistics

Tbh even if the number of life sustaining planets in the galaxy goes from 400B to 350B, that’s still rarer than we thought.

It's just another fraction to multiply in the drake equation. Start with planets, cut down to rocky ones, only in habitable zone, only rotating a certain way, only with plate techtonics, etc etc.

Or using your numbers, 1/4 factor eliminates 750 million possible candidates. That's not a happy thought.

Yeah but 25% of a few hundred billion (planets in the milky way) is still a fair lot of opportunities.

“Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is.”

Well I suppose it's fair to say that only 1/8 planets has plate techtonics, and AFAIK, none of the dozens of moons either.

I thought Titan did (though in that case, I think it's more ice quakes)

But earthquakes and plate tectonics are not the same thing

I think the point is that we know the numerator today is one, and no matter how much you cut down the denominator, increasing the numerator to two would be a huge deal. Conversely, given that we think the denominator approaches a small infinity, it seems implausible that the numerator is actually one.

Well that's not my point.

Right now the denominator is about 10 trillion.

A single 1/10 factor makes it 1 trillion. 10-ish more factors and we're down to a small number of planets before we even consider the emergence of life and likelihood of propagation.

There are likely many more 1/10 factors: Habitable zone, diurnal cycles, billions of years of geological peace, sufficient water, a stable moon (maybe only one), a particular spectra of the star ...

all those were "well they seem to happen to 1/10 or 1/100 planets" individually. They cut down the space quickly when combined.

But, the 2 may be on the other side of the universe, or even past the cosmological horizon. And since FTL speed travel is almost certainly impossible, it may very well always be 1 to the best of our knowledge.

That’s what makes this discussion so fun ;)

Even 1 in a million (planets) would allow for 100 thousand solar systems in the 100 billion stars strong galaxy to have at least one tectonic active planet; not counting moons.

As stated in another thread, it's yet another 80% reduction in the number of habitable worlds. On top of all the factors, the exponential decay is pretty steep.

That's just the nature of the drake equation. It's very much a geometric series and if all factors have to line up with 1/10 odds, you only need 10-ish "vital things" to cancel basically all chance of life except earth.

And here we have a hypothesis which might be about 1/10 odds and might be vital. 9 left. Water? Diurnal duration? Spectrum of star? A billion years of peace? A moon? A magnetic field? There's potentially lots of factors.

Seems to me like “basically” is doing a lot of work, that leaves behind what.. a few billion planets?

"basically" is doing only what I suggested: a geometric series.

Here's exactly what work it's doing:

Pretend there's 10s of trillions of planets. That's 10-13 zeros depending on whose estimate you trust most.

1/10 factor cancels one zero.

at most 13 factors accumulated means you have 1 habitable planet out of all those planets.

We have just hypothesized a 1/10 factor in this thread - that leaves 12 more - and I've lised 6 more off the top of my head.

It's just a fermi question - ballpark estimates like that are a way of thinking of the relative scale. A 1/10 chance seems like it leaves a lot of planets left (as you say - 100s of billions), but there are already many 1/10 factors floating around.

The Milky Way alone has about 2.5 × 10^11 stars. The Andromeda Galaxy has around 10^12. Let's take 0.5 × 10^12 stars on average per galaxy.

There are about 2.5 × 10^11 galaxies in the observable universe.

This gives us around 10^23 stars in the universe to fiddle with. Assume every star has an average of 2 planets; some have more, some have none.

This is a pretty large number to trim down.

I'd argue the Drake equation is excessively conservative. Note that when microbial life first emerged on Earth 4.1 billion years ago, the Earth's atmosphere was rather reducing, and the Sun was around 30% less luminous than it is today. There was free water, but no free oxygen, and an extremely high-pressure CO2 atmosphere.

The universe is arguably extremely young; the longest-lasting stars will only burn out around 10^13 years from now, and the universe is barely 10^10 years old. It's fair to say that many sun-like stars haven't even formed yet.

You can zoom out arbitrarily far to increase the odds, sure. But for discussion purposes i limited to our galaxy's 10^10-ish hypothesized planets.

Right but the unstated assumption that there is no other positive path isn’t any more well-supported than the converse. For example, observe the variety of life we have locally. I’m thinking particularly of the various life (or life-like but I digress) that exists in extremes like thermal vents or under-explored places like deep soil. So maybe it does eliminate 1/10 but maybe we forgot to add the other 1/10 for life that wants to live at 100C (or whatever) — I’m pushing back on your assuredness, not the math.

Only one does at the moment, but others might have in the past.

Mars: https://en.wikipedia.org/wiki/Tectonics_of_Mars

Venus: https://www.nature.com/articles/s41550-023-02102-w

That big thing that whacked us and created the Moon may be responsible for our being here. Amazing.

The simplest and most likely explanation for the Fermi paradox that does not rely on fanciful future great filters or even more fanciful galactic zoo hypotheses is that complex highly intelligent life capable of space flight is extraordinarily rare in space and time.

When we go out there we might end up finding some bacteria-like organisms and simple fossils but nothing close to ourselves.

But ultimately we don't know until we go see.

The sample size is very low, but it's quite the coincidence that the only rocky planet in our solar system with life is the only one with an oversized moon.

> complex highly intelligent life capable of space flight is extraordinarily rare in space and time

That feels right to me especially given the timescales we are dealing with when we consider things even just in a galactic context. Heck, even the timescales on this planet alone are such that there could be multiple complex intelligent life forms on earth separated by sufficient time that they are never known to one another! Not to mention the fact that there are intelligent life forms on earth right now that we barely understand. For all these reasons It seems to me that we are far more likely to come across an alien artifact than an alien, and we haven’t started looking for these.

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

Would we know if they went to space? How long would a geostationary satellite remain?

It's both haunting and brilliant to think about the idea of entire other intelligent species and civilisations having lived on this planet without us ever knowing. Fire and iron and cities and empires and "great people", cultures and religions and stories and families... Civilisations and species rising and falling throughout the eons. It feels a little reminiscent of Sagan's "Pale Blue Dot", but on the scale of time, not space. I wonder if they might have considered this too.

From what I understand an industrial civilization would've left evidence that would've been found by now. Building ruins, mines, unnatural concentrations of certain elements that let us determine atmospheric composition etc.

Or maybe the speed of light is a barrier that cannot be passed and therefore nobody is really that interested in going much further than their local solar system.

It's also possible that a reasonably homogenous atmospheric pressure does not promote sustained plate tectonics over geologic time scales. Since Venus has no oceans this could be why it has no plate tectonics currently.

What was the surface of Venus like before that?

I don't know. I'm unsure whether scientists do either. Chances are that it wasn't really any different than it is now. The resurfacing means that the surface is new - many fewer impact craters.

Venus' rotation is quite weird, could it be the result of an impact?

That's the fascinating thing about lava/magma's power to simply erase what was there before. Plate tectonics does a similar thing when continents scrape each other clean. For all we know, Earth had an advanced civilization on it way before us and we'd have zero ways of knowing about it.

Some bits of landmass have been around for a few billion years. From this we have atmospheric records that rule out a large industrial civilization.

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