Computer models suggest modern plate tectonics due to blobs left by collision
phys.org
phys.org
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.
We just won't know until we've found something. And for us talking now we probably just won't know.
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.
To be fair, carbon is the only base that we have (first-hand) experience with. There _could_ be other bases.
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.
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.
Or using your numbers, 1/4 factor eliminates 750 million possible candidates. That's not a happy thought.
“Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is.”
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.
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.
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.
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.
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.
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.
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.
What makes you say that? I wouldn't actually think that's as difficult as many other problems in space exploration. An uncrewed probe is free from a lot of really challenging constraints.
If they were possible, we should be having to constantly look up to avoid having one land on us.
Other things wouldn’t.
Or there are no actual aliens, except us. For whatever reason.