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Comment by blaser-waffle | Hacker News Reader

An earthlike planet is require for earthlike life.

It is fallacious to assume other life couldn't be different -- we can't prove or disprove that. Likewise, we still don't know exactly how life started here; we simply have a few good theories.

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I don't think we have "good" theories about how life started here. In particular, the complexity gap from abiotic chemicals to the smallest known organism capable of independent reproduction (about 4 billion atoms!) is extremely large, and no theory has done more than handwave about how that gap is to be crossed.

this is what has always annoyed me about basically every space movie ever made. even life a star away, let alone another galaxy away, is likely to be startlingly different from the oxygen-absorbing, ossified calcium-braced, protein-enriched, atp-powered water blobs with heads, legs, and feet we have on earth. and that's just the the biochemical stuff we're familiar with.

What bothers me about every space movie made is that all those planets have life on them (or, worse, intelligent life). Fermi says hi.

Fermi can blast out "Hi" with a 100GW of power and he is unlikely to be heard in Andromeda, which for all we know could be teeming with life and host to something like an Iain Banks Culture series space opera.

We know that fewer than about 1 in 100,000 galaxies has a Kardashev Type III civilization. So the Fermi argument has some bite even at cosmological distances.

But anyway, this was about SF movies where our heroes visit Arglebarg IV and have adventures with the natives. It's like Victorian adventure stories transplanted into space without bothering to think things through. Let's kill our dysfunctional tropes.

> We know that fewer than about 1 in 100,000 galaxies has a Kardashev Type III civilization.

But we.. don't? We just don't have anything to compare to.

A K3 civilization is one that has diverted a significant fraction of the stellar output of a galaxy to its own uses. They have to dump their waste heat somehow, and that's visible in the far to mid infrared. So we can compare the IR emission of galaxies to look for oddballs with a lot of IR.

https://www.centauri-dreams.org/2015/04/16/g-hat-searching-f...

“Our results mean that, out of the 100,000 galaxies that WISE could see in sufficient detail, none of them is widely populated by an alien civilization using most of the starlight in its galaxy for its own purposes. That’s interesting because these galaxies are billions of years old, which should have been plenty of time for them to have been filled with alien civilizations, if they exist. Either they don’t exist, or they don’t yet use enough energy for us to recognize them.”

When we assume K3 capabilities, why don't we assume they dump their waste emissions away into something like (artificial even?) black holes? Maybe they are shy and afraid of the intergalactic "Dark Forest"? ( https://en.wikipedia.org/wiki/The_Dark_Forest )

The basic problem with that idea is black holes are too small. The amount of low temperature radiation that one could dump into them would be insufficient.

I think the point is that if a large number of galaxies had Type III civilizations, we would notice, because the definition is that they use most of the available energy in their galaxy making the galaxy radiate more infrared than visible light. But that doesn’t mean life isn’t common, it may just mean that a Type III civilization is either physically impossible or very unlikely.

i think in the end it's going to come down to what you really consider "life" to be.

there is an implicit part of our definition that includes timescales and size, and this puts some limits on the type of chemistry that can be involved in order to qualify.

if you open up the definition to be things that work on totally different timescales and sizes than we consider, then i think its fair to say that the chemistry doesn't need to be similar at all.

but for things that form blobs of life on the rough range of scales found on earth, and that "do stuff" in the range of timescales that we see on earth, there are not too many choices. silicon often gets touted as an alternative because of its ability to form branched structures. do you know of any other ideas?

sure, chemistry has a somewhat limited palette to work with, but life happens in the gap between potential and kinetic, between hot and cold, between proton and electron. that's a tiny space but a huge gap.

dna is mostly a 4-note tune, and computers, the potential precursor to artificial life, 2. i'm sure something as vast as the universe can come up with a little more variation than skin type in those tiny little gaps.

I don't believe that we'd recognize computer-ish artificial life as life in the sense that we're discussing here.

ps. I have my Artificial Life III conference t-shirt from 1994 :)

Due to convergent evolution [1], I'm not sure life that originated on an Earth-like planet would look completely different from Earth life, at least on a macroscopic level.

[1]: https://en.wikipedia.org/wiki/Convergent_evolution

that's predicated on the same conditions as life on earth. a universe can surely come up with some more variety.

My point is that I think convergent evolution would be likely to produce “water blobs with heads, legs, and feet” on other Earth-like planets, i.e. planets with lots of liquid water and lots of solid land.

If you’re thinking of planets that are not similar to Earth, of course all bets are off.

> atp-powered

I think GTP-powered file is possible, even the glucose metabolism produce one GTP (and many ATP). (Perhaps I'm missing something, Chemistry is not my main specialty). Perhaps it is even possible to have NADH-powered life, but it is more different so it may be impossible.

> ossified calcium-braced

Diatoms use silica, insects have a protein exoskeleton, and octopus are weird.

> oxygen-absorbing, [...] protein-enriched, [...] water blobs

I guess these three are safe bets (with a little different selection of amino acids).

Oxygen absorbing isn't necessary, of course. Life on earth got along without it for a while until those darned cyanobacteria starting poisoning everyone else.

Eukaryote life did not appear until after the Oxygen Catastrophe though.

That's true, but how much earlier might it have occurred if the oxygen catastrophe had not occurred and caused a mass extinction? Hard to say...

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