Comment by thsksbd | Hacker News Reader

Comment by thsksbd | Hacker News Reader

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A more interesting question is if an alien race can exist with ratios that differ considerably from the ones on earth.

Outside of carbon chemistry your ability to create replicable life plummets. Biologists may speculate if non carbon life is possible, but there's no doubt it would be limited. Arsenic or silicon just don't have the chemical complexity carbon does.

As for super heavy elements - the elements alien races would have access to wouldn't be that different from ours. Heavy nuclei are terribly radioactive and thus short lived. The article points our elements heavier than 260 are too short lived, but on astronomical (and biological) scales anything past 238 (ie Uranium) is short lived.

Past Uranium (which we have on Earth naturally), only Pu-244 is relatively long lived. Its half life is 81 million years vs. U-238 at 2 billion years. 81 million sounds like a long time, but the alien race has to evolve intelligence. We've been evolving for about 4 billion years, or 40 halvings of Pu-244 initial (anyway low) concentration. 2^-40 is a small number. By comparison U-238 has halved only twice on Earth since evolution started.

To illustrate, if the entire Sun were made of Pu-244 and the entirety of the remaining Pu-244 were put on Earth after 4 billion years, the concentration would be less than 1 part per million per mass.

There's been recent talk of finding a new island of stability[1] though I don't think we imagine those super-heavy elements would have half-lives of more than thousands of years (so probably not very handy for alien engineers).

1: https://en.wikipedia.org/wiki/Island_of_stability#Possible_n...

Yea, and it is super cool to think that, but either

1. the kinetic barrier is impossibly large that even novas cant cross it, or

2. the "stable" elements are not very long lived in astronomical time scales.

Probably both are true, either way, it's a wash - no super heavy elements for alien races to play with.

> Outside of carbon chemistry your ability to create replicable life plummets. Biologists may speculate if non carbon life is possible, but there's no doubt it would be limited. Arsenic or silicon just don't have the chemical complexity carbon does.

Best argument for carbon I've seen so far is that despite carbon being just 0.02% of all elements on earth it became preferred engine of life. Although I can't rule out that under different temperature, pressures and radiation some other element might be preferred if it's reasonably abundant somewhere. In my opinion if there's mostly stable, reasonable energy gradient somewhere life will find a way if possible.

Might the low abundance of carbon on Earth compared to its ubiquity in biology be suggestive of some kind of panspermia?

Although carbon is relatively scarce on Earth, it is the fourth most common element by mass in the universe (and in Earth's solar system) [1].

The four most common elements in the human body are the same as the four most common elements in the universe, albeit in different proportions [2].

[1] https://en.wikipedia.org/wiki/Abundance_of_the_elements#Univ...

[2] https://en.wikipedia.org/wiki/Abundance_of_the_elements#Huma...

is there a known reason carbon is so scarce on earth compared to the rest of the solar system?

This doesn't seem to have been firmly established, but as far as I can tell it's currently thought to be related to how carbon in the solar system's protoplanetary disk behaved when it was vaporized after fusion began in the sun. The question was studied in this paper published in 2021 [1], which is discussed in this article [2].

The authors of the paper suggest that the material that formed the Earth was depleted of carbon early in the solar system's history due to solar activity, and that most of the carbon now on Earth was delivered to the planet later on directly from the interstellar medium.

I should note a couple of clarifications to my first comment: the elemental abundance I mentioned for the universe and Earth's solar system does not include helium and neon, which are abundant, but are usually ignored in this context as they're noble gases.

There is also estimated to be slightly more mass in the present-day universe in the form of iron than nitrogen due to the high mass of iron atoms (nitrogen is the fourth most abundant element by mass in the human body, but the body contains relatively little iron). The number of nitrogen atoms in the universe, however, is substantially higher than the number of iron atoms. The amount of iron in the early universe should also have been lower; the element is formed late in the stellar life cycle [3], whereas the other cosmologically abundant elements that are relevant to biology (carbon, nitrogen and oxygen) are formed earlier [4].

[1] https://arxiv.org/abs/2104.02702

[2] https://phys.org/news/2021-04-stardust-pale-blue-dot-carbon....

[3] https://en.wikipedia.org/wiki/Iron#Cosmogenesis

[4] https://en.wikipedia.org/wiki/CNO_cycle

it could be that carbon-based life somehow pre-empts x-based life when carbon exists in the environment, even if there’s more of x

Sure, as a matter of logic in the absence of chemical knowledge.

Once you study the various elements and realize that carbons' chemistry is uniquely rich with a set of capabilities other elements lack non carbon replication becomes meh.

Silicon comes close, but the energies suck, elemental Si is too stable, and its oxide a solid.

> oxide a solid

At room temperature and pressure of rooms built by beings made of carbon.

But we know what happens at temperatures and pressures -> things break down and become less interesting.

SiO2 is quartz. At a T high enough to melt it (never mind a vaporize it to have a cycle analogous to the C-CO2 cycle), Si chemistry breaks down.

Carbon chemistry is unique because it occupies a unique chemical niche. It makes interesting and stable compounds at T high enough to have appreciable rates, but low enough that things don't just rip themselves apart.

Not to mention at T high enough to vaporize SiO2 you won't have water, which is a pretty nifty solvent for life to have.

Think of it this way. We know the bond strength of the various chemical bondings. We cant get around that and each type of bonding is responsible for several unique and fundamental moiety in biological replication.

> Not to mention at T high enough to vaporize SiO2 you won't have water, which is a pretty nifty solvent for life to have.

Yeah, not even iron could work as a solvent at that temperature.

the logic here is extremely iffy

Sorry, to avoid embarrassment I didn't include the conclusion:

Since, as stated, non-carbon chemistry is limited, non-carbon life (very broadly defined as replication of information) is either impossible or incredibly boring.

but this is what I’m saying. perhaps it’s possible to have life from other elements, but carbon-based life simply dominates it to a degree that it doesn’t survive

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