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Comment by 3chelon | Hacker News Reader

While I agree with most of the other comments that there's little evidence for the argument of the article, it is refreshing to see a different take on the whole "earth-like planets" topic.

I am regularly shocked by how little imagination commentators in the popular science press seem to have on this subject. Even highly-respected scientists frequently refer to "requirements" for extra-terrestrial life such as carbon-based chemistry, the requirement for water, a mechanism of natural selection, etc, etc. (Actually I consider the last one, posited by Dawkins, to be probably the closest to the truth.)

I guess it's simply the anthropic principle at work, but even the nature we see on our own planet far surpasses our imagination often (especially at the microbial level), so why should we be placing any constraints on the rest of the universe?

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You can have all the imagination you want, but it doesn't change the distribution of elements by mass in the universe. The most common are hydrogen, helium, oxygen, carbon, neon, iron, and so on. The likelihood that hydrogen/oxygen in the form of water are the necessary solvent for life is simply a statistical likelihood. They form a polar solvent, which can dissolve many different molecules and move them around. Water is a great medium for life to form in, do you have a different solve to propose? I've heard some try to say liquid methane, but it doesn't have some of the great properties that water does, like solid water being less dense than liquid water (really useful for bodies of water, which keep liquid water underneath a frozen surface).

Additionally, carbon itself forms stable amino acids, stable nucleic acids, and does so not only on Earth but even in celestial samples we have taken from outside Earth origin. So, would you rather a non-scientific writer leave the confines of sound science to speculate wildly? As a scientist myself, I'd rather they confine themselves to the most likely and sensible scenarios the majority of the time, since their purpose is to inform and inspire the public. Some small amount of speculation is fine, but education should be first and foremost.

I don't disagree that water is probably a unique solvent, and you make a good point about the distribution of the elements. But once we get into nucleic acids I think we're once again into the territory of applying constraints based on our own experience.

Good answer though.

My personal bet is that carbon-based life built on amino acids, proteins, fats and sugars is the low-hanging fruit in the universe, more-or-less for the reasons said above. Fats and sugars are pretty basic carbon compounds; amino acids arise spontaneously in the right inorganic conditions, and it's a hop and a skip from amino acids to the incredibly useful proteins.

From there it probably gets a lot more divergent. I'd also bet that cells and something like DNA is pretty common wherever carbon-based life is found, but my understanding is that the exact way DNA codes for proteins is pretty arbitrary, and the fact that all life on Earth more-or-less uses the same coding scheme is an artifact of the common origin of life rather than because it matters.

Body plans and the details of multi-cellular life are probably going to be wildly divergent, with some caveats. For instance, eyeballs evolved multiple times on Earth, so it would be surprising if eyeballs didn't evolve pretty often elsewhere.

(Incidentally, if aliens are made of proteins, sugars/starches and fats, that means that whatever they look like, we can probably eat 'em, barring the usual toxins and allergens.)

I want to add to saal's point-- Carbon is _fantastic_. It easily forms bonds in 4 directions, but can instead form bonds in 3- or 2- or occassionaly 1- direction if need be. If you are choosing an element to be the "lego backbone" of a bunch of molecules, carbon is a great choice because it gives that versatility. Nitrogen can easily form 3 bonds, and Oxygen 2, and Boron 3.

So, you ask, what about Silicon? Silicon also has 4 valence electrons, so it ought to be a nice substitute, right?

The problem is that Silicon is in the next row down.. so it's more massive and it's electron cloud takes up more room. So you still only have 4- connectors, but it's a larger element.

There's clearly an advantage into being able to form precise and small molecules. Carbon, with it's small size and 4 valence electrons, is a clear choice for a backbone.

> The problem is that Silicon is in the next row down.. so it's more massive and it's electron cloud takes up more room

I'm a complete layman, but how does that play out in planets with gravity force significantly stronger or weaker than Earth? Woul that skew which elements make the best building blocks?

No, gravity for microscopic purposes may as well not exist. Surface tension matters for very small creatures.

Silicon is basically impossible as a building block for life. It forms bonds that are too strong, meaning it requires much more energy to fuel life processes and reducing the rate of chance collisions which lowers the prospects for abiogenesis.

Carbon is the only realistic choice; it is the only element abundant enough with the right balance of stability and versatility.

I guess I was thinking more about pressure than gravity (though the two are related). At extremely high or extremely low pressure, I would imagine things to play out differently

I think that in most situations the pressure would still not change much as far as atom-to-atom distances go. And in those situations, carbon is still smaller and workable.

Carbon-based chemistry isn't the worst requirement to throw in there, since the only plausible analogue - silicon - for example doesn't form stable analogues of the usual amino acids at room-temperature.

Now it would be reasonable to say "well what about extreme environments" but we are specifically looking for planets with temperatures and "ambient energies" similar to ours - so it's reasonable to think that whatever chemistry is there probably has to follow Earth to a large degree.

If instead we were talking about high-pressure hot Jupiters or something, then it gets more interesting - but that's going to be something so different we're unlikely to recognize it at all (what consciousness does sentient life which evolved in a gas-environment have?)

Exactly. We think of things recognisable to us, in scale and environment, but as the article pointed out before it got too fanciful, it seems far more likely we'll find actual life by looking outside these constraints.

Well carbon based compounds are significantly more stable when they grow in size than any other atom.

That bond stability had to play a gigantic role over the last billion years of evolution.

Well, the problem with life that doesn't look like ours is that we'd find it very hard to recognise it as life in the first place, even if oozed to our doorstep and rang the doorbell. So it makes sense to look for habitable planets that could harbour life like ours first- because we're much more likely to recognise them if we can find them.

And knowing that life like ours has already developed on a planet like ours, it makes sense that there will be others like it out there, so we might as well look for them first.

I'm not saying that ET life can't be very different to ours, or develop in conditions radically different to ours- but it makes sense to start searching at the most likely place for the most likely thing, no?

> it makes sense to start searching at the most likely place for the most likely thing, no?

It may be the most likely to be easily recognized but not necessarily the most prevalent. You have two variables to balance: (a) likelihood that the planet you're looking at contains life and (b) likelihood that you'll know it when you see it.

If you can increase (a) significantly then it may not matter if (b) is very low or not

I taught an undergraduate class on this topic once and was pleased to find that, in the early 1990s, existing textbooks and papers and articles in things like Scientific American had a ton of imagination about non-liquid-water, non-carbon, etc.

One thing I don't get is why nobody ever points out that these are the kinds of planets we're finding because these are the kinds of planets we can find.

We'd have no way of identifying an earth-sized planet at an earth-sized distance from a sun-sized, star, as far as I know.

There could be a huge amount of exactly earth like planets out there that we have no way of finding. In fact -- all the sun-sized stars we've looked at that we can't find planets at all around, might be ones with solar systems just like our own.

My understanding of the current situation is that statistically it appears that nearly every star has planets, but smaller stars are more common and longer-lived than medium and large stars, and that planetary formation models suggest they should be more likely to have rocky planets in the habitable zone. So, the presumption is based more on statistics and models than observations, although of course observations still stand a chance to obliterate the models when we have better instruments to find exoplanets.

But I am not an astronomer, would love to see some data on this.

It’s our knowledge of chemistry at work. I think https://en.m.wikipedia.org/wiki/Hypothetical_types_of_bioche... is pretty disenchanting.

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