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Comment by loxias | Hacker News Reader

Over the course of the past 2 decades we've gone from "perhaps some day we'll find extra solar planets" to "most if not all stars have planets".

JWST's already found water.

At this point it's no longer a matter of if but when extrasolar life will be confirmed, and with which instrument. Might be JWST, might be WFIRST, might be HabEx.

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> At this point it's no longer a matter of if but when extrasolar life will be confirmed

There is absolutely no data to confirm this, simply because no matter what equations or variables you throw around, we have no way of knowing the probability of life. We have only earth, and we still don't fully understand how that all worked out. We do not know what (or if!) any filter is present that prevents complex life. From a philisophical perspective, we'd be much better off as a species that the harder we look, we should hope to find _no_ life: https://nickbostrom.com/extraterrestrial.pdf

I never claimed to have proof, just that it's true. ;-)

Personally, it "feels true" that life exists as a undeniable consequence of statistical thermodynamics (see: http://www.englandlab.com/uploads/7/8/0/3/7803054/2013jcpsre... referenced in https://www.quantamagazine.org/a-new-thermodynamics-theory-o...).

But I know I'm an engineer, not a scientist or mathematician. I'm perfectly comfortable with things being true even sans iron clad rigorous proof. If you limit your worldview to only that which has been rigorously proven you miss out on a lot of necessary and interesting truths, I think.

I always go back to people like Drake and Shostak. They seem pretty convinced that "they" are out there, but the problem is that there’s no known way to communicate with them due to distance and time. If there’s a way to solve this problem, please let me know. Sagan and others seemed to think that if we could communicate (a very big if), then we could exchange our knowledge, but how exactly could you do that? The problems appear insurmountable.

Humanity has undertaken massive projects that took hundreds of years before (~2,000 years for all of the great wall of China).

As long as they're less than a few thousands light years out, I don't think that communication would be impossible.

I’m with you. But is it reasonable to assume that we can understand each other and share knowledge between the stars? From what I understand, it’s extremely unlikely. After all, we can barely communicate with species other than our own.

I think we'd better hope that we're alone. If creatures like us are common then that's bad new for us, because "life that behaves like humans and exists for millions of years" and "a galaxy that hasn't been completely filled long before now" are mutually-exclusive states. If we have any chance at all of conquering the stars we'll be doing it alone.

This is a reformulation of the Great Filter argument. Are there any good counterarguments?

Sure, plenty. They just mostly require us to change over time. Maybe sapient species mature and don't feel the need to ever expand, for millions of years. Maybe interstellar travel is so hard nobody ever does it nor do they make machines that do. What worries me, though, is it doesn't seem that impossible. It seems like if humans exist long enough one of us is going to make and launch a Von Neumann probe that eats the entire galaxy. The fact no alien species has makes me think we're far and away more likely to go extinct long before attaining that capability.

I remember a line from a sci-fi novel: "We only consider a species mature when you could put the power to destroy the world into the hands of every single individual with complete confidence that none of them will ever use it." I think we're, eventually, doomed.

I still have my doubts. I agree it feels true, and yet every "earthlike" planet found so far is nothing like the Earth. How many exoplanets have been found, and how many of those are Earth-size planets with a large moon in the habitable zone of a sun-like star? Because it's entirely possible that all of those are necessary.

Because as inevitable as life may seem, it's also fragile. Practically the entire universe is extremely hostile to it, with the exception of a small layer on the surface of this one planet. Radiation will destroy life, so we need a thick atmosphere and a strong magnetic field, something of the rocky planets in our solar system, only our planet has. The focus is mostly on water and the Goldilocks Zone, but it's entirely possible that there are a lot more requirements to make life possible.

I want to believe there's lots of life out there, but I don't want to be suckered into wishful thinking; evidence does matter.

> I'm perfectly comfortable with things being true even sans iron clad rigorous proof. If you limit your worldview to only that which has been rigorously proven you miss out on a lot of necessary and interesting truths, I think.

A sensible outlook in general. And I frankly think it's true of everybody, even if some people claim otherwise. Most of the human experience is not provable or falsifiable, yet we experience it and believe it.

> How many exoplanets have been found, and how many of those are Earth-size planets with a large moon in the habitable zone of a sun-like star?

I think this is down to how we look at the moment. One of the main ways we find exoplanets is to notice the star dimming with a regular frequency. This gets harder as the orbital time increases, as you need multiple observations of the planet transiting the star to conclude that it's a planet. This means that the vast majority of the exoplanets we know about have relatively short orbital periods, and the characteristics that go with that (e.g. they're close to their stars, therefore less likely to have moons, and if we consider them to be in the 'habitable zone', then their stars are relatively dim).

Actually, the fact that we've found so many exoplanets that fit in the relatively limited criteria that we can detect, suggests to me that there are probably a lot of exoplanets of all kinds, that we're just not as good at detecting yet.

By the standards of astrobiology, none of those instruments are capable of detecting life. But they might detect telltale signatures of biosphere in extrasolar atmospheres, which will trigger decades of debate as to whether there is life.

I am not a biologist. I could be entirely wrong in which case please educate me. :)

My understanding was that life would be "detected" by finding chemical signatures in the atmosphere which could not exist at a "steady state" without biological creatures there doing things. Is this what you mean by telltale signatures?

(A++ username)

Yes, astrobiologists distinguish between direct detection of life and biosignatures. The latter are things for which we don’t have non-biological explanations at this time, but which we don’t consider adequate evidence by themselves for life.

FWIW we’ve already detected bio signatures for life in the atmosphere of Mars (seasonal methane emissions). And maybe Venus, though that is less clear.

(Thanks!)

Thanks for this, your phrasing makes the concept more clear.

Can you give a (fictitious as necessary) example of what would constitute "direct detection of life", from an astrobiological perspective? Given our window to the universe is through telescopes I'm having difficulty imagining/distinguishing how to draw the line between "observed data for which we don't have any non-biological explanation" and something even stronger.

Like, does "direct detection of life" only include "we sent something there and found life"?

> we’ve already detected bio signatures for life in the atmosphere of Mars (seasonal methane emissions).

And ~Europa~ Enceladus too, right? ~ISTR seeing some paper about plumes of water vapor, as well as chemistry for which we don't have any non-biological explanation.~ I found it -- I was thinking of in 2017 when H2, CO2 and CH4 were detected at a ratio in thermodynamic disequilibrium .

Edit: I'm guessing "direct detection" in an astrobiological context would be something that builds on "direct detection" in an exoplanet context, a concept I already understand a bit -- friend of mine wrote her thesis on direct detection and imaging of exoplanets 15 years ago. :)

https://astrobiology.nasa.gov/research/life-detection/ladder...

Yeah.

I found and read that before my above comment (and skimmed some of the associated paper[1]) but all that says is direct detection hasn't been attempted since Viking, and that most good signals require in situ or sample return.

It still doesn't answer my question! "astrobiologists distinguish between direct detection of life and biosignatures".

The highest methods in terms of strength of evidence obtainable with remote sensing would be finding "deviations from thermodynamic equilibrium and/or kinetics" of "co-located reductant and oxidant" or "element or isotope fractionations indicative of metabolism". Which sound a whole lot like a detection of a biosignature, which "astrobiologists distinguish" from direct detection . :)

I suspect direct detection in an astrobiological context must mean either "direct, as in we physically touched it" or maybe "a signature found using direct imaging".

Definitions are hard. :)

1: https://www.liebertpub.com/doi/full/10.1089/ast.2017.1773

Direct observation means, well, observing it directly. Seeing living cells in a microscope. Growing it in a Petri dish. Genetic testing, if it turns out it has DNA or RNA like us.

Within our own solar system that generally means sample return, or sending people with a full biology lab. For an extrasolar planet? I don't think the astrobiology community is of a single mind regarding what should count as direct observation there. We're not sending people or doing sample return for thousands of years. One common opinion I've heard is that seeing an atmospheric spectrograph indicating the presence of biomarkers like oxygen and methane within the habitable zone would be a bio signature, but multi pixel imaging of the planet would be required to claim direct observation. I'm not sure this is a very strong scientific distinction, however.

I'm not an astrobiologist btw, but I used to work at astrobiology.nasa.gov. Small world!

> multi pixel imaging of the planet would be required to claim direct observation.

Here is an interesting proposal for direct imaging of an exoplanet with 1 km resolution:

https://arxiv.org/abs/1802.08421

...and an interesting youtube video about it:

https://www.youtube.com/watch?v=NQFqDKRAROI

It's a cool idea. Unfortunately getting something out to 550 AU and stopping there would take the better part of a century even with advanced technology. And once you're there, you're really stuck in terms of what you can look at. You can only see what's on the other side of the sun, and with an orbital period of 12800 years, it's not like you're going to sweep out much of the sky in a reasonable timeframe.

If you skip to 14:38 in the video, they describe the solar sails that are supposed to be able to accelerate things to 22 AU/year, which get things to 550AU in 25 years. But yes, you are only going to be able to look at one thing once you get there.

https://www.youtube.com/watch?v=NQFqDKRAROI&t=883s

One thing that gives me pause for thought: even here on Earth, we are all, likely, descendants of a single organism: https://en.wikipedia.org/wiki/Last_universal_common_ancestor .

It doesn't necessarily mean that the 0-1 creation of life happened exactly once on Earth (maybe it happened many times but the other lineages died out), but I think it makes this argument likely. We see a near-infinite menagerie of different organisms, from archaea and bacteria to reptiles and mammals, each finding its own niche, yet we don't see, on our own planet, any other blueprint for 0-1 life creation. Everything is DNA/RNA and cells, based on carbon and water, etc. And here we know conditions are just right, and we have studied the Earth like no other alien world.

I'd expect, if the universe is in fact teaming with life, that we would see "aliens" on our own planet. But all we see is descendants of LUCA. It seems to me that either "life" is in fact vanishingly rare, or there's lots of other life forms we don't notice (including on Earth). My gut feeling is on the former.

I think once you have one particular "life framework", it likely makes it super-difficult for other frameworks to develop in any major way.

Maybe, but why? We have everything from archaea to mammals living and thriving alongside. Why not DNA life alongside non-DNA life?

Is DNA-based life some kind of cosmic inevitability, like gravity? Or substitute for "DNA" any other common property of all life in Earth.

It may be the 'LUCA' is very nearly the ideal early life form and RNA/DNA are the right way to do things. Just like water is pretty unique.

Or alternatively, the first successful organisms on a planet out compete and literally eat the other organisms to extinction.

Water is incredibly common in space, so doesn't really prove anything.

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