[throwup238]

> Parts of our own Earth aren't in the habitable zone, by that definition.

What parts would that be? Even the polar caps have huge liquid water oceans underneath. Unless you’re talking about the mantle or molten core, there are no uninhabitable areas on earth as per astrobiology (not even miles underground).

> Not all life in the universe may require liquid water, nor require it 24/7.

You might as well be talking about leprechauns and unicorns and Horta. Water is the universal solvent and has at least five unique properties that are as critical to life as carbon’s ability to form four chemical bonds.

You’re correct that moons experiencing tidal heating can contain liquid water, but that’s irrelevant to a planet. The habitable zone is specifically talking about planets (rocky ones at that), not any arbitrary satellite. It’s a term of art in astronomy, not a colloquialism.

[ceejayoz]

> Unless you’re talking about the mantle or molten core, there are no uninhabitable areas on earth as per astrobiology (not even miles underground).

We've found microbes that can survive at 120 Celsius, -25 Celsius, very high and very low pH, large amounts of ionizing radiation, intense pressures, etc. Habitability is a wide range encompasing scenarios not conducive to liquid water.

> Water is the universal solvent and has at least five unique properties that are as critical to life as carbon’s ability to form four chemical bonds.

None of that rules out life on other chemistries. It makes water+carbon-based life the most likely scenario on planets with liquid water, but hardly rules out other potential biologies.

> You’re correct that moons experiencing tidal heating can contain liquid water, but that’s irrelevant to a planet. The habitable zone is specifically talking about planets (rocky ones at that), not any arbitrary satellite.

But we should absolutely be looking at planet-sized moons with potentially habitable conditions, which we believe to be quite common. They are, after all, more common than the single "habitable zone" planet even within our own system.

[glenstein]

>None of that rules out life on other chemistries

Is the upshot of this observation supposed to be that PLATO should change its plans and direct its telescope in a different direction because it has more promising places to look than the habitable zones around stars?

If not, and if you can understand why it's prioritizing that, then why do you take this definition of habitability to be tantamount to denying the possibility of discovering other forms of life? For those possibilities to be relevant to a research program, they need to be motivated by something more than "gee, hey, you never know."

So it's not for lack of reflection on those possibilities that we arrive at this operative definition of habitability. There are pertinent reasons for moving forward with this definition that don't amount to denying other boutique possibilities. Construing it that way I think is just an uncharitable interpretation.

[throwup238]

Dude, just read the Wikipedia article: https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi...

It’s not impossible, but we’ve got a ton of evidence why it’s extremely unlikely. It’s a long list including stuff like possible quantum transition states enabling biochemistry, reactivity with oxygen (the third most abundant element), and spectroscopic transparency. It’s an active area of research that keeps coming up with dead ends.

Ammonia and methane are the best candidates but those would only be possible at low temperatures that preclude lots of other reactions.

[ceejayoz]

The Wikipedia article supports my point - plenty of smart folks think it's at least plausible for alternative chemistries to work.

An ammonia-based life form at our stage of exploration is probably gonna scoff at the idea of scaldingly hot liquid water as a basis for life, too.

> It’s an active area of research that keeps coming up with dead ends.

So's SETI so far, but I'm not willing to conclude extraterrestrial life is impossible just yet.

[throwup238]

And none of those smart people have come up with any experimental evidence that it’s actually possible. No equivalent to amino acids or nucleotides or saccharides or… the list goes on.

I’m not talking about SETI, I’m talking about basic chemistry experiments. There are tons of experiments that can spontaneously form amino acids and nucleotides, even way outside the parameters normally considered habitable.

[ceejayoz]

That's similarly true for carbon-based life; abiogenesis remains a hypothesis in search of concrete evidence.

[throwup238]

There is tons of concrete evidence, you’re just ignorant of it. Start with Stanley Miller’s seminal 1953 paper “Production of Amino Acids Under Possible Primitive Earth Conditions” and go from there. There’s been a lot of work on the topic since then, several of which have made it to the HN front page.

[JumpCrisscross]

> ammonia-based life form at our stage of exploration is probably gonna scoff at the idea of scaldingly hot liquid water

Ammonia-based life exists within water habitable zones; Mars is within our Sun’s conservative habitable zone [1]. (Also, “ammonia boils at 98°C instead of –33°C” at “60 atm, for example, which is below the pressures available on Jupiter or Venus,” meaning “ammonia-based life need not necessarily be low-temperature” [2].)

One reason to suspect ammonia-based life is rarer than carbon-based life is the universe contains a fifth of the nitrogen that it does carbon [3]. (This is why silicon-based life is also almost written off.)

[1] https://en.m.wikipedia.org/wiki/Habitable_zone

[2] https://www.daviddarling.info/encyclopedia/A/ammonialife.htm...

[3] https://en.m.wikipedia.org/wiki/Abundance_of_the_chemical_el...

[Davidzheng]

How is it clear at all that extraterrestrial life is more likely carbon based than not. Is there any evidence other than it's the only one we know?

[ziddoap]

PBS Space Time has a very good & informative episode covering this. I'll try to dig it up.

Most elements are missing some key properties that carbon has.

Silicon is the next most-likely element, but it's still missing out on a few properties that carbon has.

Here:

https://www.pbs.org/video/what-if-alien-life-were-silcon-bas...

Or on YouTube:

https://www.youtube.com/watch?v=469chceiiUQ