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> 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 undernea

> 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 2y ago
> 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.
- throwup238 2y ago
  Dude, just read the Wikipedia article: https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents 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 2y ago
    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 2y ago
      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 2y ago
        That's similarly true for carbon-based life; abiogenesis remains a hypothesis in search of concrete evidence.
        throwup238 2y ago
        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.
        ceejayoz 2y ago
        I'm aware of the Miller-Urey experiment. It's only one piece of the puzzle, and we're aided significantly in it by knowing what the results are supposed to look like.
- glenstein 2y ago
  >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.
Davidzheng 2y ago
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 2y ago
  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-based-xijric/ https://www.pbs.org/video/what-if-alien-life-were-silcon-bas... Or on YouTube: https://www.youtube.com/watch?v=469chceiiUQ https://www.youtube.com/watch?v=469chceiiUQ
- deleted 2y ago
  [deleted]

ceejayoz 2y ago

> 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.

throwup238 2y ago
Dude, just read the Wikipedia article: https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents 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 2y ago
  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 2y ago
    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 2y ago
      That's similarly true for carbon-based life; abiogenesis remains a hypothesis in search of concrete evidence.
      - throwup238 2y ago
        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.
        ceejayoz 2y ago
        I'm aware of the Miller-Urey experiment. It's only one piece of the puzzle, and we're aided significantly in it by knowing what the results are supposed to look like.
glenstein 2y ago
>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.

Davidzheng 2y ago

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 2y ago
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-based-xijric/ https://www.pbs.org/video/what-if-alien-life-were-silcon-bas... Or on YouTube: https://www.youtube.com/watch?v=469chceiiUQ https://www.youtube.com/watch?v=469chceiiUQ
deleted 2y ago
[deleted]