[ceejayoz]

Parts of our own Earth aren't in the habitable zone, by that definition. Even here we get big surprises - undersea vents were unexpected oases, microbes miles deep underground, microbes living in boiling water in Yellowstone...

Not all life in the universe may require liquid water, nor require it 24/7. In our own solar system, some planetoids outside our supposed habitable zone likely have some liquid water - Europa and Enceladus, for example.

[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.

[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

[patmorgan23]

Do any of those zones actually have life?

The two examples you gave... Include liquid water.

As far as I know there is no life native to the coldest parts of the earth that have no liquid water.

It is entirely possible there is some other form of life that does not require liquid water, but we have yet to discover it.

[ceejayoz]

> The two examples you gave... Include liquid water.

This specific planet spends half its orbit in said zone. Here on Earth, we have creatures like https://en.wikipedia.org/wiki/Mudskipper that can survive severe dry spells, and fish that can happily freeze sold in ice for months.

Europa spends zero time in our solar system's "habitable zone", but because of its conditions, may still possess large amounts of liquid water. It's a perfect example of why the "zone" may be overly narrowly defined, even for Earth-like water-dependent life.

> It is entirely possible there is some other form of life that does not require liquid water, but we have yet to discover it.

And we certainly won't if we only look in Earth-defined "habitable" zones.

[dylan604]

You seem to be deliberately trying to pervert the definition of habitable zone.

Just because there are regions on a planet in the habitable zone that contains ice does not mean it is not in the habitable zone. If it were further out beyond the habitable zone, there would be no liquid water at the surface.

To quote my friend Andy Dufresne, "How can you be so obtuse?...Is it deliberate?"

[Dylan16807]

> You seem to be deliberately trying to pervert the definition of habitable zone.

They're fine. Some people here are trying to overextend the definition and it's good to push back.

The habitable zone is about surface water. Pointing out that parts of the earth lack surface water for extended periods is a really good analogy to a planet that drifts in and out of the habitable zone.

[ceejayoz]

> You seem to be deliberately trying to pervert the definition of habitable zone.

No, I'm saying an Earth-life centric metric is a bit of an odd choice when evaluating extrasolar planets.

It's like an African elephant declaring Norway uninhabitable.

[pc86]

Earth is the only environment in which we've found life, so it makes sense to first focus on other Earth-like planets. Your point is not without merit but taken to the extreme it would be like sending exploratory teams to the Mariana Trench to look for un-contacted human tribes.

We have incontrovertible evidence that water + carbon + time sometimes equals life. We have no evidence of any other non-carbon or non-water chemistries resulting in life so why wouldn't we focus on locations potentially rich in water and carbon first?

[ceejayoz]

The first exoplanet detection was in 1992; in my lifetime, "there are no planets outside our solar system" was roughly as supportable as "there is no life outside our solar system" is today.

Here on Earth, we can barely decide if viruses are life or not, and discover new things within our own bodies pretty regularly, despite... a lot of direct access to them. (Example: https://www.science.org/content/article/it-s-insane-new-viru...)

We should be casting a pretty wide net.

[pc86]

You seem to be talking past everyone else in this thread because nobody has disputed a single thing you've said however you're ignoring (perhaps unintentionally) the basic statistical reality that if we focus on potentially water- and carbon-rich environments we are as a matter of course more likely to find life sooner.

[ceejayoz]

But the "basic statistical reality" is also "there are a lot more known/accessible planets outside of the zone in which liquid water exists naturally on the surface", which leads to a quantity vs quality consideration for which we don't have enough information to decide right now.

[glenstein]

There's a meaningful operative definition and you're muddying the waters over that definition on the grounds that, hey, who knows, maybe it's different somewhere in some way.

I just think you're confused if you think that observing a specific definition of habitable zone is tantamount to a specific denial of that possibility.

[ceejayoz]

I'm not confused by its definition, I just dislike the use of the term. It leads to significant confusion in laypeople - "they found a habitable planet!" is something I've heard breathlessly repeated multiple times, and "but Earth is so perfectly placed, it can't be by chance!" used as an argument for creationism.