> It's not in our habitable zone.
The habitable zone is defined as the area around a star where liquid water could be found, there is no "our" habitable zone and "their" habitable zone.
The habitable zone is defined as the area around a star where liquid water could be found, there is no "our" habitable zone and "their" habitable zone.
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.
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.
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.
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?"
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.
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?
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.
Or maybe we start with what we know (carbon-based), and keep our minds open to other possibilities. Like we do now.
But we know more than one thing. One of those things is the Fermi paradox - that the universe should statistically be full of evidence of life, and yet we struggle to find it. That may be evidence we're making the wrong assumptions.
> keep our minds open to other possibilities...
Yes, and I'd argue that means including scenarios like Jupiter's moons in our search. (As a bonus, Jupiter-style planets, being larger and far from the star, are substantially easier to find.)
We haven't even had the chance to fail yet, the Fermi paradox is not yet in play when we're considering essentially our first move. To extend an analogy from further up thread, it'd be like looking in the Mariana Trench for un-contacted human tribes after taking a quick glance around the neighborhood and deciding there's nothing else to be found anywhere else.
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.
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.
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.
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.
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.
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.
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.
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.
The specific chemical details are irrelevant. We have no evidence of other monomers that could enable non-water based life.
I’m done, have a great day! (Monomers)
Edit: My apologies for being dismissive. I’d like to get into the specifics of why amino acids (amino and carboxylic groups specifically) are special, and interesting exceptions like hydroxy and alpha-hydroxy acids, but I’ve got to get to work and I could spend an entire year explaining the nuances. The deeper you get into the details, the more the anthropic principle rears its ugly head.
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...
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.
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:
Well, one constructive way to take it out of the realm of subjectivity is to put forward a specific definition.
Sure, but not in the context of "habitable zone" which is a specific term of art in astrobiology.
And that's fine, but when communicating outside the speciality, I'd really like to see some other term used.
https://www.cjonline.com/story/news/politics/government/2025... for example says "Kansas tuberculosis outbreak is now America's largest in recorded history", where "recorded history" is apparently the CDC's "term of art" for "since 1950", which isn't what a layperson hears.
The IAC, where this article is from, is the Instituto de Astrofísica de Canarias (literally the "Institute of Astrophysics")
That's about as far into the specialty as you can get.
That's why the article's breadcrumbs say "Home > Outreach > News".
We saw the same issue during COVID - scientists talking to the general public often talk like scientists instead of science communicators, and that causes people to misunderstand. Fauci's "no evidence" (yet) masking prevents disease incorrectly becomes evidence masking can't prevent disease.
Once it makes its way to PBS Space Time, sure, maybe you avoid terms of art. Or explain the particular definition when it is first introduced.
That's just my opinion anyways. I always try to familiarize myself with the common terms of art when learning about a new discipline.
But they will get linked to it, or read articles by reporters using it as a source without enough domain knowledge to make the distinction. I, after all, didn't seek this out - it just popped up on the HN home page.
> What seems weird to me is to be interested in a discipline, seek out news and conversation about it (from university press releases!) but then reject and/or argue about any terms of art that are established within that discipline.
I think science communication, post-COVID, needs to take a serious look at how to better explain things to the public. "Habitable zone" is simply one example of it.
However, I think there's some serious slack to cut when you're viewing an article on the Institute of Astrophysics website, compared to reading Fox/CBS/whatever.
Edit: In my re-reading of the article, I see they define it! I'm no longer sure what all this back and forth is even about. "This orbit places it within the habitable zone of the system, _meaning it is at the right distance from its star to sustain liquid water on its surface_" Do you want them to not use the term even when they define the term?
Yes, I do. I think it's a needlessly confusing term to use in stuff intended for public consumption.
For a similar example of the issue, I often get radiology reports in my healthcare provider's portal. My dad is a radiologist and they're still quite scary/bewildering to read - they frequently use various terms of art for "looks fine and normal" that sound terrifying.
That was all fine when the intended audience was other doctors, but these days I can pull them up myself. I at least know enough to not freak out and ask my dad; many don't.
I don't want to see NBC/BBC/NYT articles using the term, and that means being careful with the sources from which they receive their info.