"Water is the only molecule that fits the bill, and happens to be the second most common molecule in the universe." [citation needed]
As to the first part, there are uncountable examples of water-based life, and zero examples of non-water-based life. That doesn't rule out its existence, but basic chemistry makes it unlikely, and it won't look anything like life as we know it. No other liquid has the properties needed for any kind of life observed.
As to the second: https://www.google.com/search#q=%22second+most+common+molecu...
The most common molecule is diatomic hydrogen and the second most common is helium. That'll likely be true for hundreds of billions of years, if not trillions, due to the way stars burn their atomic fuel. Even if you don't include atomic helium as a molecule, the second most prevalent would likely be the most reactive combination of hydrogen and nitrogen/carbon.
https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elem...
Also see this delightful comment by logfromblammo: https://news.ycombinator.com/item?id=10292067
Assuming your definition holds: Do you have an actual (peer reviewed) citation that demonstrates that H2O is the second most prominent molecule in the universe, for any real definition of molecule? If not, do you have a citation showing that diatomic oxygen is less likely to form than water in the interstellar medium (considering the momentum of hydrogen atoms will be significantly greater than that of oxygen atoms and thus impact the chemical reactions drastically)? If not, do you have evidence to show that our capabilities to detect polar molecules like water are able to account for the absurd abundance of nonpolar molecules (for which electromagnetic detection is practicaly impossible) based solely on first principles of astro or even quantum chemistry?
I'm not trying to play devil's advocate or prove you wrong. These are questions I'd love to have the definitive answers to but I'd rather have no answers at all than a random HN post which only cites a Wikipedia article on the prevalence of atoms (not molecules, missing the entire point of chemistry) and another unreviewed HN post on astrochemistry from first principles without a single significant citation.
The purpose of [citation needed] is not to take the wind out of your statement but to force you to provide the necessary evidence to back up your assertion.
If you can't provide a single citation to support such fundamental statements as "H20 is the second most common molecule in the universe" outside of irrelevant Wikipedia articles, they are worthless, HN moderator and community standards notwithstanding.
[0] - https://en.wikipedia.org/wiki/Carbon_monoxide
[1] - https://en.wikipedia.org/wiki/Nuclear_fusion#Nuclear_fusion_...
Diatomic oxygen is extremely rare in the universe, because it's extremely unstable. Oxygen would rather be bonded to pretty much anything rather than itself, including hydrogen. [citation: go burn some hydrogen]. It's common round these parts due to some rather bizarre nonequilibrium chemistry [aka life].
What we know about the prevalence of various molecules is a combination of what we know about the prevalence of atoms combined with what is thermodynamically stable. It goes hydrogen, water, methane, ammonia, carbon monoxide, I think? At this point it depends on local conditions, temperature and so forth, but the stability of water combined with the prevalence of oxygen makes it basically unassailable as the second most common molecule after H2.
Can you see why instead of "forcing someone to provide references" it would be better to ask someone nicely for references?
No it doesn't; https://en.wikipedia.org/wiki/Molecule cites that as an earlier, less precise definition than the one that's commonly used today. The definition right at the top of the page is quite clear that by the usual definition, a single atom by itself does not constitute a molecule.
> The purpose of [citation needed] is not to take the wind out of your statement but to force you to provide the necessary evidence to back up your assertion.
In polite discussions, attempting to "force" your conversational partner to do anything is kind of discourteous. If you're honestly seeking information, then by all means feel free to ask questions. But whether you intended to or not, your responses are coming across as very adversarial and dismissive.
> In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules despite being composed of a single non-bonded atom.
Unless you're saying that water (or any other molecule for that matter) can form inside of a star, which would open an entirely new field of chemistry, this is the definition to use when talking about astrophysics.
True, we've only seen water-based life firsthand, but our "sample" of reality is so infinitesimally minute relative to the whole that we cannot properly use that as a basis to make assertions about what the overall proportional makeup of types of life might be. Doing so is pure speculation.
It would be sort of like randomly picking up one tiny pebble at random from anywhere on the earth, identifying its elements, and then concluding that it's "unlikely" any other non-identified elements exist on the entire planet because they aren't in that particular pebble.
You're going to have quite a lot of H, some He, and ever decreasing quantities of larger atoms. The abundance of atoms in our galaxy by mass is 74% H, 24% He, 1% O, 0.5% C, 0.1% Ne & Fe & N, 0.05% Si & Mg & S. When you break it down by quantity of atoms, the spread only grows wider. 92% H, 7.5% He, 0.1% O, 0.05% C, 0.01% N & Ne, and ~0.003% Mg & Si & Fe & S.
Now, for each atom, you're going to work the room at the universal party. If you're an H, you show up early to the party, and you're very likely to run into another H and really hit it off. So most of the H in the universe is going to pair up as H2 before anyone else even shows up. He and Ne don't really want to be at the party. They hang out in the corners and stare at their own quarks. So let's just pair up all those H atoms into H2 and ignore the He and Ne.
We're now at 99.6% H2, 0.2% O, 0.1% C, 0.02% N, ~0.005% Mg & Si & Fe & S among atoms that actually want to bond.
So C, N, Mg, Si, and Fe are pretty chill. If they happen to meet up with a lone H, they get along together pretty well. Otherwise, they're content to circulate around the party and meet other atoms looking for a good bond.
But O and S are a little crazy. They are not going home alone, ever. And usually, they're going to hook up with the first other atom (or molecule) they meet at the party. Almost every O that joins the party is going to end up as the meat in an H sandwich and leave the party. By coincidence, some will become O2 and be content. A bit less will become CO, or NO, and a very tiny amount will end up in MgO, SiO, FeO, or SO. And those will stay at the party, looking to bond more.
So just by random meetings based on relative abundance, it is clear that H2O is going to be third among discrete chemical units, and second among bonded molecules. If O were less abundant, or less ravenous for electrons, it might be different, but it's pretty safe to say that O will try to react with anything it meets, and it's very likely to meet an H2.
As far as I understand it's much easier for us to know distribution of different elements that it is to know distribution of different molecules in the universe. I assumed that it wouldn't be that popular given that it does not take part in a nuclear fusion.