The co-author (Janusz Petkowski) stated:

> This means either this is life, or it’s some sort of physical or chemical process that we do not expect to happen on rocky planets.

> We really went through all possible pathways that could produce phosphine on a rocky planet. If this is not life, then our understanding of rocky planets is severely lacking.

So it's a matter of probabilities, not absolute certitudes.

My question is, why couldn't our current understanding of rocky planets be lacking?

Because "must be harboring life" rules this option out unconditionally.

> My question is, why couldn't our current understanding of rocky planets be lacking?

I've got an analogy for you. Lets say at one point you came to the conclusion that 2+2=4 and later I give you two apples and two apples, it would be quite unproductive to stop and ponder "Wait, my question is, why couldn't our understanding of addition be lacking?" Not when you have knowledge on hand to predict that you have four apples. Under-using your knowledge is not something conservative or cautious or in any way virtuous, it's simply wrong.

Searching for new hypotheses is cool, coming with new evidence is cooler, but doubting the hard-earned knowledge without a reason is not. In other words I'd suggest to stubbornly ignore unknown unknowns.

There are people who put their names on "we really went through all possible pathways" and you've put your name on a version of "huh?", excuse me for shortening it so brutally.

Agreed, any knowledge our civilization has is probably lacking to some degree. We kinda succeed by playing what we presently have on hand (and correcting it on the way).

I know it's probably not satisfying, but they spent 10 years on this research asking this very same question. If the only remaining possibility is biological origin, then we should probably take the risk of ignoring an unknown unknown.

I don't think such approach is fully consistent with scientific methodology.

The abstract clearly presents the question as an alternative, not "the only remaining possibility", as you stated:

"PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life"

It doesn't strike me as scientific to "ignore an unknown unknown", which would boil down to an assumption that we already know every possible reaction mechanism there is to know.

Discovering unknown chemistry would be groundbreaking of course, but the same goes for confirming extraterrestial life.

Indeed not 100% pure science, but I like the fun side science too where I can just forget about the rigours and entertain ideas that are likely to be true.

I've just read the paper and all this is right. Before that I haven't seen these claims so we are now in maybe life territory again. Sigh.