An earthlike planet is require for earthlike life.
It is fallacious to assume other life couldn't be different -- we can't prove or disprove that. Likewise, we still don't know exactly how life started here; we simply have a few good theories.
I don't think we have "good" theories about how life started here. In particular, the complexity gap from abiotic chemicals to the smallest known organism capable of independent reproduction (about 4 billion atoms!) is extremely large, and no theory has done more than handwave about how that gap is to be crossed.
this is what has always annoyed me about basically every space movie ever made. even life a star away, let alone another galaxy away, is likely to be startlingly different from the oxygen-absorbing, ossified calcium-braced, protein-enriched, atp-powered water blobs with heads, legs, and feet we have on earth. and that's just the the biochemical stuff we're familiar with.
i think in the end it's going to come down to what you really consider "life" to be.
there is an implicit part of our definition that includes timescales and size, and this puts some limits on the type of chemistry that can be involved in order to qualify.
if you open up the definition to be things that work on totally different timescales and sizes than we consider, then i think its fair to say that the chemistry doesn't need to be similar at all.
but for things that form blobs of life on the rough range of scales found on earth, and that "do stuff" in the range of timescales that we see on earth, there are not too many choices. silicon often gets touted as an alternative because of its ability to form branched structures. do you know of any other ideas?
sure, chemistry has a somewhat limited palette to work with, but life happens in the gap between potential and kinetic, between hot and cold, between proton and electron. that's a tiny space but a huge gap.
dna is mostly a 4-note tune, and computers, the potential precursor to artificial life, 2. i'm sure something as vast as the universe can come up with a little more variation than skin type in those tiny little gaps.
I think GTP-powered file is possible, even the glucose metabolism produce one GTP (and many ATP). (Perhaps I'm missing something, Chemistry is not my main specialty). Perhaps it is even possible to have NADH-powered life, but it is more different so it may be impossible.
> ossified calcium-braced
Diatoms use silica, insects have a protein exoskeleton, and octopus are weird.
> oxygen-absorbing, [...] protein-enriched, [...] water blobs
I guess these three are safe bets (with a little different selection of amino acids).
Oxygen absorbing isn't necessary, of course. Life on earth got along without it for a while until those darned cyanobacteria starting poisoning everyone else.
Due to convergent evolution [1], I'm not sure life that originated on an Earth-like planet would look completely different from Earth life, at least on a macroscopic level.
My point is that I think convergent evolution would be likely to produce “water blobs with heads, legs, and feet” on other Earth-like planets, i.e. planets with lots of liquid water and lots of solid land.
If you’re thinking of planets that are not similar to Earth, of course all bets are off.
Fermi can blast out "Hi" with a 100GW of power and he is unlikely to be heard in Andromeda, which for all we know could be teeming with life and host to something like an Iain Banks Culture series space opera.
We know that fewer than about 1 in 100,000 galaxies has a Kardashev Type III civilization. So the Fermi argument has some bite even at cosmological distances.
But anyway, this was about SF movies where our heroes visit Arglebarg IV and have adventures with the natives. It's like Victorian adventure stories transplanted into space without bothering to think things through. Let's kill our dysfunctional tropes.
That's also discussed in the article. The author states that Mars may have qualified before Earth did.
Edit: Also, the article you linked specifically states "Scientists debate a range of ideas about how life on earth began", undermining your first point.
It is fallacious to assume other life couldn't be different -- we can't prove or disprove that. Likewise, we still don't know exactly how life started here; we simply have a few good theories.