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peter_hurford comments on The Great Filter is early, or AI is hard - Less Wrong
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Going off of this, what if life is somewhat common, but we're just one of the first life in the universe? That doesn't seem like an "early filter", so even if this possibility is really unlikely, it still would break your dichotomy.
The problem with that is that life on Earth appeared about 4 billion years ago, while the Milky Way is more than 13 billion years old. If life were somewhat common, we wouldn’t expect to be the first, because there was time for it to evolve several times in succession, and it had lots of solar systems where it could have done it.
A possible answer could be that there was a very strong early filter during the first part of the Milky Way’s existence, and that filter lessened in intensity in the last few billion years.
The only examples I can think of are elemental abundance (perhaps in a young galaxy there are much fewer systems with diverse enough chemical compositions) and supernova frequency (perhaps a young galaxy is sterilized by frequent and large supernovas much more often than an older one’s). But AFAIK both of those variations can be calculated well enough for a Fermi estimate from what we know, so I’d expect someone who knows the subject much better than I would have made that point already if they were plausible answers.
Even within the Milky Way, most "earthlike" planets in habitable zones around sunlike stars are on average 1.8 Billion years older than the Earth. If the "heavy bombardment" period at the beginning of a rocky planet's life is approximately the same length for all rocky stars, which is likely, then each of those 11 Billion potentially habitable planets still had 1.8 billion years during which life could have formed. On Earth, life originated almost immediately after the bombardment ended and the earth was allowed to cool. Even if the probability of each planet developing life in a period of 1 Billion years is mind-bogglingly low, we still should expect to see life forming on some of them given 20 Billion Billion planet-years.
How do you know? (Not rhethorical, I have no idea and I’m curious.)
It was in a paper I read. Here it is
Thank you, that was very interesting!
That would push many of them over into Venus mode seeing as all stars increase in brightness slowly as they age and Earth will fall over into positive greenhouse feedback mode within 2 gigayears (possibly within 500 megayears).
However, seeing as star brightness increases with the 3.5th power of mass, and therefore lifetime decreases with the 2.5th power of mass, stars not much smaller than the sun can be pretty 'sunlike' while brightening much slower and having much longer stable regimes. This is where it gets confusing; are we an outlier in having such a large star (larger than 90% of stars in fact), or do these longer-lived smaller stars have something about them that makes it less likely that observers will find themselves there?