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Luke_A_Somers comments on Quickly passing through the great filter - Less Wrong Discussion
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I don't get the reason panspermia needs to be involved. Simply having a minimum metallicity threshold for getting started would do the job.
It might do, except that the recent astronomical evidence is against that : solar systems with sufficient metallicity to form rocky planets were appearing within a couple of billion years after the Big Bang. See here for a review.
Hmmmm. (ETA: following claim is incorrect) They're judging that the planets are rocky by measuring their mass, not by noticing that they're actually rocky.
If you don't have a Jupiter-sized core out there sucking up all the gas, why would gas planets need to end up as giants? They naturally could do that - that happened with the star, after all, but it doesn't seem inevitable to me, and it might not even be common.
In that case, the earth-mass planets would be gas planets after all. If you think this is a stretch, keep in mind that these are specifically in systems noted to be low metallicity. Suggesting that they might not be high in metals after all is not much of a stretch.
Actually, Kepler is able to determine both size and mass of planet candidates, using the method of transit photometry.
For further info, I found a non-paywalled copy of Bucchave et al's Nature paper. Figure 3 plots planet radius against star metallicity, and some of the planets are clearly of Earth-radius or smaller. I very much doubt that it is possible to form gas "giants" of Earth size, and in any case they would have a mass much lower than Earth mass, so would stand out immediately.
I forgot about photometry.