alt Hacker NewsWe know 2 things that are apparently incoherent:
1 - Abiogenesis is incredibly rare. We don't know how much exactly, but it's a lot.
2 - Abiogenesis happened on Earth about as soon as it became possible. Where "as soon as" means within half a billion years, but it's still way quicker than its rarity implies.
A lot of people think panspermia is what made those two happen. Life had about a full billion years to appear in meteors before they could appear here.
There are some problems, e.g. that each meteor only stayed chemically active for less than that half-a-billion years Earth had. Or that all the meteors that fell on Earth had only a fraction of the material that was later available here. But IMO, the largest issue is that just doubling the time is absolutely unsatisfying.
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People make the inference that "early occurence of life" implies "life must be easy to start". But that inference requires the assumption that the chance of OoL (origin of life) remains mostly constant with time. An alternative would be that the conditions under which life could arise are transient, so life either starts early or not at all. We don't know enough about OoL to rule this out. Some chemicals that might be needed for OoL, like ammonia, are not stable for long. And if life originated in small asteroids, this might have only a few million years for it to occur while they are still warm enough from early short lived radioisotopes like Al-26.
Life cannot appear in any of the small bodies that become meteors, because there is no source of energy for it.
Life can appear only on big planets or on big satellites, like the big satellites of Jupiter and Saturn, if they have a hot interior and volcanism.
Volcanism brings at the surface substances that are in chemical equilibrium at the high temperatures of the interior, but which are no longer at chemical equilibrium at the low temperatures of the surface, providing chemical energy that can be used to synthesize macromolecules.
Solar energy cannot be used for the appearance of life. Capturing light requires very complex structures that can be developed only after a very long evolution and which cannot form spontaneously in the absence of already existing living beings.
The only theory of panspermia that is somewhat plausible is that life could have appeared on Mars, which had habitable conditions earlier than Earth. Then, some impacts on Mars have ejected fragments that have fallen as meteorites on Earth and some remote ancestors of bacteria have survived this interplanetary trip.
There are many meteorites on Earth that have their origin in impacts from Mars, so at least this part is known as being possible.