Not to mention, we have zero evidence that comets can really seed life and we also have absolutely no reason to believe that any place in the universe is better at spawning life than earth.
So I wonder, why is this idea so attractive?
Also, life on Earth started very fast, almost as soon as it was physically possible. That also seems quite unlikely.
We've got goldilocks conditions here on earth for creation of complex chemistries and emergence of life.
We've got all sorts of chemical elements, nice warm distance from the sun - lots of energy, but not too much to destroy things. Lots of liquid water, getting stirred up by a moon. A hot molten core providing both geothermal energy and a magnetic radiation shield. Plenty of lightening to initiate higher energy reactions...
Does it get an better ?
All those chemicals, getting mixed in water, stirred up by tides and thermal currents, heated up by the sun, are going to react ... no way to stop them.
In a few decades of trying a small number of labs experimenting with early earth type conditions have already been able to see organic chemicals created out of inorganic .. the building blocks of life, so imagine how common this would have been on early earth itself, with billions of micro environments chemically evolving 24x7 for millions of years ...
It's hard to say what we should regard as the beginnings of evolution, but this type of ever changing, ever complexifying stage of emerging organic chemistry on earth - essentialy an inevitability - could even be considered as the start.
If almost inevitable, then I'd also expect life to form readily in many other places too. Unless Earth was just very, very unusual in some way.
More convoluted possibilities are less likely, but that doesn't render them impossible. You have to rule them out by evidence, not by unfounded bias.
I would argue that this isn't precisely true. We know of some forms of relatively complex terrestrial life that can survive for indefinite (as far as we know) periods in vacuum - tardigrades are the most common example given.
For some other good candidates, I'd start with the list included the (failed) LIFE mission: https://en.wikipedia.org/wiki/Living_Interplanetary_Flight_E...
> What scientific problem would actually be solved by panspermia? Obviously, the product of two unlikely events is not more likely than one unlikely event by itself.
Panspermia is the belief that life is pervasive across the Universe. You don't have to believe that to believe that terrestrial life may not have originated here. From a statistically perspective, if we assume that life has arisen exactly once, it's no less likely that it arose on Mars as on Earth. Maybe some or all of the early history of life had already happened before the accretion of Earth. If that's the case, we'll likely never find direct evidence of it.
In short... I'd argue that panspermia doesn't "solve" any problems at all. If life is unique to Earth, we should be able to find evidence of its earliest forms. If it originated on Mars and was "seeded" here, then we should eventually find that evidence there. If it's universal, it still had to have originated _somewhere_; it's just going to be much more difficult to discover where.
To the best of my (lay) understanding, while we have a fair idea of how life might have began, we don't have very good evidence that it actually began on Earth.
What we do know:
* Earth is ~4.5b years old.
* The oldest fossils we know of are ~3.5b years old: https://news.wisc.edu/oldest-fossils-found-show-life-began-b...
* There exists evidence suggestive of life that is ~3.8b years old, in the form of what we believe to be precipitate from a oceanic hydrothermal vent that shows signatures of biological processes.
This means that in order for life to have arisen on Earth, it must have done so in the first ~1b years - because those oldest fossils mentioned above included multicellular life. To go from "primordial soup" to "multicellular microorganisms" is a pretty big leap, and we don't know the specifics.
Then there's the fact that _all_ known life either contains DNA or, in the case of RNA viruses, requires access to DNA-based life to reproduce. We don't know what came before that. Did primitive metabolism in come first, or genes (encoding structure in a durable and reproducible way)? Did they arise in tandem?
Either way, we are very confident that 1b years after the matter that we now know as Earth coalesced, there were multicellular organisms alive. Since all known life requires DNA, that means DNA was around at that point as well. That doesn't seem long enough to me, and apparently others share that intuition.
(edited for coherence)
A path like: (pre-TNA world ?) -> (TNA world ?) -> RNA world -> Ribonucleoprotein world -> Simple cells & viruses with DNA & RNA
https://en.wikipedia.org/wiki/RNA_world https://www.ncbi.nlm.nih.gov/books/NBK26876/ https://www.newscientist.com/article/dn21335-before-dna-befo...
It's one of the steeps before the cells than may have come from outer space because we did not find trace of them on earth. The other hypothesis is than they have disappear since then, because of later steeps in life history like the Great Oxidation Event.