Comment by schiffern
1 day ago
One thing about panspermia math: why assume current travel conditions between the stars?
Today the Sun is very far from other stars, but early in star formation (within nebula) the stars are
1. much closer together
2. have a higher rate of material ejection (mass exchange)
3. likely have more benign radiation environments unless near a star, due to GCR shielding by nebula gas
4. likely have higher equilibrium temperature in interstellar/interplanetary space
Combined, these factors yield orders of magnitude improvement in panspermia efficiency vs the Sun.
This has occurred to people! But if you run the numbers, they are still not good enough in that early (~100 My) window when stars in the parent nebula are close together, you get something like 1 chance in 1000 of possible panspermia events (based on modeling lumps of rock flying back and forth). Not terrible odds, but nothing like the Mars -> Earth odds.
A more extreme version of this is the idea of panspermia in the early universe, when the cosmic microwave background radiation was warm enough to keep water liquid: https://lweb.cfa.harvard.edu/~loeb/habitable.pdf
I knew I couldn't be the first, but I was more commenting on the lack of discussion I've found in the peer-reviewed literature!
The key was to change "nebula" (returning results for "stellar nebula") to "star forming region."
https://journals.sagepub.com/doi/abs/10.1089/ast.2005.5.497
Yes indeed, I've thought panspermia in the Sun's natal cluster is an interesting idea. What could make it even more interesting is that small bodies in our Solar System (and likely its sister systems) were warm enough for liquid water to exist for some millions of years then, so there would be an enormous volume of material in which Origin of Life might have occurred (and, if it did, to then spread after the parent body froze and then fragmented.)
It would make a fine science fictional scenario that could get around the Fermi problem: those 10,000 or so systems could have life, but the rest of the universe not have life.
That's also neat to contemplate, thanks.
I was thinking bigger: if panspermia gets dramatically more viable, then nebula could switch over in their probability distribution from "default sterile" to default teeming.
In this scenario, we could find (related!) life essentially everywhere, and caused by panspermia no less, despite numbers still saying panspermia is wildly improbable for mature stars.
"Default teaming" really isn't viable due to the Fermi problem.
But if OoL is extremely unlikely, the cluster panspermia idea still allows extraterrestrial life to reasonably exist and be reachable, because that life didn't require a separate highly unlikely OoL event.
I think SETI should focus on stars that look like the Sun's siblings.
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Forgive my ignorance here, but at such an early period in the solar system, wouldn't there have been few if any rocks with sufficient gravity for life to form on?
I mean we as humans have a way to approach light speed (with nuclear explosions) today, with our primitive technology. Anything even 500 years more advanced than us can probably travel very long distances.