I'd argue you are also substantially underestimating the scale and numerousness of biology: your body alone has more bacterial cells than there are stars in the observable universe - all you need is one viable cell to seed an entire planet. A planet teeming with life if it was shattered due to a collision would catapult billions or trillions of seed containing rocks across the galaxy. We have three unfathomable scales competing each other - light years in distance and 100s of billions of stars, billions of years and quadrillions or more of seed cells potentially floating in space. Of course any of these factors could he off by a few orders of magnitude but it's hard to suggest one hypothesis is more valid than other without data.

Assuming some of the life-containing mass ejected from the collision had enough velocity to fully escape the gravity of not just the colliders but the entire star system it started in, that life would have to then survive for probably hundreds of thousands or millions of years of transit _and_ the re-entry and impact on the other life-sustaining planet it hits (assuming it hits anything, or doesn't fall into a star instead).

I'm not saying it couldn't have happened, but even on a cosmic scale it sounds like it would have to be an exceedingly rare outcome.

Maybe it doesn't need to survive per-se, but the ejecta could have enough life building blocks (aminoacids, etc) to achieve abiogenesis easily again.

Reentry develops sufficient heat that any complex organic molecules are torn apart by their thermal energy. And the impact is severe enough that it would likely destroy what was left.

Tell that to the water bears: https://en.wikipedia.org/wiki/Tardigrade

Not only is their original habitat 420 K and 1 K (absolute zero) they’re described as “colony” species.

Quite interesting.

1K is not absolute zero, 0K is. 420K is 150°C, barely above boiling.

In space, your temperature may range from 0.01K to over 2000K depending on solar radiation levels over the spectrum.

You're exposed to higher levels of radiation and vacuum will tear at organic structures.

A tradigrade can probably survive a trip to mars if they're very lucky. Better to send a huge amount of them, maybe one makes it.

But interstellar distances are too vast and the time taken is too long. The DNA within tradigrades, even when hibernating, will have been torn to shreds and with no energy to repair there is no way back to life from that.

I will quote form the article:

> Back on Earth, over 68% of the subjects protected from solar UV radiation were reanimated within 30 minutes following rehydration, although subsequent mortality was high; many of these produced viable embryos.

And that is after 10 days of exposure. If it was 400 years, it's unlikely that even 1 is able to reanimate.

The hydrates species fared much worse and hydrated is more likely the state of the tardigrade when they are blown into space, rather than carefully dehydrated.

Woops:

> Some of them can withstand extremely cold temperatures down to 1 K (−458 °F; −272 °C) (close to absolute zero), while others can withstand extremely hot temperatures up to 420 K (300 °F; 150 °C)[36] for several minutes, pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space.[37] Tardigrades that live in harsh conditions undergo an annual process of cyclomorphosis, allowing for survival in sub-zero temperatures.

And we're not even talking about 400 years, we're often talking about tens of thousands

Nitpick: you certainly do not actually mean the observable universe

Bacterial cells in one adult human body: ~100 trillion

Stars in observable universe: ~1 billion trillion

You're off by seven orders of magnitude.

I did mean it but obviously my memory failed me and I was wrong

A quick DDG search says 10^13 bacterial cells in a human, 10^12 galaxies in the observable universe and 10^8 stars per galaxy. Your hyperbole is very off. The rest of the argument is not unreasonable, but not evidently convincing either. Certainly inspiring to think about though.

And I think you are underestimating how hostile space is to life. Tardigrades can make it, to some extent, for a limited amount of time. Doesn't mean all life is invulnerable to near absolute zero temperature and constant radiation. Add to that the temperatures a meteor reaches when they crash through an atmosphere, granted without much an atmosphere there isn't much to generate friction. Yet the impact would still be devastating.

It's certain we still don't have enough data to make any conclusions about how easy it's for life to spread. I'd wager it's less than expected yet in our galaxy, my guess would be that either hundreds or dozens of planets/moons contain life. Depends on the statistical distribution of the occurrence of life.

> Tardigrades can make it, to some extent, for a limited amount of time.

Yeah but likely not for tens of thousands of years in the void between space + cosmic radiation?

Also overestimating the possible speed of travel.

*life that needs water probably has a monopoly on water

4 billion years life has been on earth.