[antonvs]

Is is possible? Yes. Is it likely compared to the alternative? No.

Comets are theorized to have brought us water from within the solar system, so the comparison doesn't really apply unless life evolved in our solar system but somewhere other than Earth.

Here's some of what has to happen for life to cross naturally between planets in different solar systems:

0. Life evolves somewhere else

1. Somehow, some of that life ends up on an asteroid heading out of that solar system. The mechanism for this is pretty iffy. In the case of the ISS, we took those bacteria with when we launched, and they still haven't left Earth's gravity well.

2. That life survives the multi-millennia trip to another solar system, on an airless rock exposed to interstellar radiation for millennia.

3. The asteroid with life on it happens to hit a planet that can support that life.

4. The life isn't all destroyed in the blazing fireball caused by its descent through an atmosphere.

Each of those events has a probability, most of which are very low. To get the probability of the overall chain of events, multiply all the probabilities together, giving a much lower probability.

For life to evolve on Earth, all that's needed is step 0. Therefore, without even trying to assign actual numbers, we can see that the probability of life having originally evolved on Earth is far higher than it having come from somewhere else.

[JoeAltmaier]

Bad conclusion. There are billions of 'other planets' and only one Earth. You'd have to add up the marginal probabilities of all the other scenarios, to compare with just the Earth scenario.

[AnimalMuppet]

But of those billions of other planets, as they get further and further away, the probability of an asteroid from them finding Earth goes down. So does the probability of life on the asteroid surviving the trip.

I think that the number of planets within a distance R is O(R^2), but the probability of an asteroid from such a planet reaching us is O(1/R^2). But if the probability of life on the asteroid surviving the trip decreases with a longer trip, the net effect is probably that the further-away planets contribute little to the probability of life reaching us in this way.

[JoeAltmaier]

The panspermia idea is, that it started somewhere and somewhen, and has been planet hopping ever since. We only need one nearby life-bearing planet to seed earth; it got seeded from further away and so on.

Similar to how ancient Romans wore silk but didn't have trade with China. It only took each trader to trade with their neighbor, and silk could travel thousands of miles.

[AnimalMuppet]

OK, but at that point, the "billions of planets out there" argument doesn't work. You're left with the probability of it transferring from a nearby planet.

[JoeAltmaier]

Right, and the inevitability of it being present in a nearby planet went way up. Because once it started somewhere (and there are billions of 'somewheres') it would spread like a virus and infect every possible receptive environment.

The statistics is hard. No simple counting of planets is enough. All the vectors have to be accounted for.

[AnimalMuppet]

No, the inevitability of it being present in a nearby planet went way up if the probability of it moving from one planet to another planet is decently high - and not otherwise.

[antonvs]

> You'd have to add up the marginal probabilities of all the other scenarios, to compare with just the Earth scenario.

There aren't even close to billions of other planets that could have been candidates for sending a life-bearing asteroid to Earth. Some of the restrictions are:

1. A planet capable of having developed life.

2. A planet close enough for an asteroid from it (I have trouble typing that because it's such an unlikely scenario) to have reached Earth around the time that life on Earth began. That was ~4 billion years ago, which seems like a lot, but consider that if the recently spotted asteroid 1I/2017 U1 had come direct to us from Proxima Centauri, the closest star, it would have taken at least 48,000 years to get here. That puts a limit on candidate sources of less than 100 times the distance to Proxima, and limits the number of star systems to ~2 million.

Even if you multiply the probabilities in question by a factor of 1e6 to 1e7, it's not going to change the conclusion.

[JoeAltmaier]

Add in : asteroids in close orbits around suns developing life elsewhere (or here). Precursor chemicals to life surviving the asteroid trip even if whole organisms do not (accelerating life development on Earth by millions/a billion years).

We started with, it took an Earth to develop life, but if it came from elsewhere we'd have to expand our ideas to include all other possible sources of life, not just Earthlike. Because our premise changed.

[wil421]

How is your list any more remarkable than life starting on Earth in the first place?

What’s the probability of adding water and heat to a planet then waiting a few billion years for life? I hope we can send some probes to Saturns moons and find out. Even Mars could have a trace of life in the dirt.

[PeterisP]

Assuming that the above list has a 10% chance of happening, that would be 10 times more remarkable than life starting on Earth in the first place; since you'd have the probability of adding water and heat to a (non-Earth) planet and waiting a few billion years for life and that whole list as well.

[bdamm]

We don't know precisely what the ideal concoction is for life to get started, nor can we measure how close to ideal our planet is relative to others. So maybe Earth is not the most efficient planet at developing life, and in that case, the probability equation could tip back to panspermia.

[antonvs]

> How is your list any more remarkable than life starting on Earth in the first place?

Because the list includes that probability and adds a number of other extremely unlikely events that have to take place.