What if it is extraterrestrial in origin but also exists on earth (originally coming from elsewhere)? How would they know the difference then?
We haven't yet found any organisms on Earth that have evolved independently, so an organism found in space that has a different genetic structure from Earth life would be a good bet for being alien in origin.
An alternative, albeit unlikely, possibility is that Earth life was originally seeded from somewhere else - panspermia, https://en.wikipedia.org/wiki/Panspermia.
In that case, Earth life and (some) alien life would have a common origin, and we'd have to examine the differences in its DNA to find out more about its history.
Even in that case, it ought to be possible to determine that the organism had branched off from Earth life long enough ago to make it a candidate for alien life.
Maybe there was a Prometheus-esque comet with some single celled life in it.
[1]http://time.com/5033026/mysterious-object-confirmed-another-...
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.
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.
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.
The statistics is hard. No simple counting of planets is enough. All the vectors have to be accounted for.
You keep trying to say "but big numbers of planets, and big amounts of time" to make this idea reasonable. But it doesn't work. You have to deal with how low the probability actually is of it happening from one planet in one year. If that's low enough, you can't salvage it just by saying "billions of years and billions of planets".
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.
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.
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.
Because the list includes that probability and adds a number of other extremely unlikely events that have to take place.
I've seen this hypothesis before. I've also seen the oft repeated assumption that other intelligent life might be out there, more advanced than ours.
Crazy though: what if we are the most advanced life in th universe?
It would be plausible for us to be alone, and it would be plausible for us to be one of the less advanced lifeforms among many, but it would be quite surprising if we'd to be the most advanced civilization among many; since many/most of them would have gotten a significant head start.
One theory I'm fond of is that we are not alone, but life is so rare that the distances between intelligent civilizations are too vast for us to ever observe them (or vice versa).