[
Replying to post 22 by Goose]
1. There are many billions of planets.
2. Life exists on one planet.
From those two premises I'm not following how you are arriving at the conclusion that, "Statistically, the probability is very high." Can you flesh this out please?
Premise #2 is to note that life does, in fact, exist on at least one planet. So we can say with 100% certainty that it is possible for a planet to support life.
Premise #1 notes that there are likely many billions of planets (based on having detected over 4000 exoplanets to date, the limited regions that have been investigated, and the number of galaxies that are observed).
Some of these exoplanets are in the "habitable" region where liquid water can exist. This does not guarantee that life exists of course, but if life can arise on one planet in the habitable zone of a star (ie. Earth), and there are likely billions of other planets, then the statistical probability that life could exist on at least one of them is high, given that these premises are true.
I am making a purely statistical argument, independent of any mechanism for how life arose on this planet, or how it might have arisen on another planet (if it ever did). There may very well be no other life outside of Earth, but since the event occurred once on a planet that is 1 of 8 planets in a solar system (ie. at least 12.5% of the planets in our solar system have life), the probability that it could happen again on another planet is proportional to the number of planets that exist. The greater the number of planets, the higher the probability the event could happen. Including moons as well (eg. there are ideas that certain moons of Saturn and Jupiter might have harbored microbial life) then the probability is even higher.
At this point we don't have the ability to investigate the atmospheres of exoplanets well enough to help with the question, and given their distance from Earth we can't get probes to any of them any time soon. There have been attempts to estimate the probability of intelligent life elsewhere (eg. the Drake equation, which can lead to such a huge range of possibilities it is practically useless).
https://www.discovermagazine.com/the-sc ... iens-exist
Sara Seager estimates that there are about 25 billion exoplanets just in our galaxy:
https://www.nationalgeographic.com/maga ... or-aliens/
The above article also has more discussion on some of the upcoming missions aimed at this question. Here is another article addressing the question:
https://www.space.com/37157-possible-re ... liens.html
Life of any kind, vs. "intelligent" life, is also a factor. But I'm referring to
any form of life with the "high probability" comment, not just intelligent life. There are millions of species of life on Earth now, and many times more than that having gone extinct, but out of those large numbers only one (humans) have evolved a level of intelligence capable of building equipment to search for extraterrestrial life (or even of asking the question). So the probability that intelligent life exists (intelligence meaning something of the order of humans) is, of course, far less than the probability that life of any kind exists elsewhere if the fraction of living entities that achieves our level of intelligence is anything close to that of Earth.