How likely are we to find extraterrestrial life?

[otseng]

Many NASA scientists think we're on the verge of finding alien life.

Ellen Stofan, NASA's former chief scientist, said in 2015 that she believes we'll get "strong indications of life beyond Earth in the next decade and definitive evidence in the next 10 to 20 years."

Many astrophysicists and astronomers are convinced that it's not a matter of if we'll find life — it's when.

https://www.businessinsider.com/nasa-pl ... de-2019-11

Questions for debate:
- How likely are we to find extraterrestrial life?
- What empirical evidence is there that any extraterrestrial life exists?
- What are the implications if extraterrestrial life exists or do not exist?

[DrNoGods]

[Replying to post 19 by ttruscott]

Looks like the prior discussion I remembered is here:

viewtopic.php?t=34669&postdays=0&postor ... &start=230

I engaged in some back and forth with Guy Threepwood on SETI and the "WOW" signal, and quantified some of the issues on this and the Fermi Paradox which he also brought up.

[Goose]

[Replying to post 1 by otseng]

- How likely are we to find extraterrestrial life?

I think the primary support for this (in favor of finding life) is the sheer number of planets that we now know are out there. Some 4,500 exoplanets have now been found and the implication from these relatively recent findings (first exoplanet found was in 1992) is that there must be many billions of planets out there. This, combined with the fact that we do have one planet that supports life, suggests a high likelihood that life exists elsewhere in the universe. Statistically, the probability is very high.

• 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?

[DrNoGods]

[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.

[Goose]

[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.

Of course given the right environmental characteristics the possibility that planet can support life is 100%. This establishes a baseline of possibility. But the question asked wasn’t one of possibility. It is one of probability that we will find 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).

Granted, there are lots of planets, probably billions of them.

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.

Life does exist on one of them and there are billions of planets. It seems to me, if these premises are true, this implies the statistical probability that life could exist on at least one of them is quite low, one in a billion(s). I’m still left wondering how you are getting “the statistical probability that life could exist on at least one of them is high� from those two premises?

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.

Okay now we are getting somewhere. However, given your argument here, the probability life exists on planet X would still be one in eight or 12.5%. The number of planets that exist doesn’t change that ratio given your argument. A 12.5% probability seems relatively low that planet X would have life. Every time a planet is observed to not have life the probability that life exists on planet X decreases. Before we had the ability to make such observations of other planets, the probability that life existed elsewhere was 1, or 100% (life on earth). The moment we found no life on, say, Mars the probability that life existed on planet X fell to .5 or 50% and so on down to 12.5%. The current trend is towards zero probability given your argument here.

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.

That’s all fine. I’m granting billions of planets and willing to accept any form of life. I’m still wondering how you get to, "Statistically, the probability is very high" from your premises.

[DrNoGods]

[Replying to post 24 by Goose]

But the question asked wasn’t one of possibility. It is one of probability that we will find life.

My original comment (post 2) was:

"This, combined with the fact that we do have one planet that supports life, suggests a high likelihood that life exists elsewhere in the universe. Statistically, the probability is very high."

The comment on a high statistical probability referred, specifically, to the likelihood that life exists elsewhere in the universe, not a high probability that we would find it, which is an entirely different problem (and the question posed in the OP, but not the issue I stated there was a statistically high probability of). Given that we can't yet claim that life positively does not exist on another planet or moon in our own solar system, or that it did not exist in the past, I'd say the probability of us finding extraterrestrial life anytime soon is indeed low. It is virtually impossible outside of our solar system using the technology we have available now as we can't investigate the atmospheric spectroscopy well enough to make any inferences on biological activity, and we certainly can't get to any exoplanets with physical probes.

The closest exoplanet to us is 4.2 light years away, which is about 24 trillion miles:

https://en.wikipedia.org/wiki/List_of_n ... exoplanets

If we sent a spacecraft about the same size as the recent New Horizons Pluto probe, traveling at its speed of about 40,000 MPH, it would take almost 70,000 years to get there. And that is just the closest exoplanet. But who knows what technology may exist thousands of years from now ... we may well be able to get to the closest exoplanets with physical probes to search for life, but I agree that right now, the probability of finding extraterrestrial life is extremely low. Our only hope in the near term (next few decades) is somewhere in our own solar system where we can get to the planets with probes. Or, some kind of detection of electromagnetic signals like SETI is trying to do. But that requires intelligent life which, if Earth is any analogy, is far less probable than life in general.

I’m still wondering how you get to, "Statistically, the probability is very high" from your premises.

Again, this referred to the probability that life exists elsewhere in the universe ... not of us finding it. And I'd argue from the two premises that they do imply a statistically high probability that life in some form exists elsewhere in the universe.

[brunumb]

[Replying to post 19 by ttruscott]

The Fermi paradox: if Earth, a fairly young planet has life and life is so easy then the vast numbers of older planets should have produced ancient civilizations by now...where are they?

They could be everywhere but the tyranny of distance means that it is unlikely that any civilisation out there will ever meet up with any other.

The way human civilisation is going it is possible that it will only last for a few millennia before destroying itself. That may be the ultimate destiny for most advanced civilisations. I think that someone has actually formalised this hypothesis.

[Menotu]

[Replying to post 26 by brunumb]

They could be everywhere but the tyranny of distance means that it is unlikely that any civilisation out there will ever meet up with any other.

Based on out current understanding, maybe. But our understanding of everything is so small it's almost immeasurable. There could be all kinds of alternate means of travel, natural or unnatural, for travel in the physical form, much less other forms.

The way human civilisation is going it is possible that it will only last for a few millennia before destroying itself.

That seems over optimistic But true none-the-less.

That may be the ultimate destiny for most advanced civilisations.

Again, based on our experience (which is all we have but still).

[William]

[center]SCIENCE

COULD INVISIBLE ALIENS REALLY EXIST AMONG US? AN ASTROBIOLOGIST EXPLAINS[/center]

Link:

William: More interesting data on the subject. The part about Silicon based life could support the idea that The Earth Itself is alive - as a living intelligent entity.

[Clownboat]

[center]SCIENCE

COULD INVISIBLE ALIENS REALLY EXIST AMONG US? AN ASTROBIOLOGIST EXPLAINS[/center]

Link:

William: More interesting data on the subject. The part about Silicon based life could support the idea that The Earth Itself is alive - as a living intelligent entity.

We know how planets form.

For what reason would we even consider planets being alive? There must be some observation that you can share with us that would suggest such a thing? What would be the (suggested?) mechanism that makes planets alive?

Or is the statement that planets may be alive as valid as claiming that they were once made of jello?
I ask because I'm aware of no observation that would even make me consider either scenario. Perhaps I have missed it?

[Diagoras]

The Fermi paradox: if Earth, a fairly young planet has life and life is so easy then the vast numbers of older planets should have produced ancient civilizations by now...where are they? Hiding behind a Vulcan decree to not show themselves until we get interstellar capability?

For anyone interested in exploring the Fermi paradox properly, there are a couple of excellent YouTube videos from the Kurzgesagt team which explain it very clearly:

Part 1 (6 m 21s)
Part 2 (6m 17s)