On the abundance of extraterrestrial life after the Kepler mission Amri Wandel

Some recent calculation of the Drake Equation with estimates of the likelihood and logevitiy of civilizations

Related: The Great Filter and Planets in the habitable zone, the Drake Equation, and the Great Filter

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So what happens if we find all these biologically feasible exoplanets that just don't have any life on them?

BTW, you might want to give Matthew Stewart's book Nature's God a read. He points to the unexpected fact that many of the Americans in revolutionary times who wrote down their thoughts on the matter believed in "space aliens," as Stewart calls them, on exoplanets throughout the universe, and that these colonial Americans considered this arbitrary belief "rational" because of the peculiar way early modern philosophy originated from the revival of Epicureanism around the beginning of the 17th Century.



what happens if we find all these biologically feasible exoplanets that just don't have any life on them?

That would be evidence for an early filter over a late filter, so it would probably be good news.

s/probably/really, really/

This is indeed unexpected. It appears the belief in aliens has been waning instead of waxing as we find out more and more about the universe.

"So what happens if we find all these biologically feasible exoplanets that just don't have any life on them?"

We go forth and put some, of course!

How very human of you,

How very human of you.

...Why not Zoidberg?

Due to the the decapodian mating tendencies (which include standing on beaches attracting their mates after which they die) I don't think they would be driven to cause life on other planets. However, it might be a good idea to send the mutants from the sewers. They could reproduce and improve their evolution within the constraints of that new environment.

My own favorite hypothesis goes like this: Our universe is most likely to be the simplest one that contains me (us, observers, conscious beings, whatever your favorite rendition of the anthropic principle). It is not likely to be much larger than necessary for creating me. The reason it is as large as it is, then, is that that's what it takes. The answer, then, is that something like me exists only once. More would be a waste of universal size and/or complexity, and Occam forbids it.

Is this as crazy as it sounds?

That doesn't sound crazy at all. I mean at least not to me. At least not at 1 o'clock in the morning. It sounds like the most likely solution given complexity considerations. It is the most likely Tegmark 4 instance with weights inverse to complexity/size as in Solomonoff induction.

Yeah, pretty much. It would be my default assumption, but only if I was completely ignorant about anything beyond the atmosphere. And if we're going to put ourselves in that position, it's not entirely unreasonable to conclude that Marduk grew the world from a weed.

If you are referring to complexity, then I think it's almost common sense.

So many things have to come together for life to happen and for intelligent life to rise up and dominate as we have done. How many other places this has taken place could be very interesting to discover as time goes on.

Estimates? Here some quotes from the paper on those "estimates":

"Also Lc, the average longevity of a communicative civilization, cannot be inducted from its short history on Earth and could be anywhere between a few hundred years and billions of years."

"Bayesian analysis demonstrates that as long as Earth remains the only known planet with biotic life, any value could be assigned to Fb"

You tell me how valuable these estimates are, in view of their precision....


We may not have good measures for estimating Fb or Lb let alone Lc, but the Kepler mission gives us a pretty good estimate of Rb. You should update your estimate of the closeness of a biotic planet depending on whether your Rb prior was higher or lower than the result.

That is true. However, if "any value could be assigned to Fb", then any value can be made to come out of the Drake equation, except for an upper bound. Updating on Rb can shift around that upper bound, but it tells you nothing about the really small values that decide whether we are alone in the universe or not.


Two questions:

I assume a biotic planet is defined as a planet which currently contains living bacteria or something living derived from bacteria. Is this correct?

I think I've spotted an error, but wanted to check. On page 3, Rb is defined as "the rate at which stars suitable for the evolution of biotic life are formed in the Galaxy." Based on the parameters given, shouldn't this be the rate at which planets suitable for the evolution of biotic life are formed in the Galaxy?


Also, seeing stuff like this really bugs me:

top of page 2: "Recent analyses of the Kepler statistics showed that about 20% of all Sun-like stars have Earth-sized planets orbiting within the habitable zone [Petigura, Howard and Marcy 2014]."

2nd paragraph of page 3: "Analyses of the Kepler results shows that 7-15% of the Sun-like stars have an Earth-sized planet within their habitable zone [Petigura et al., 2014]"

That's a pretty glaring error to be making. This isn't a top journal, but it isn't an obscure one either. http://eigenfactor.com/rankings.php?bsearch=International+Journal+of+Astrobiology&searchby=journal&orderby=eigenfactor

I agree. However, considering that Kepler is not actually sensitive enough to detect Earth sized planets in the habitable zone of sun-like stars, both these numbers are extrapolations and it must be assumed that the 7-15% or 20% are well within each other's error bounds.

Could the latter be a lower-bound?

'7-15%' does not look like a lower bound to me.

I wonder if the resolution is that the first is supposed to read 'the ratio of Sun-like stars to Earth-sized planets orbiting such a star within its habitable zone is around 5:1'. That would double-count stars with two such planets.

'7-15%' does not look like a lower bound to me.

If there is uncertainty in the lower bound produced by an argument, how else would you write it?

Certainly not

"Analyses of the Kepler results shows that 7-15% of the Sun-like stars have an Earth-sized planet within their habitable zone [Petigura et al., 2014]"


"Analyses of the Kepler results yields a minimum fraction of Sun-like stars with an Earth-sized planet within their habitable zone, of 11 +/- 4% [Petigura et al., 2014]"

Notice how much more labored and pedantic your version is - the sort of writing that one would not do unless one could see into the future that there would be nerds somewhere nitpicking exactly that sentence.

It is more labored, because it's attempting to convey a more complicated concept. However, the distinction is not pedantic. This is saying 'there is one fence near here, somewhere within this range'. The other statement means 'there are two fences here enclosing this range.'. These are not at all interchangeable statements.

In what setup would the difference between the two be measurable?

I dunno. Not an astronomer. But there are lots of different strategies for measuring things, which come with their own particular strengths and weaknesses, so I wouldn't be surprised if some available measures of some fraction had different inherent bounds or precisions based on available data.

(For example, in genomics, it's not uncommon to have a lower bound with a confidence interval; in fact, every GCTA study using SNPs produces a lower bound with a somewhat loose confidence interval, and this has tripped up some commentators who, upon observing an estimated heritability of, say, 0.25-0.30 for intelligence from one study, triumphantly declare that the glass is more than half-empty - forgetting that it's a lower bound, and different GCTAs using differing levels of comprehensiveness of SNPs will turn in different lower bounds and so one could easily have a GCTA estimate 0-0.20 and another 0.25-0.30, in contradistinction to twin studies with heritability of 0.5 or higher - based on how many SNPs were included and how many samples there were!

Or to take a physics example from my reading yesterday, Meehl 1990. Meehl, discussing philosophy of science & statistics, notes that in the book Atoms (early 1900s) are covered 13 different ways of estimating Avogadro's number which result in different numbers of the same magnitude but that treated in terms of random sampling error, the 13 ways would yield confidence intervals that would often exclude each other's. Surely, he asks, we would not reject the 13 consilient arguments for the existence of atoms solely because of this slight discrepancy, and instead regard the slight disagreement as purely springing from systematic error such as the differing approximations and simplifying assumptions made?)

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