I think that number was thrown out there as a "small" percentage, to highlight the (literally) astronomical numbers of possibilities. One model of getting to some percentage though is called the Drake equation: https://en.wikipedia.org/wiki/Drake_equation

The problem is, as you said, that it seems Drake and others like the sound of billions of stars in billion of galaxies, but when it comes to an unknown number for existence of life in a planet they pick a very optimistic number, to justify their thesis of abundance of life in universe.

Maybe they take the earth as an example, the life existing under very harsh conditions and deduce that it should be a good probability given enough time. But I am skeptical of this.

If you plug values in drakes equations ou can make anything comes out. But historically with any like real numbers you get many civilizations right up until you state how long each civilization lasts. This is why Great Filters are such an important topic of discussion.

Try a calculator [1] and puch in your pessimistic and see what chances you actually get. It is really hard to get 10^20 without just getting 0, and we know the chances are higher than 0.

1 - http://www.bbc.com/future/story/20120821-how-many-alien-worl...

The calculator link you gave is doing exactly the same thing, smallest percentage is 1%. It doesn't make sense.

Well sorry, for that. Get a piece of paper and try it. The drake equation isn't too hard. It is just a bunch of probabilities multiplied by an amount of stars. You can even add your own terms if you don't like the one you have, but generally doing this takes yo further from the science...

Anyway, Here is a simple description I snagged from google search [1]:

> The Drake Equation is: > N = R * fp * ne * fl * fi * fc * L > > where: > N = The number of broadcasting civilizations. > R = Average rate of formation of suitable stars (stars/year) in the Milky Way galaxy > fp = Fraction of stars that form planets > ne = Average number of habitable planets per star > fl = Fraction of habitable planets (ne) where life emerges > fi = Fraction of habitable planets with life where intelligent evolves > fc = Fraction of planets with intelligent life capable of interstellar communication > L = Years a civilization remains detectable

Now grab a pen and cocktail and make the math do whatever you want. I think that is you stick to realistic numbers you will get reasonably high values. But we might just disagree there.

[1] http://www.astrodigital.org/astronomy/drake_equation.html

well, lets say fi < 1e-9 fl < 1e-12 and its a mostly a desert outside.

My post had the formatting screwed up, but you didn't even try to show your work. You are just presuming what you think or want to be true without putting even the theoretical work into it.

Something interesting I found in the wikipedia page:

> One reply to such criticisms is that even though the Drake equation currently involves speculation about unmeasured parameters, it was intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.

The existence here on earth of fungi growing within the melted down Chernobyl reactor and possibly using Gamma radiation for it metabolism, makes me a bit more optimistic.

https://en.wikipedia.org/wiki/Radiotrophic_fungus

The difference is that existing life adapted to those conditions. It doesn't mean that it could start in them.

Essentially, we have no clue. I'm optimistic that a bacteria like level is quite usual (like > 1e-6), but it baseless optimism. IANAA IANAB, IANAAB, ...

This is the same problem that has the Drake equation: we don't have a good estimation of most of the factors. https://en.wikipedia.org/wiki/Drake_equation#Criticism

Carl Sagan talks about this point in Cosmos. Life arose here on Earth pretty quickly after it cooled. So the thought is that it's not difficult for life to appear on favorable, habitable planets. A probability of 0.1% may be far too low.

Remember too, an earth-like planet can have lots of organic chemicals floating around, and you only have to hit the right combination once over millions of years to end up with life.

A sample size of 1 means: not a clue.

True that. The argument (at that level) is speculative. But sometimes it can be interesting to speculate.

Even the most basic life seems super complicated to me, I am not sure the first spark of life has that high probability of happening even under right conditions. But maybe I am over estimating its complexity.

People have studied this, and basic building blocks (amino acids) arise spontaneously when electric current is applied to chemical soups for example. Also when we look around today, we see the end prodcut of billions of years of evolution, we don't know what truly primitive life may have looked like in the first few millions years, life forms even more primitive than viruses, say.

Still, a few amino acids are far far away from proteins, let alone all proteins that work together and makes a simple living cell. Or mechanisms like: https://www.nature.com/scitable/topicpage/mitochondria-14053...

it might still be possible "given enough time".

Some of the chemo-synthesizing lifeforms around volcanic vents might be close.

These days we have an industry that makes DNA "oligo"'s that should be somewhat close to how life would have looked at some point in the distant past. The chemical reactions involved in the entire lifecycle are entirely understandable with university chemistry knowledge. Starting the reactions takes a remarkably small number of chemicals present, and none of these are too far fetched.

You have to keep in mind that, on earth, today, life cannot just exist. It must compete successfully with other very complex lifeforms. This effectively imposes a minimum complexity level that any lifeform must attain to survive in the current environment. But, conceptually, there are many things that would count as life even though they have zero hope to exist on earth (outside of labs). Such simple lifeforms today can only exist in places like the clouds of Venus.

One explanation is that life was seeded from space by spores of some sort, and they may be quite common throughout the universe.

We're talking about radio in the article. Which means we're talking about a very small subset of possible life.

I suppose that's why "even if" precedes a number most people will accept as a common enough phrase: one in one thousand.

Exactly. It's rather hard to estimate the probability of an event for which there is only one known example.

Life started on Earth relatively early in it's history, so a high percentage isn't unreasonable to assume.

It took 4 billion years after that and several mass extinction events for a species to evolve that can send radio signals (and so far only briefly ), so even on an astronomical scale we might be rare.

Agreed, also consider the importance of a large moon in our system, which cause massive tidal flooding, and churning geology for all these years. How important was periodic deposit of water based life on dry land for the eventual evolution of a technological civilization? One might suppose very important.

Thus one could posit that technological civilizations, requiring both a planet in the habitable zone of a star and also an appropriately large satellite companion, could be even less probable. It would be interesting to know how probable similar earth/moon systems are in the universe.