* an iron core that produces a magnetic field surrounding the planet, which allows the atmosphere to remain in place, rather than being blasted off the planet by solar winds.

Not only that - the outer core is also molten to a large degree. Without it, we wouldn't get such a strong dynamo effect.

YES. A major one I forgot. This is the first thing I think about when I hear about colonizing Mars, which has no iron core to speak of.

And timing! With humans only being 150,000 to 200,000 years old, and really only intelligently processing input from the stars for less than a century, we're basically talking about two people trying to wave at each other from airplanes that aren't even flying over the same country.

Isn't Homo Sapiens > 300K years old? Not to detract from your wider point which is completely valid.

It's a spectrum, really. Homo sapiens are generally about that old, but when you start talking modern anatomy, behaviors, etc, what we would recognize as "human" is much more limited.

If anything, what you bring up kind of goes right along with my point! There may be life out there, but what is life, or intelligent life for that matter? How would we know what we were looking at if we even saw it?

> what is life, or intelligent life for that matter?

Dolphins?

> How would we know what we were looking at if we even saw it?

We try very hard not to see it!

We only know one system that "worked", and thus may have a biased view of what conditions can create advanced life.

The article mentions "toxic atmospheres", but oxygen was toxic to 99% of the existing life when it first started increasing on Earth. Life switched strategies to use it instead of choke on it. "Toxic" is relative. The other factors you mentioned may have similar issues.

Some bacteria can thrive under high radiation because they have four nuclei and related error correcting. It costs some energy to have that feature, but that's not a show-stopper to the critter.

I don't see this as black and white (life either happens or it doesn't at all).

My pet theory is that there have been many places where life took root, in very simple form, but failed to hit the "home run" like the Earth did, because the other factors stifled it.

that's nuts. ... do we just need more nuclei to be immune to cancer or is that a stupid question?

Could be. In the wild it may not be advantageous because the extra hardware would make you too slow to compete relative to those who are fast but live short. Living on a planet where everyone is in the same radiation boat could make such a feature worth it.

Such efficiency issues may not matter to modern humans. However, changing that would be too big of an upgrade to our current system to simply bluntly "add".

That's a bit too earth-centric at this point. Could be life on the gas giants for all you know or elsewhere in other esoteric configurations.

Douglas Adams - a puddle

Imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!'

> The Moon, just the right size, just the right distance, to keep us in stable rotation, with seasons.

I wouldn't mind an explanation on that one, seasons are a result of the axis tilt of the planet, not something to do with the moon.

Without the moon the tilt of the axis of rotation of the Earth to the ecliptic plane would vary a great deal more than it does today, and the axis of rotation would sometimes lie on the ecliptic, which would mean 6-month-long (very hot) days and (very cold) nights.

The moon's orbit acts to keep the Earth's obliquity in a very tight range, between 22.1 and 24.5 degrees (where 0 means perpendicular to the plane of the Earth's orbit).

Keeping that obliquity angle range keeps the seasons roughly what they are, avoids the extremes that come with a 90 degree obliquity, and stabilizes climate. An obliquity angle of zero might work too, but I think life on Earth is much more interesting with seasons than without.

Also the moon provides tides. Which meant that the first billion years of wet Earth, every grain of sand on every beach was a little chemical laboratory of wet/dry, sun/shade, hot/cold and with local conditions of pH and salinity. An organic chemistry lab for trying to invent life.

Without the moon it would have just sat stagnant.

Definitely the tides are a huge factor in the development of life but given the fact that the earth would still rotate the wind would still agitate the water and the waves would come in to the shore. Life mostly developed in the oceans from what I've read, not on the land/water interface though there is a good chance that without the tides the first animals to really adapt to living on the land would have had a much harder time to make the transition.

This creature still lives like that:

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

Super interesting, a bit like a planet sized phase-locked-loop. Given the predicted effect of a couple of degrees centigrade on average on our climate you have to really respect the impact a degree more or less in axial tilt would have. Thank you for the explanation.

At least three astronomic cycles affect climate on Earth: the varying eccentricity of Earth's orbit around the Sun, the varying obliquity of the Earth's axis of rotation, and the precession of the equinoxes. The sum of these waveforms are known as the Milankovitch cycles, which is a theory about the correlation of glacial/interglacial periods with those astronomic cycles. I'm not sure what the effect of these cycles would have been before the current ice age, but in this ice age it really does seem like glacial and interglacial periods correlate with the Milankovitch cycles. Specifically, interglacials correlate with maximum insolation during the summer in the Northern hemisphere, specifically around the 65th parallel, while glacial periods coincide with minimum insolation in the summer of the Northern hemisphere -- not every maximum/minimum leads to a state transition, of course.

Is it possible that we have too narrow definition for “life” as in “similar to what we define as life on Earth, and by proxy what we see as life in humans”?

I find that argument a bit difficult. Yes this is how we evolved, but do we have any evidence that that's the only constellation? Eg is having the moon just a lucky coincidence or a necessity. We can argue why it was necessary for us to come to be the way we did, but extrapolating to make that the only option seems rather a stretch.

has someone adjusted that famous Carl Sagan equation taking into account the above points?

Do you mean the Drake equation? The relevant factor would be n_e "the average number of planets that can potentially support life per star that has planets"

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

The thing with the Drake Equation is that it sort of plays with our lack of intuition around extreme probabilities. It includes some terms around the size of the universe and the number of planets that we (now) know are huge. And some others that we suspect aren't all that unlikely (like evolution of "life" in some form or other). But then it includes terms that, given a sample size of one, we really have no idea about--and just because 1 in a trillions odd seems a hugely conservative guess doesn't mean it's reasonable.