Neat and convenient coincidences of planet Earth (and other planets)

[Yora]

I was just talking with someone about living conditions on other planets, and a few times things came up that just happen to be pure coincidences that have no universal law behind them

Most people probably never think about it, but Earth's moon happens to have 1/400th the diameter of the sun and a distance from Earth 1/400th the distance to the sun. By purely random chance, the Sun and moon look the same size in the sky. A total eclipse in which the sun's corona is visible around the shadow of the moon is an astronomically rare phenomenon in the universe. And getting that on a planet that is also inhabited? Most people probably never think of it, but this is one of the most unique natural events in the galaxy.



Related to it: Earth having a gigantic moon. None of the other inner planets has anything remotely comparable. (Though Pluto has Charon, which in comparison is even bigger. So it might not be that super rare throughout the universe.)

Another fun one that is even more obscure, is that the Earth's atmosphere has 21% oxygen. The burning of wood and paper requires a minimum of 14% oxygen. Human bodies (that have evolved to work optimally at 21%) start to have impaired thinking below 14% and only die below 6%. It's very possible that there are countless of planets that are full of life but don't have fire. (They might have other things that burn, but there's only so many compounds that can easily be made biologically from common basic elements.) That certainly would be an obstacle to develop industrialization.

Speaking of which. Wood. After plants started covering the land surface of the Earth, wood didn't evolve for a 100 million years or so, or about the first 20% of the existance of plants on land. Most plants are doing perfectly fine without wood, and there were plenty of big vertebrates on land before that. There is no necessity for a planet that produces sentient life to have wood. Aside from the making fire thing, I think the whole process of tool making and construction might hit a serious dead end if all you have to work with are stone and bone.
Also related: It still took millions of years after the evolution of wood for the evolution of wood consuming microbes. This lead to huge buildups of dead plants that didn't rot, which were buried and turned into fossil fuels over millions of years. Not many planets with complex biospheres might have much coal and oil, which also could seriously hamper technological progress.

I guess you could also count coasts and beaches. Most planets have no surface liquid, but it's believed that many do. But it's also hypothesised that planet bigger than Earth would likely have so much water that the entire surface would be ocean floor. It seems that the theory that life started on Earth on the ocean floor is more popular now (have not heard anyone talk about tidal puddle on the coast for a long time now), so they could have plenty of life and maybe evolve sentient species from something like dolphins.
But I think without land to make fire, developing metalworking is out of the question.

Speaking of another planet, there's the cool idea that sounds a bit far out, but also totally plausible, of having floating cities (or at least stations) hanging from big balloons in the skies of Venus. This also seems inviting only because of a pure coincidence that at altitude where the air pressure at Venus is the same as on Earth, the average temperature is a comfortable 23°C. It's neat, but other planets with crushing atmospheric pressures could easily have freezing cold or boiling hot temperatures that would make the whole idea impossible.

I guess I am kind of a party pooper for space colonization and meeting friendly aliens, but I think this is still a really interesting subject. Any other things about Earth that are actually extraordinary when you think about them?

[factotum]

If you think about it, the eclipse thing is kind of cool, but also makes eclipses on Earth relatively rare. If the Moon were much larger in the sky than the Sun then eclipses would be far more commonplace, which would also reduce their mystique somewhat.

I don't think the oxygen thing is that special, though. The actual percentage of oxygen in the atmosphere has varied throughout history--it's been anything between 10% and 35% over the course of the last half billion years or so, and it's declined something like 0.7% in the last million alone. Furthermore, the actual percentage of oxygen required for something to burn is dependent on pressure, temperature, *and* whatever it is you're burning (hydrogen gas will burn much more readily than paper!).

[Yora]

If you think about it, the eclipse thing is kind of cool, but also makes eclipses on Earth relatively rare. If the Moon were much larger in the sky than the Sun then eclipses would be far more commonplace, which would also reduce their mystique somewhat.

Oh, yeah. The day-night cycle on the moons of Saturn and Jupiter is probably quite crazy. During a new moon there is no daylight at all, and during full moon you have this huge reflective planet covering half the sky.

[hamishspence]

Jupiter would look a bit more impressive from the innermost big moon (Io) than the outermost (Callisto).

It would be 18.5 degrees wide from Io. Big, but not super-big - not "Yavin-esque". For comparison, the two brightest stars in the constellation Leo, at either end (the "head" and the "tail" are 24 degrees apart.

On Io there would be devastating amounts of radiation from Jupiter. On Callisto, there'd be next to none (more than Earth's background radiation, but not enough to worry about), so watching Jupiter would be a lot safer, if less impressive.

Apparently all the big moons are tidally locked to Jupiter, so it will never "move in the sky" from the point of view of an observer on one of them.

During a new moon there is no daylight at all, and during full moon you have this huge reflective planet covering half the sky.

During "full moon" (full moon from the point of view of the observer in Jupiter's atmosphere, at least) when on the moon itself, there would be a huge black sphere covering a chunk of sky - since the planet would be between the observer and the Sun. During new moon, there'd be no daylight, but the planet would be fully illuminated.

[Peelee]

I was just talking with someone about living conditions on other planets, and a few times things came up that just happen to be pure coincidences that have no universal law behind them

Most people probably never think about it, but Earth's moon happens to have 1/400th the diameter of the sun and a distance from Earth 1/400th the distance to the sun. By purely random chance, the Sun and moon look the same size in the sky. A total eclipse in which the sun's corona is visible around the shadow of the moon is an astronomically rare phenomenon in the universe. And getting that on a planet that is also inhabited? Most people probably never think of it, but this is one of the most unique natural events in the galaxy.

Is is really that rare? Or is it just that rare at this exact moment in time? The Moon is getting farther away, after all; it may only be a couple cm a year, which is nothing on our time scale, but astronomical time scales are radically different. Let's say that the Moon will get 2 cm farther away from the Earth every year at a constant (which is not true, but holy crap will it make the math easier). After 1 billion years, it will be an additional 200,000 km away from the Earth. One theory posits that when the Moon first formed 4 billion years ago, it was only about 40,000 km away from the Earth.

It's not really a coincidence that the Moon is 1/400th the diameter of the Sun and a distance from Earth 1/400th the distance to the Sun. The coincidence is that this happened during human's time period. And even then, it's only really a coincidence because we, as a species, are super egocentric.

[Fyraltari]

The moon is huge though. As far as I know we've never found a planet with a natural satellite that size. If it were alone, it'd be a dwarf planet, and not the smallest one at that.

I remember how in Foundation that is one of the identifying features of the lost Earth and it is theorized that the tides it created helped bring radioactive material closer to the surface which is why life mutated more on Earth (hence the suspicious lack of aliens in the serie).

[Peelee]

The moon is huge though. As far as I know we've never found a planet with a natural satellite that size.

Do you mean compared to the planet it orbits? Because the Moon isn't even the biggest moon in our solar system, let alone ever discovered.

Do t get me wrong, it (and bigger moons) are definitely huge. But dwarf planets can still be moons if a big enough planet snags them in orbit.

[Fyraltari]

Do you mean compared to the planet it orbits? Because the Moon isn't even the biggest moon in our solar system, let alone ever discovered.

Yes, I do.

Do t get me wrong, it (and bigger moons) are definitely huge. But dwarf planets can still be moons if a big enough planet snags them in orbit.

Dwarf planet is just a way to say "big asteroid" without hurting the feelings of the Pluto-as-planet supporters to much.

[Peelee]

Yes, I do.

Then yeah, absolutely. Dude is a chonker for us.

Though I'll defend dwarf planets, since they meet more planet criteria than asteroids. And Im apathetic towards Pluto.

[hamishspence]

Yup - "rounded by gravity" is the main break point between dwarf planet and asteroid.

Most of the really big moons could do what Mercury did when it comes to "clearing out their orbit" if they were where Mercury was.

[Fyraltari]

Then yeah, absolutely. Dude is a chonker for us.

I do not understand.

[Peelee]

I do not understand.

Indeed, I agree. That celestial body is indeed quite a large size compared to the parent planet.

[Fyraltari]

Indeed, I agree. That celestial body is indeed quite a large size compared to the parent planet.

Ah, the Moon is the dude in this sentence? That makes sense, now.

[Peelee]

Ah, the Moon is the dude in this sentence? That makes sense, now.

Sorry, I may have gotten a bit too colloquial there.

[Fyraltari]

Sorry, I may have gotten a bit too colloquial there.

Honestly, the colloquialism wasn't the issue as much as "the Moon" being female in French I'm not used to anthropomorphizing it as a man.

[Peelee]

Honestly, the colloquialism wasn't the issue as much as "the Moon" being female in French I'm not used to anthropomorphizing it as a man.

Ha! Actually, I tend to use "dude" gender-neutrally, though I try to avoid that when I know another party in the conversation doesn't.

[Khedrac]

Ha! Actually, I tend to use "dude" gender-neutrally, though I try to avoid that when I know another party in the conversation doesn't.

I used to have a colleague who would talk about a "night out with the guys" - every person involved was female which tended to confuse me.

More (or less) back on track, one of the huge coincidence factors is water. I know this is less about our planet and more about our universe but...
Water breaks (well bends into a pretzel) a huge number of physical "laws".
Water cheats on thermal expansion - unlike nearly everything else water's version of "the colder the more dense" isn't simple. For liquid water the densest temperature is about 4 degrees C, and then on forming ice the density drops again quite a lot. This means that lakes freeze at the top first and the ice then forms a protective cover, resulting in warmer (4 degrees C) water being trapped at the bottom of the lake where life can survive in it unless the lake manages to freeze solid. Without the density inversion the coldest liquid sinks to the bottom as does any frozen liquid - a good way to ensure life does not survive the first winter.

And that's just the start of water's weirdness.

[Peelee]

I used to have a colleague who would talk about a "night out with the guys" - every person involved was female which tended to confuse me.

More (or less) back on track, one of the huge coincidence factors is water. I know this is less about our planet and more about our universe but...
Water breaks (well bends into a pretzel) a huge number of physical "laws".
Water cheats on thermal expansion - unlike nearly everything else water's version of "the colder the more dense" isn't simple. For liquid water the densest temperature is about 4 degrees C, and then on forming ice the density drops again quite a lot. This means that lakes freeze at the top first and the ice then forms a protective cover, resulting in warmer (4 degrees C) water being trapped at the bottom of the lake where life can survive in it unless the lake manages to freeze solid. Without the density inversion the coldest liquid sinks to the bottom as does any frozen liquid - a good way to ensure life does not survive the first winter.

And that's just the start of water's weirdness.

Science: "Cold contracts, heat expands."

Water: "....no."

[Khedrac]

Science: "Cold contracts, heat expands."

Water: "....no."

Science: "Now on to 'Specific Heat Capacity'; in general..."

Water: "....no."

(Water has a ridiculously high specific heat capacity - how much energy it takes to change its temperature - for what it is, which probably has a major moderating effect on the earth's temperature and climate.)

[Eldan]

Water still mostly behaves normal outside of a tiny bump in the curve.

[Peelee]

Water still mostly behaves normal outside of a tiny bump in the curve.

Sure. If you ignore capillary action. Or the Mpemba effect. Or that it should be a gas at room temperature. Or...

Science: "Ok, so how it works is-"

Water: "...no."

[Khedrac]

Water still mostly behaves normal outside of a tiny bump in the curve.

If you mean the temperature : density ration it's actually 2 bumps - a small one for liquid water and a larger one for the liquid solid transition.

And I agree that they are just bumps in the straight line, but we probably wouldn't be here without them as they permit complex life to survive in shallow water temperate environments, which makes them exactly the sort of thing this thread is about.

[Tvtyrant]

How about the fact that life exists so far into the cold category we survive at highest 315 degrees hotter then absolute zero, but 5,480 degrees cooler than the majority of mass in the solar system? We are much closer to being made of vacuum levels of energy than what the majority of atoms exist at.

[Peelee]

How about the fact that life exists so far into the cold category we survive at highest 315 degrees hotter then absolute zero, but 5,480 degrees cooler than the majority of mass in the solar system? We are much closer to being made of vacuum levels of energy than what the majority of atoms exist at.

That's still effectively due to water. In all environments on Earth, no matter how cold or hot or acidic or hostile to life in any other way, if there is liquid water, there is life.

[Peelee]

When I use those numbers I get only 20,000km in 1 billion years. From what I've heard it's believed that our sun will expand and burn the Earth to cinders in just a few billion years, so the astronomical timescales here aren't necessarily that big. The sun-moon size correspondence might well be true for the entire time that the Earth is able to sustain life.

You got 20,000 because I am stupid and calculated 100 meters to the kilometer. Regardless, there's still that 40-80km distance 4 billion years ago, while life on earth existed at least 3.5 billion years ago.

It's not a coincidence that it's happened in the universe, it's a coincidence that we are around to see it.

[Peelee]

Was that not part of OP's point?

No, that was my point:

Is is really that rare? Or is it just that rare at this exact moment in time?

Regardless of whether you look at "rare across time" or "rare across space" it is still likely that even if there is intelligent life elsewhere, the experience of watching a total solar eclipse is one of the most unique* experiences in the galaxy.

That's fair. Though for all we know, all life-sustaining planets that eventually breed intelligent life might do so while the moon is an equal ratio size and distance for whatever reason. I doubt it, but it's certainly possible.

And I don't consider it bad grammar at all.

[Tvtyrant]

No, that was my point:

That's fair. Though for all we know, all life-sustaining planets that eventually breed intelligent life might do so while the moon is an equal ratio size and distance for whatever reason. I doubt it, but it's certainly possible.

And I don't consider it bad grammar at all.

It might help explain our very comfortable tidal forces. Life is possible with some really extreme environments; intelligent life not so much.

And I mean intelligent as in chordates with memories, not necessarily humans.

[Bohandas]

Speaking of which. Wood. After plants started covering the land surface of the Earth, wood didn't evolve for a 100 million years or so, or about the first 20% of the existance of plants on land. Most plants are doing perfectly fine without wood, and there were plenty of big vertebrates on land before that. There is no necessity for a planet that produces sentient life to have wood. Aside from the making fire thing, I think the whole process of tool making and construction might hit a serious dead end if all you have to work with are stone and bone.
Also related: It still took millions of years after the evolution of wood for the evolution of wood consuming microbes. This lead to huge buildups of dead plants that didn't rot, which were buried and turned into fossil fuels over millions of years. Not many planets with complex biospheres might have much coal and oil, which also could seriously hamper technological progress.

On a related note, it's long been my opinion that in alien invasion films this should be the kind of thing that the aliens are after, rather than something like water or metal that's stupid common on uninhabited planets

[gomipile]

On a related note, it's long been my opinion that in alien invasion films this should be the kind of thing that the aliens are after, rather than something like water or metal that's stupid common on uninhabited planets

Back in the 1980s some science fiction writers wrote a treatment on how they'd fix the V miniseries in a proposed sequel series. Part of it was revealing the aliens to be the descendents of an intelligent dinosaur species who had escaped Earth during an extinction event. They're back because Earth still "feels right" to them biologically, so they'd rather conquer Earth than live anywhere else.

[Tarmor]

On a related note, it's long been my opinion that in alien invasion films this should be the kind of thing that the aliens are after, rather than something like water or metal that's stupid common on uninhabited planets

I agree that coming to earth for water or metals is stupid. If they have the technology for travel between stars, I'd expect them to be using energy sources well beyond oil and coal too.