400 million years to build a planet?

[Cray]

I'm stuck writing up a planet for another science fiction setting that is easily inhabitable by humans (Earth-like ecosystem) and has native life has reached mammalian-ish levels of sophistication.

Problem: the planet is only 400 million years old. I can't change that, nor can I invoke artificial intervention. The planet is not terraformed, nor are there any ancient aliens I can invoke.

So, never mind the probabilities, is 400 million years enough time for a terrestrial planet to solidify and produce an oxygen atmosphere, assuming life started early?

If I'm reading the history of Earth article on wiki correctly, it takes some 10 to 20 million years to form a crust and oceans. However, Earth's environment got more stable around the beginning of the Archean 500 million years after formation. The biggest spike in oxygenation happened in a 300 million year period. Most multi-cellular life developed in a 500 million year period...actually, it was pretty sophisticated within 250 million years of the Cambrian explosion. So, it LOOKS like a planet could get habitable in a 400 million year period, but I'd like the story to make sense.

I'm also constrained by canon to a 0.92G surface gravity, 16% land area, and Earth-like atmospheric pressure and composition, but I can vary density, diameter, and planetary composition within those limits if it helps.

[stevebowers]

This would be a very short history for an Earth-like planet. You are supposing that an oxygen-rich atmosphere and advanced life in an era that (on Earth) barely had single-celled organisms and a reducing atmosphere. The development of complex metazoan life might be crammed into a single gigayear, but not half of that period.

...unless you imagine a completely different mechanism for evolution. If somehow the primitive biosphere can be persuaded to follow an accelerated path somehow, allowing the rapid development of new species. Perhaps the early microbiota develop neural nets inside their microbial mats, and start to actively design new lifeforms. Active evolution, or (semi-)intelligent design of sorts. Some weird organisms might result, but they might be quickly replaced by more sensible designs.

[Bear]

In what sense is it 400M years old? A system that's only been around for 400M years could have captured an interstellar rogue planet that's actually much older and, while it would have to survive a heavy bombardment period as the local accretion disk settled out, it could skip the molten-surface Hadean era.

That said, much faster development of complex life could all happen as a result of a single happenstance; Sex could be invented on, say, January 20 instead of November 11.

By way of explanation: We got single-celled life on Earth about 3650 million years ago. Map that to 365 days starting on January 1, and sex was invented on November 11. (We used to have an annual "Big Bang" party a couple weeks before Thanksgiving to commemorate the event). Between the origin of life and the invention of sex, only a little bit of biological evolution happened. The most significant event was the development of photosynthesis around March 30. But there were no real barriers to the invention of sex at some earlier point; the timing, as far as we know, is mainly a happenstance.

Anyway, once you get a method of genetic recombination working, evolution speeds up immensely and you can get complex life within a couple of "months."

[Matterplay1]

If the planet got photosynthetic eukaryotes (or the equivalent) from somewhere else that would speed up the timeline considerably. However, it appears that it took quite a long time to oxygenate the Earth's atmosphere. There were huge volumes of dissolved minerals in the water and a large volume of exposed minerals on the surface that needed to be oxidized before any significant amount remained to add to atmospheric gases. This is one of the challenges of terraforming, too.

[stevebowers]

That's right. At least ten times as much biogenic oxygen as currently exists in our atmosphere was absorbed by the crust before the air reached its current equilibrium; this did not happen overnight.

[Cray]

That's right. At least ten times as much biogenic oxygen as currently exists in our atmosphere was absorbed by the crust before the air reached its current equilibrium; this did not happen overnight.

Yep. Oxygen appeared in the atmosphere about 2 billion years ago, but it took a long time to spike to habitable levels.

Bear, the system is 400 million years old in the sense the bright primaries have 500 million years on the main sequence and I didn't want to deal with red giants, so I picked 400 million. I didn't mention the stars because I know they're a bugger, but I don't have to worry about the stars until I get past issues like "Will the planet have enough time to form a solid crust?" (Yes) and "Is that enough time to form life?" (Probably) and "How about an oxygen atmosphere and mammals?" (Er...)

An interstellar wandering planet has some disadvantages, like the improbability of capture. But at least I've got 4 stars (it's Mizar) to arrange something. On the other hand, then you need to defrost it and produce an oxygen atmosphere in 400 million years.

[stevebowers]

One interesting possibility in a cluster is panspermia or capture from other nearby stars in the same cluster. So even if Mizar is only 400 my old it could pick up life-bearing rocks or even a whole planet from another star system in the same cluster.

Star formation in a cluster doesn't last very long, so all stars in a cluster are about the same age- but if there are a lot of stars with planets then the chance of abiogenesis occurring at an early stage on at least one of them is somewhat greater.

Mizar is in Collinder 285, also known as the Ursa Major Moving Group, which is the closest cluster-like object to Earth. At one time this cluster was much more compact and the stars could have swapped rocks, planets and lifeforms much more easily.

[Cray]

Like you said, the entire moving group is also of a similar age: 400-500ish million years old. On the other hand, it does have foreign stars crossing through it. A rock from a X-billion year old planet would give me a jump start, at least allowing well-evolved algae to take root earlier.

On the gripping hand, before the atmosphere is oxygenated there's a ton of UV light to justify rapid mutation.

[stevebowers]

Like you said, the entire moving group is also of a similar age: 400-500ish million years old. On the other hand, it does have foreign stars crossing through it. A rock from a X-billion year old planet would give me a jump start, at least allowing well-evolved algae to take root earlier.

This possibility merits further consideration. A typical young cluster is anything from less than one to ten light years across, and might exist in a closely bound state for up to a hundred million years; in that time and with that cross-section it is likely to intercept a small number of older stars, which will take a few thousand years to pass through. So the possibility of cross-infection is non-zero.

For some reason a few clusters never disperse, such as Messier 67;
https://en.wikipedia.org/wiki/Messier_67
this ancient cluster will have had numerous close encounters, and it is not impossible that the entire cluster is infected with life- perhaps of several different kinds.

[Drashner1]

Hm. If I'm reading the Wikipedia article correctly, this cluster is inside Terragen space. Over 1000 start in a 10ly radius. Lots of potential crunchy goodness in that from garden worlds to mega engineering and one or more polities.

Todd