400 million years to build a planet?

[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.