It might be an impossible task, but at least it was a well-defined one. The rules for performance were no problem. He had learned them long ago on Te… - Charles Sheffield

The answers come pat and fast. You see, what the downsiders want isn’t an explanation; it’s a catchphrase they can use instead of an explanation.

A long time ago humans talked of terraforming Mars and Venus, but we never did it. Just too busy blowing ourselves up, I guess, ever to get round to it.

The universe is all extremes. Monstrous gravity fields, or next-to-nothing ones; extreme cold, or heat so intense that solids and liquids cannot exist; multimillion atmosphere pressures, or near-vacuum.
Ice or fire. Niflheim or Muspelheim: the ancient alternatives, imagined by humans long before the Expansion.
It’s planets that are the oddities; the strange neutral zone between suns and space, the thin interface where moderate temperatures and pressures and gravity fields can exist. And if planets are anomalies, then planets able to support life are rarer yet—a zero-measure subset in that set of strangeness.
And within that alien totality, where do humans fit?