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Redefining habitable zone for advanced Terran animals
(06-17-2019, 05:12 AM)Noclevername Wrote: Would these atmospheric conditions necessarily remain stable on a biospheric world? Earth's toxic-gas period covered most of our planet's existence, but when photosynthesis came along, it changed our entire atmosphere, purged out most of the toxins (from our POV) and allowed life to adapt to free oxygen and develop complexity.

Couldn't an exoplanet be capable of the same process?

 Possibly. But there are IMHO two problems with this. One is that the resulting biosphere might generate minor amounts of gases that would be dangerous to Earthly life (the OP mentioned Earthly animals) such as chlorine, ammonia or CO.

The other is that a planet like Earth might be quite rare, in particular the fast spin which largely generates the magnetic field that protects our atmosphere from the solar wind. It's seriously postulated that the rotation of proto-Earth was quite slow, until the Thera event that spun Earth up and created the Moon. After all, of the terrestrial planets only two have reasonably fast spin. And the magnetic fields of Mercury and Venus are negligible. Venus in particular, which is virtually a twin of Earth in most respects, has almost no magnetic field - because it has a rotation period approximately equal to its year.

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RE: Redefining habitable zone for advanced Terran animals - by iancampbell - 06-18-2019, 12:38 AM

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