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A Tight Fit: Planets in the Habitable Zone

4 Aug 2020, 14:05 UTC
A Tight Fit: Planets in the Habitable Zone
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How many habitable planets should we expect in the average stellar system? One sounds like a good number to me, even an optimistic one. But it’s a tough question because we don’t exactly know what an ‘average’ stellar system is, there being such a wide range currently being discovered.
There was a time less than a century ago when the idea that there might be three habitable planets — i.e., habitable by humans — in the Solar System was current. Imagine Venus as something like French Polynesia, or maybe what was then the Belgian Congo. Imagine Mars with a thicker atmosphere and ancient seas, Edgar Rice Burroughs territory.
Today we think of multiple habitability here in the Solar System as perhaps including ocean life under the ice of the moons of giant planets, but we’ve ruled out anything a human could walk around on in relative comfort. The question of what makes our Solar System able to support just one planet in the human habitability range bothered Stephen Kane (UC-Riverside) because of examples like the seven planets around TRAPPIST-1, three of which seem to be in that planet’s liquid water habitable zone. Why so many? Why so few around ...

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