Space/Science — HabitableZone

Space/Science » in reply to Proposed Rule of Thumb

Re: Proposed Rule of Thumb

Nobody Knows !

I run a course in planetary evolution, as part of which I get the students to build their own solar systems and try for a terrestrial planet.

Despite researching this, there are no good rules for the likely form of a Solar system. Everything is very random and wide ranges of outcomes are possible.

Nobody knows, for instance, if bigger stars have bigger planets, or more planets, or if it's random.

Since the search for extra-terestrial planets has concentrated on Sol-like stars, we don't have enough data on larger or smnaller stars to see what trends exist.

You do need a star of sufficient Metallicity to have enough solid debris when it formed.

You need to be in the "habitable zone of the galaxy" - too far out, there is still too low metallicity. Too far in, there is too much violence in the star creation-destruction cycle

You need a stable orbit over geologic time, so binary and multiple star systems are difficult, but not impossible.

You need M(star) < 2 M(sun) for a lifetime of > 1Ga, to permit evolution of reasonably complex life.

You need M(star) > ~0.1 M(sun) to avoid tidal lock (unless you go for the moon-of-a-gas-giant route)

Together, these constraints on Solar mass rule out 90% of stars immediately

You need a planet in the habitable zone, which is narrower than most folks admit (There is less than 10% contingency in Earth's orbit over the life of the sun, despite people claiming that both Venus and Mars may have been in the habitable zone at some time)

You need a planet larger than Mars and smaller than about 5x Earth Mass (otherwise it starts becoming a gas giant). OR you need a similar sized moon of a gas giant and you have to survive the radiation belts and the immense impact velocities in that gravity well.

You need to be far enough away from the nearest gas giant not to get disrupted or displaced. But you probably do need a gas giant somewhere to clean up the debris and stabilise the whole set of orbits.

You need plate tectonics or some other sort of progressive and regular volcanic acticity to constantly provide new land and new minerals as the old land areas erode. (marine civilisations are possible, but technological and communicative marine civilisations are very unlikely, IMCO)

You need luck to avoid an impactor that resets the clock.

You need more luck to evolve intelligence and tool-using.

And there's much more that we don't even know that we don't know...

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