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Old 06-26-2016, 05:02 PM
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lpetrich lpetrich is offline
Join Date: Jul 2004
Location: Lebanon, OR, USA
Gender: Male
Default Re: Fermi Paradox thread; aliens welcome

I'll now consider the question of what sizes are possible for organisms on other planets, and what that implies for their biology.

Access : The limits to tree height : Nature
Maximum plant height and the biophysical factors that limit it

Maximum tree height likely constrained by gravity. The sap pressure at the base of the tree is (density)*g*(height), where g is the acceleration of gravity. Since the feasible pressure is constrained by the tree's physiology, we have

(height) ~ 1/g

One finds the same equation for the maximum height of a mountain, and the scale height of the atmosphere, and maximum land-animal size is likely given by that equation also.

Maximum mountain height: 10.2 km (Mauna Kea and Mauna Loa, Hawaii, related to the ocean floor) Mars has higher mountains, like Olympus Mons at 21.9 km, but scaled to the Earth's gravity, it's 8.3 km.

Maximum tree height: 112.7 m (California redwoods, Sequoia sempervirens).

Maximum land-animal height: about 7 m (shoulder height of sauropod Brachiosaurus)

The maximum size of an animal has other constraints, like absorption of food and heat rejection. The blue whale is the largest known animal and likely the largest possible. Largest mass: 173 metric tons, longest: 33 m.

So on a planet with low-enough gravity, land animals could grow to blue-whale size. But I'll have to check on how low an Earthlike planet's gravity can get.
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Thanks, from:
The Man (06-26-2016)
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