[ExI] Double-Earth (Was: kepler study says 8.8e9 earthlike planets)

[Anders Sandberg]

On 2013-11-22 09:28, Kelly Anderson wrote:
> On Wed, Nov 20, 2013 at 5:36 AM, Anders Sandberg <anders at aleph.se 
> <mailto:anders at aleph.se>> wrote:
>
>     OK, now I have a better writeup:
>     http://www.aleph.se/andart/archives/2013/11/greetings_from_doubleearth.html
>
>
> That was an incredibly detailed writeup. Thank you Anders. So if I 
> understand, the physics of a "twice as large" earth lead to no 
> continents? Does that assume the same proportion of water as earth was 
> formed from?

Thanks! Yes, I assumed the same proportion of water went into the 
formation of Dry. Wet of course is much wetter.

> It kind of seems like if there were less water for some reason, you 
> still might get continents, but perhaps I'm missing some rule of thumb 
> that suggests the ratio of water is similar everywhere???

We don't know for certain. When a solar nebula coalesces into planets, 
the innermost parts get baked by the sun and gas and volatiles are blown 
away. Outside the "iceline" a few AU out lots of water remain, and 
planets that migrate inwards during formation will turn into inner 
system wet planets - but the details of early migration are complicated.

I think the likely answer is that Earth formed roughly where it is, and 
has a typical composition. So if it had been bigger it would have been 
like my Dry double-Earth. In theory, a Dry that formed from material 
even closer to the sun and then got moved out a bit (say by a passing 
hot Jupiter) might be a heavy world with "normal" ocean sizes.


-- 
Dr Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

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