[ExI] kepler study says 8.8e9 earthlike planets

[Anders Sandberg]

On 2013-11-08 15:17, spike wrote:
> Reason: of all those Goldilocks planets, I have having a far easier 
> time imagining a Goldie completely covered in ocean, probably deep. 
> That shouldn't be so hard for us to imagine: this planet is almost 
> deluged.

In fact, the models of waterworld exoplanets I have found in the 
astronomical literature have *really* deep oceans. Earth is mildly odd 
in that it has just so much water it would cover the entire surface to a 
few kilometres: most of these have hundreds of kilometres of water.

That means a few interesting things. The ocean floor will be covered by 
a mantle of high-pressure ice, likely with veins of mineral erupted from 
the rocky core (a planet has volume ~R^3 and area ~R^2, so the heat flux 
grows as ~R - big worlds will tend to be much more volcanic and 
plate-tectonic than Earth). The latest results on metallic ices suggest 
that they will not occur, since they require 5 TPa, and that moves us 
into gas giant territory - but non-metallic warm ice is still odd. The 
atmosphere may have plenty of water vapour as a greenhouse gas. The 
surface temperature distribution depends on the rotation rate; if it is 
fast winds will be more zonal and the poles will be cooler (possibly 
with ice), while a slower rotation produces an even temperature. If it 
is warm enough you can get some amazing hurricanes or hypercanes. Most 
of the ocean will be totally dark, heated only by a geothermal gradient 
or volcano-induced currents - surface heating only penetrates down to a 
thermocline a few hundred meters down, and the lack of undersea 
mountains reduces mixing.

(See Charles Stross' "Neptunes' brood" for a fictional world that gets 
close to this - although he admitted he choose some parameters for 
simplicity, and I have my doubts about his awesome blue smokers.)

I don't know if general technology could emerge here. Sure, it looks 
near-impossible to build a lab underwater. But that might just be our 
limited imagination. Many compounds are volatile in air and dissolve, 
yet we have found ways of managing them. It might be that a waterworlder 
would have an equally hard time imaging how a dry lab could work.

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