[ExI] kepler study says 8.8e9 earthlike planets

[Anders Sandberg]

On 2013-11-13 19:24, Kelly Anderson wrote:
>  The loss of atmosphere is of course related to the loss of the 
> magnetosphere.

Is that really obvious? I know a fair bit of planetary science, and I 
always thought it was more UV-dissociation of water into hydrogen that 
does a Jeans escape or hydrodynamic escape that dried out the planet and 
led to most atmospheric loss.

> For planets very close to their sun would the increased density of the 
> solar wind require an even stronger magnetosphere to preserve an 
> atmosphere? Would larger planets with greater gravity be able to hold 
> onto an atmosphere more tightly?

The solar wind intensity goes down as 1/a^2 where a is the orbital 
radius. The magnetosphere doesn't seem to scale in a simple way with 
planet size: I have tried to find some neat rule of thumb, but it mainly 
looks like big planets with fast rotation have a better chance of being 
more magnetic, but it all depends on iffy magnetohydrodynamic properties 
of the core. One possible scaling law I don't trust is that field 
strength goes as sqrt(density/period).

A heavy planet will have a shorter atmospheric scale height; 7400/g 
meters, where g is the surface gravity in Earth gravities. But a low 
atmosphere doesn't necessary help, it is temperature that matters most. 
However, the escape temperature for each gas scales as 1/R - large 
planets need to be hotter to lose as much atmosphere.


>
> The dynamic interaction of atmosphere, magnetosphere and solar wind 
> might make it difficult to have enough atmosphere for enough time to 
> evolve intelligent life. Also, if the atmosphere is too thick, that 
> seems like it would cause its own set of problems. You can't live on 
> Jupiter for example, thought that is an extreme example.

Even thick terrestrial planet atmospheres cause a lot of complications - 
they distribute temperature evenly, they convect in ways different from 
our thin atmosphere (thinness in this case is all about how many optical 
length constants deep the atmosphere is - can an IR photon get through 
it without scattering?)

>
> I don't know the necessary physics, but if you had a planet 2x the 
> size of earth with the same proportion of water and atmosphere, but a 
> much greater magnetosphere because of the size of the core, would you 
> have problems with the atmosphere being too dense? Would the oceans be 
> too deep in some sense? Would it be harder for continents to arise 
> from the deep?

A double-Earth would likely be a waterworld, from what I have read. Rock 
after all contains water, and if you squeeze it enough it will be 
released to the surface (plus cometary water). So double-Earth will at 
least have eight times the water volume but just four times the surface 
area.

Plenty of volcanism, and over time I think continents will develop. 
Mountains would be half of our height (since gravity would be twice as 
large), but that is still enough to get some to poke up.

Atmospheric density is the hard part: it depends on how the planet 
formed, and its temperature. It is to some extent a free variable which 
also affects the temperature that constrains it - it is a real headache 
in my worldbuilding program ;-)

Quick guess: the atmosphere would be thicker but not super-thick, as 
long as surface temperatures are fairly low and like Earth's. Might be 
unbreathable to humans (too high oxygen partial pressure) but not to 
local life.

>
> So many questions. I'm sure there are people at NASA who have been 
> scratching their heads about this stuff for decades.

Indeed. I have to rush, but I have a library of papers and books on 
this. Am writing a short guide for worldbuilding.

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