[ExI] kepler study says 8.8e9 earthlike planets

[Kelly Anderson]

On Wed, Nov 13, 2013 at 5:17 AM, Anders Sandberg <anders at aleph.se> wrote:

> On 2013-11-13 09:32, BillK wrote:
>
>> On Wed, Nov 13, 2013 at 8:32 AM, Kelly Anderson  wrote:
>>
>>> So nobody knows NASA's definition of goldilocks planets well enough to
>>> know
>>>
>>> if mars would count as one of the eight billion or not?
>>>
>> I think the confusion is between 'habitable zone' and 'habitable planet'.
>> A goldilocks planet must be in the habitable zone, but for other
>> reasons (like being too small) may not be habitable.
>>
>
> Goldilocks is not even a proper scientific term, just a shorthand and
> journa-splaining word.
>
> Whether Mars is too small to be habitable is best phrased as a timing
> issue: Mars-sized worlds will stop continental drift early (crudely: total
> internal energy ~R^3, radiation rate ~R^2, so the time until things stop is
> ~R) and then become dry and lose atmosphere. But once it had oceans, and no
> doubt life could have lived in them (given what Earth-life can do). The
> zone where water can exist also moves somewhat across the lifespan of the
> star (the inner and outer radii scale as sqrt(luminosity)) but depends on
> planet mass and atmosphere.  So we might talk about habitability in terms
> of time and space.
>

If intelligent life had evolved on mars in the distant past, then they
might have been able to escape the death of their world. The loss of
atmosphere is of course related to the loss of the magnetosphere. 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 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.

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?

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

-Kelly
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