On Sep 8, 6:51 am, Naraht wrote:
> This is more sort of a research question.
>
> Presuming a planet with more or less the same makeup of the solid
> portion, the same age, and same distance from the sun, how large or
> small can the planet be to be habitable by humans? I *think* the
> primary issue is that if it is larger than a certain size then the
> planet won't lose the Hydrogen/Helium in the atmosphere and become a
> close in Gas Giant and if it is smaller than a certain size, it won't
> keep it's Oxygen long enough (which puts the minimum somewhat larger
> than Mars (or would Mars have kept its oxygen if it had a
> Magnetosphere)).
>
> Any ideas where to look for info on this?
It is not just the Oxygen. The main thing is
the geologic cycle. Mars has the ability to
keep its atmosphere on the planet. The
problem with Mars is that it doesn't have
enough volcanism to keep the Carbon
Dioxide from being locked into the planet's
crust as carbonate rocks.
Earth and Venus have similar potential
amounts of Carbon Dioxide. The difference
between Earth and Venus is that on Earth
there is water, which allows vast amounts
of CO2 to be stored in the Earth's crust as
carbonate rocks. On Venus, UV light from
the sun in the upper atmosphere gradually
bled the hydrogen away from water vapor
in the upper atmosphere, destroying all the
water. This meant that there was no
way of weathering the CO2 into the crust,
which left virtually all of it in the atmosphere.
The atmosphere of the Earth has further
been modified by photosynthesis from life,
taking nearly all of the rest of the Earth's
carbon dioxide and fixing it in plants or
fossil fuels. Still the amount that has been
fixed by plants is a very small amount in
comparison to that which is locked up in
the crust as carbonate rocks.
Supposedly in about a billion years the
Sun will increase its radiation output enough
to start bleeding the Earth's hydrogen away
into space in a similar fashion to that of Venus.
Then if intelligent life forms at that time do not
do something to stop the process, the lack
of any water on Earth will make all of the
carbonates bake out of the rocks and the Earth
will be very similar to Venus. This is only about
a billion years into the future, and the Sun is
still supposed to have about 5 and a half billion
years or so before it uses up enough hydrogen
to go off the main sequence. Then it will go
giant star phases for about half a billion years,
and then become a white dwarf.
If Mars had a lot more volcanism it would
be a lot more similar to Earth because
more of the CO2 would be in the atmosphere.
When it comes to stars, clearly the determining
factors are: 1 too small, and tidal locking forces
the planet to show its face to the star at all times
if the planet is close enough to get enough radiation
similar to that of Earth. This cuts out about 95%
of all stars since small red dwarf stars smaller
than the Sun are much more numerous than
the Sun is. 2. Too large, and the brightness
of the star will cause it to loose its hydrogen
and go off the main sequence in too short a
time. The lifetime of the Sun is about 9 or
10 billion years, but when stars get through
into the F, A, and B ranges their lifetime can
drop to less than a billion.
When it comes to planet mass it would have
to be somewhere between Earth and Mars
to enable volcanism to persist up to the
current age of the solar system. Volcanism
does exist on Mars, but plate tectonics
and volcanism do not push the CO2 back
into the atmosphere on Mars to the extent that
it does on the biosphere of Earth.
My guess is that terrestrial planets can in
theory be much larger than Earth, because if the
star ignites before Jovian like planets
have formed in an inner solar system area,
and drives off the helium, and there are
significant amounts of heavy elements in
the nebula in comparison with the hydrogen
and helium initially, then smaller planetesimals
and ring fragments can remain without the
hydrogen, which can then accrete to form
terrestrial planets and moons and ice moons.
This however would be only from the nebula
of 'metal rich' stars, which started with
significant amounts of heavy elements
to begin with. My guess is that rocky
terrestrial planets could be much larger than
the Earth, if there were enough planetesimals
and ring fragments around to accrete after
stellar ignition had blown away the hydrogen.
The Earth is the most massive non-gas-giant
in our solar system. It might be that you could
get a terrestrial-like rocky moon orbiting a
hot Jupiter. A hot Jupiter in the inner solar
system, however, might accrete a lot of
planetesimals and ring fragments, however,
leaving less for rocky planets and moons.
It is hard to say how massive a non-gas-giant
could get, however it would have to form
from a metal rich nebula, because only
they would have the heavy elements to begin
with, which would enable them to form.
If you have a planet gradually coming together
from smaller ring fragments, then the ring
fragments can loose the hydrogen before
actually forming the planet. The ignition of
the star could be a factor in driving the
hydrogen off.
My guess would be a web search, combined
with a search of abstracts and articles of
science magazines and periodicals, along
with simple college astronomy textbooks. |
