Re: Help with worldbuilding
- From: brdavis@xxxxxxxx
- Date: 26 Aug 2005 08:20:14 -0700
Denni wrote:
> I still have to <i>read</i> ["Worldbuilding" by Gillet]
Do, it's well worth the time. And don't be scared, or embarassed, or
anything, in this forum - this is a GREAT place to ask this sort of
thing, and there's a good number of informed folks here who love
helping out.
> at 0.85 solar masses as an example, with my
> world circling at 0.4 AU, about 0.5 Terra years orbit
The orbital period at 0.4 AU from a 0.85 solar mass star should be
sqrt(a^3/M) = sqrt( 0.4^3 / 0.85 ) = 0.27 years, or about 100
terrestrial days. And yes, "Worldbuilding" will show you how to do this
& make it simple.
Where did you get the 0.85 solar masses from? And no, that's not at
all a criticism - there are so many different references for this stuff
I'm not sure who's "right". I've got written down 0.71 solar masses (&
I didn't note where I got that, either - my bad).
I also have listed the luminosity at about 0.292 solar, meaning at
0.4 AU from the star, the planet will receive 1.8 times the insolation
(amount of light) as the Earth - it's going to be really, really warm,
like warmer than Venus, by my rough estimate.
If you still want to use Eps Eri, I'll assume a luminosity of 0.292
solar and a mass of 0.71 solar. To get Earth-normal insolation, put the
planet 0.54 AU out from the star. At 0.54 AU from a star with 0.71
solar mases, the orbital period would be 0.47 terrestrial years.
> The planet is a low gravity world, about 1/4
> Terra's, half its size...
I'll assume here by "half its size" you mean half the radius. That
gives use enough to figure the planets density at about half Earth's.
Earth is at 5,520 kg/m^3; this 0.25 G half-the-Earth's-radius planet
would have a density of 2,760 kg/m^3, a little on the light side for
comfort. Even the Earth's Moon is denser, at 3,340. Probably the only
way to get the density down that low is to have a significant
percentage of low density ices in the mix, and that's not going to make
for a very stable bulk planet (if it's that warm).
To keep the 0.25 G surface gravity, but using a reasonable density
(say, that of Mars; 3,940 kg/m^3), the planet would be 0.35 Earth's
size, or a radius of 2,234 km. Really rather small, smaller than Mars.
> but with a similar magnetic field...
As another poster mentioned, that generally requires a conducting,
convecting fluid core - i.e., the planet still needs to have internal
heat production due to radioisotopes. Do you *need* a magnetic field?
It's not needed (IMS) for biology, nor to protect the atmosphere.
> and a breathable, <i>much</i> denser,
> atmosphere (gliding plays a part in the story)
Gliding will be easier on such a low-gravity world. If you want it
human-breathable, 400 mb pO2 (that's "partial pressure of oxygen", one
atmosphere is 1013 mb) is about the upper limit, while for pN2 higher
than about 3,100 mb isn't a good idea (unless you *want* your
characters to be "narked out" with nitrogen narcosis). Combining those,
the total surface pressure for a N2/O2 atmosphere would end up about
3,500 mb (just under 3.5 Atm) with around 11% oxygen (fires would be
much tougher to make on this world). As a fun story note, the humans
might want to stay near the mountaintops; going down (a lot; see below)
to the lowlands would increase the total pressure (not a problem) as
well as the partial pressure of N2 (a problem; narcosis again).
The gas density (what you want to worry about for gliding) will
depend on temperature, but we can rough it out assuming a nice 293 K
(about room temperature) at around 4.1 kg/m^3; Earth-normal is about
1.4 kg/m^3, so flyers are in *great* shape on this world. Due to the
low gravity, they only need to produce 1/4th the lift of an Earthly
bird, but each patch of wing can now deflect 2.9 times the mass of that
same Earthly-analog bird; assuming other things being equal*, wings
could work with less than 10% of the surface area of their terrestrial
counterparts.
Note that ballooning would be even easier here; on Earth, the
greatest density contrast you could get is 1.4 kg/m^3 (vacuum inside
the balloon, normal atmosphere outside). Here you could, again, get
nearly three times that density contrast, and, again, the gravity being
lower... fun.
This was essentially the setup some folks (including for a while me)
were trying to use to flesh out a world called "Velvet", with a mostly
airborn biosphere. It was great fun while it lasted (although, it turns
out hailstones were a *serious* problem).
This would also be an amazingly thick atmosphere, in that the
pressure would not drop as rapidly with altitude, nor would the
temperature drop as rapidly with altitude, as on Earth. Both these
things, again, are tied to the gravity. Convective storms are usually
only as wide as they are high, with their height limited by the
tropopause; on your world, the tropopause will be very high, resulting
in very very wide, very tall convective complexes... think
"superthunderstorms", black as night beneath and extending for perhaps
hundreds of kilometers wide, with denser air in the longer updrafts. A
rough calculation predicts hailstones could be 14 times as wide, or
more than 2000 times as massive as Earthly counterparts... ouch.
> what proportion of it has to be land?
What proportion do you want to be land? More ocean means storms have
a greater potential to grow in many cases, and the wind-driven waves
will be higher (more fetch, more time to build waves). Some oceans are
probably a good idea.
> It is very poor in radioisotopes
Why? Is that needed for the story? Radioisotopes buy you an internal
heat source, which allows magnetic fields, volcanic and
mountain-building activity, and lots of other useful things. The fact
that this planet is so small means that given an Earth-normal
concentration of radioisotopes the heat per unti surface area radiated
is going to be *less* than Earth's, so you might want to go with a
"radioactively-enriched" planet in some cases.
> The dominant colour of the vegetation in
> the dimmer light might be red/purple.
It probably will be dimmer near the bottom of this atmosphere, but
remember I set up this "thought experiment" by assuming (ass-u-me) an
Earth-normal insolation level. Also, planets don't have to be very
efficient (Earthly photosynthesis is terrible, for instance, only about
3-5%), they just have to beat out the neighbors. If you just want
"other-worldly", I'd go with chlorophyll-based with phycobilins as
accessory pigments. Or even a rhodopsin-based system. But the color is
probably up to your imagination here, not biochemistry.
> massive trees, any idea how high they'd
> grow?
There was some nice work on this recently published in Nature
(within the last couple years?). On Earth, tree height is limited by
water stress, with the very tops of the trees looking for all the world
like desert plants. IMS, the conclusion was that with current system
redwoods etc. are near the limit. Drop the surface gravity and you can
get a lot higher (at 0.25 G less than 4x as high, due to capilary
forces and friction), but nowhere near the ultimate tensile strength of
water (equivilent to around 2 km on Earth). So maybe 3x times redwoods,
or in the 1 km range (wow... that seems high).
> It turns out that every couple of 100 standard
> years or so, an outside event (comet?
> Another star coming close?) triggers a complete
> turnover.
Hmm. 100 years or a very very short time (even 1000 years, in this
context). A comet wouldn't do it, because a comet is only going to last
10-100 (being really really optimistic) pericentron passes before
becoming inactive, if not completely vaporising. That's "only" 0.1
million years for these events to have been happening, *VERY* short in
terms of evolution.
A passing star is worse. To get something to come by every, say,
1000 years, the semi-major axis of the orbit would be less than 9 AU.
Assuming a really eccentric, cometary-type orbit (e = 0.99) means it
would only get at *most* less than 18 AU from the main star. Heck,
Uranus is further than that from the Earth, and from a truely dark-sky
site it's naked eye visible. There's NO WAY to hide a star, or even a
moderately sized planet, at those distances.
How about episodic vulcanism?
> I would be grateful for any advise.
Oh, I don't have any of that handy ;-)
> I find this world-building lark vaguely scary :}
That's OK - you just posted to a place where most of us find it at
least as facinating as our day jobs (speaking of which...).
--
Brian Davis
*they aren't; drag is going to go up drasticly as well in this
atmosphere, due to the increased density, but we'll be foolishly
optomistic ;-)
.
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