Re: planet help


Tux Wonder-Dog wrote:
> Kevin wrote:
>
> > In rec.arts.sf.science chornedsnorkack@xxxxxxxxxxxx wrote:
> >
> >> S. Palmer wrote:
> >>> brdavis@xxxxxxxx wrote:
> >>> > S. Palmer wrote:
> >>> >
> >>>
> >>> > > Here's what I've got... Two planets, A and B, tide-locked
> >>> > > together... around a fairly Sol-like sun... A is larger [1.1 G's
> >>> > > surface gravity]... B is smaller [0.8 G's surface gravity]
> >>> >
> >>> > For "Earth normal" climate on an Earth-sized planet around a
> >>> > Sun-like star, you'll probably want it, well... in an Earth-like (1
> >>> > AU) orbit.
> >>>
> >>> Makes sense. I just wasn't sure if having two planets threw stuff off,
> >>> but I guess now that I think about it more it's pretty obvious.
> >
> > If there are eclipses, then that will slightly reduce the heat they
> > receive from their sun, and hence they will be slightly colder. But I
> > suspect that effect will be very small for any reasonable orbital
> > parameters.
> >
> >>> I seem to recall that there was a fixed range of distance where the
> >>> system would be stable -- any closer and they smash together, any
> >>> farther apart and they don't stay together.
> >>>
> >> The inner limit? You cannot have the month at 90 minutes like the
> >> satellites at LEO, because the Moon would be disrupted tidally. But
> >> Phobos orbits Mars in less than 8 hours - and stays in one piece. I
> >> suppose you could alternatively start wih imagining Mars tidelocked to
> >> Phobos and see how big you can make Phobos. Basically, the minimum
> >> rotation period is a few hours, and depends on the square root of
> >> density (of the satellite, mainly, unless the sizes are very close).
> >
> > Wikipedia has a good article on "Roche limit."
> >
> >> The outer limit? The so-called Hill radius. If you look at the
> >> dimensioning, then effectively it is defined in terms of the number of
> >> months.
> >
> > IIRC, being within the Hill radius is a necessary but not a
> > sufficient
> > condition for stability, and to be comfortably stable a satellite should
> > be
> > closer than a third of the Hill radius. Our own Moon is close to that.
> > So as regards the original question, I think the bottom line is that
> > planets A and B should have a distance between them at least equal to
> > about 3 times the radius of planet A, while the distance between them
> > should not be much greater than the distance of the Moon is from Earth.
> > My personal suggestion would be to have the distance be equal to
> > about
> > 6 times the radius of planet A, which would give both of the planets
> > rotational periods comparable to good old Earth's.
> > The weird thing is that each planet would be fixed in position from
> > the
> > perspective of the other. From an entire half of planet A, you wouldn't
> > be
> > able to see planet B at all. Might there be entire cultures that regarded
> > the existence of planet B as mere myth? Or maybe all the continents are
> > on the side where you can't even see planet B---only sailors get to see
> > it? Or maybe there are a few islands from which planet B is visible, but
> > the
> > inhabitants of those islands keep it secret? I can imagine being on a
> > sailing ship that slowly crosses over into the hemisphere from which
> > planet B is visible...you will see it slowly emerge up from the waves...it
> > will be this enormous thing, something like 20 times the apparent size of
> > our own Moon. Would the sailors be terrified of it, or would they regard
> > it as a good omen?
> > Oh well, I don't want to write your story for you. :)
>
> But if there is a land-tide for each planet raising land surface above the
> barycentre, I would expect it would mean that the continents would clump on
> the tidal bulge immediately facing the other planet, tectonics or no
> tectonics. It's not something we see on Earth, because the Moon's rotation
> is tidally locked to the Earth, not vice versa.
>
> Dig up something on the Moon's barycentre sometime, and its relation to
> MASCONs.
>
Well, the Moon does have a difference between near and far side. But
the result is that there is actually more seas on the near side.

Moon, of course, lacks a clear marker of equipotential surface... and
does not appear to have had pieces of surface wandering around.

> I suspect there would still be tectonics, particularly if the planets were
> as young as Earth. But they would swirl rather than wander. That's my
> guess at any rate.

Would the non-sphericality of the surface seriously guide the wandering
continents? Earth is not a sphere, either.

> >
> <snip>
> >
> > Either you've made a mistake in algebra above or I have. The above
> > equation only agrees with my answer if a is the total distance between the
> > two planets.
> > Since it is specified that the two planets are tidally locked, we may
> > reasonably presume that their orbit around each other is practically
> > circular. Planet A will orbit around the barycenter in an orbit with
> > radius r_A, while planet B will orbit around the barycenter in an orbit
> > with radius r_B. If we let d be the total distance between the planets,
> > then d=r_A + r_B. If we change a to d in your version of Kepler's 3rd law
> > above, the result will be the equation that I find.
> >
> >>> okeydokey, so if I follow you correctly I need to use the complex
> >>> formula to figure out how long they take to rotate around one another,
> >>> but I can use the simpler formula to figure out how long it takes them
> >>> to go around the sun once. Now, since the orbit isn't around a point in
> >>> the exact center, I assume I can describe the orbit as an ellipse? (or
> >>> can I?) What, then, is the semi-major axis? An average of the radius
> >>> from the point of rotation to object, or is it more specific than that?
> >
> >> It is an ellipse which may or may not be a circle. And the circle would
> >> be in the centre of gravity - between the two planets.
> >
> >> The semimajor axis is half the sum of the minimum and maximum distance.
> >
> > As I mentioned above, it is safe to assume that the orbit of the
> > planets
> > around each other is practically circular. You can also reasonably assume
> > that the two-planet system itself has a circular orbit around its sun.
> > The semi-major axis of a circular orbit is simply the orbital radius.
> > But if you do want to add an extra complication, you could make the
> > orbit
> > of the two-planet system around its sun be a more eccentric ellipse. That
> > would make for more extreme seasons.
> >
> >>> > > Can I have A have an atmosphere but not B?
> >>> >
> >>> > Maybe. Especially if they are very very close (have you ever read
> >>> > "Rocheworld"?).
> >
> > I don't follow why being very close would make it more likely that
> > one
> > would lack an atmosphere. (I haven't read "Rocheworld" myself so maybe
> > that provides an explanation?)
> >
> >>> I haven't. I could theoretically have atmosphere on both, but I want A
> >>> to be life-sustaining but B not so much. Otherwise, certain plot points
> >>> involving suddenly finding onself outside the domes become much less
> >>> harrowing than I'd like (-:
> >>>
> >> There are plenty of reasons why a planet with comparable to Earth size
> >> and substantial atmosphere may not be friendly to life. Venus has over
> >> 0,8 mass of Earth, and even more of escape speed and gravitational
> >> acceleration. The very Earth had substantial, but unbreathable
> >> atmosphere before Cambrian - meaning 90% of all history.
> >
> > I agree! Personally, I would find it far more interesting for a
> > character
> > to try to survive on a planet similar to Precambrian Earth rather than on
> > some lunar-like planet.
> >
> >>> > Yes, but... if they are tide-locked, the tides don't move around
> >>> > the
> >>> > planet. In other words, the tidal bulges will be fixed in one place,
> >>> > so you won't "see" any tides,
> >
> >> But still have the solar ones. Roughly a third of the total spring
> >> tides on Earth.
> >
> > I believe that the tidal force will have a substantial influence on
> > atmospheric circulation, somewhat analogous to the Coriolis force. What
> > the upshot will be in terms of climate I can't say.
> >
> >
> > Kevin
>
> Wesley Parish
> --

.

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