Planet Building Help Needed

[Basil]

Re: Planet Building Help Needed

 

A few notes:

 

Surface gravity is related to density times radius. With 3/4 the radius, you need 4/3 the density; with 3/4 the radius and 9/10 the surface gravity you need 6/5 the density. Earth is the densest large object in the solar system. I kinda doubt your satellite is gooing to happen.

 

=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

 

Earth's diameter is 3 2/3 that of the Moon. So its apparent size is 3 2/3 that of the Moon at any particular distance (OK, OK, so not quite, and there's a lot of trig to figure it out right. But at planetary distances the difference is meaningless).

 

What's this mean? Weeelll....

First column is mass of the jovian planet, in Earth masses. (Uranus, the lowest-mass of the SOlar System jovian planets, is 14.6 in these units; Jupiter is 318;

...

Assumed mass, implied distance, for p held fixed at 28 days:

[EDIT: added 3rd column, which is distance in AU]

 

15.0 --> 23.235 --> 0.060

28.0 --> 28.318 --> 0.073

52.1 --> 34.667 --> 0.089

97.2 --> 42.543 --> 0.110

181.2 --> 52.278 --> 0.135

337.9 --> 64.289 --> 0.166

...

The diameter of Neptune is 3.8 times Earth; the diameter of Jupiter is 11.2

...

the planet structure theory I've seen suggests that the diameters of jovian planets have a broad maximum at about Jupiter's mass (so more massive things are actually slightly smaller in diameter), so the 4 and 12 Earth diameter numbers make useful limits.

 

So if we put Jupiter where Earth is and put Earth orbiting Jupiter in 28 days, Earth will be 64.289 Earth-Moon Distances away from a body about 41 Moon diameters in size. Therefore its apparent diameter will be 41/64.289 ~ 0.638 times that of the Moon, and thus ~ 0.638 that of the Sun! That means there will never be a total eclipse of the Sun. So much for "a period of darkness every revolution." In fact, in the event of a full annular eclipse, the light will decrease by only about 40%. If you use other reasonable values for the star's size, the gas giant's size and mass, you'll get to pretty much the same situation.

 

While "zero tilt, orbiting at zero degrees from the equator" sounds nice, it's not really achievable. And, if the angle between the gas giant's orbit around the star and the satellite's around the gas giant is more than a degree or so, there will be passages of the satellite "behind" the gas giant where there won't be even a silver of an eclipse. Going back to the situation in the last paragraph, an angle of 0º 52' 4" is enough.

 

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The light reflected from the gas giant onto the "near side" of the satellite will illuminate, but it will not warm. There simply isn't enough energy involved. Further, for the gas giant to be emitting significant heat energy means it is now in the fuzzy, undefinable border between superjovian and brown dwarf. Most or all bodies in that range "flicker"; that is, the heat energy output varies through a wide range; enough to scorch life off your satellite if (at usual output) it is contributing to the satellite's warmth.

 

=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

 

Let's repeat something:

[EDIT: added 3rd column, which is distance in AU]

337.9 --> 64.289 --> 0.166

0.166 AU is more than half the distance from Earth to Venus when they're closest. The satellite's orbit is unstable. The period must be shorter than 28 days or the gas giant smaller or in an orbit much farther from other planets (which probably means much farther out than 1AU). The first and the third methods mean little if any seasons; the ratio of distances kills the idea.

 

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1. Can I get away with a planet that is roughly 75% the size of Earth, but has similiar gravity? What does that do to composition?

2. What kind of star would be needed to produce a life-zone that would contain a gas giant and a moon that had warm-tropics in some areas? (at least, from looking at our own solar system, gas giants are "outer planets")

3. How far from the primary should the gas giant be to sustain the correct climate?

4. How far from the gas giant should the moon be? Is 28 days revolution doable based on that - or even related?

5. Is the variance in distance from the primary based on the moons revolution be significant enough to create notable seasonal effects, or will they be unremarkable? What about the long day-night periods?

6. Will temperatures on the planet-side be affected by the gas giant?

7. Will the planet side have some kind of night-light from the gas giant?

8. Will the sun-side grow exceedingly hot during the day and exceedingly cold during the night? Too much so for habitation or plant life?

9. Does the rotation/revolution of the moon need to be cut to a single day to make life sustainable?

 

This Posts Big Question:

 

Can anyone help me map out what the day and night cycles for the respective sides of the planet?

1. Not really, unless your satellite has odd compostion.

2. Just about any kind, as others have said.

3. Take the star's energy output as compared to the Sun's, take the square root, and multiply by 1AU. That distance gets you the same incoming radiation. If the satellite's greenhouse effect or albedo are different from Earth's, you can fudge the distance a bit.

4. Cancer covered this one.

5. The difference in distance could be enough to create seasons, but doing that will probably mean the satellite's orbit is easily perturbed by other planets in the system, and will be unstable.

6. I highly doubt it, unless you change the distance/period a boatload.

7. Cancer got this one too.

8. Yes, and yes. This is the biggest flaw in your idea. According to an article I read a few years ago, by either Asimov or Pournelle, a "solar" day (daytime plus nighttime) of more than 100 hours will create such intense weather than life would be impossible.

9. No, a "solar" day of 100 hours should be possible, though I suspect half of that will be needed it you want buildings to stand and other parts of primitive tech to be possible.

Big Q. No need. The far side is obvious, the near side is the far side plus a REALLY BIG "moon" that stays in one place in the sky, and every once in a while blocks out part of the sun.

[Vondy]

Re: Planet Building Help Needed

 

Big Q. No need. The far side is obvious, the near side is the far side plus a REALLY BIG "moon" that stays in one place in the sky, and every once in a while blocks out part of the sun.

 

Fair enough. Makes sense.

 

Based on the rest of your post, which I ruthlessly snipped, we can change the rotation and/or revolution periods. One reason I wanted the planet tidally locked is that I didn't want to have to work out the day-night cycles in relation to the orbit around the moon. Also, most moons that we are aware of are tidally locked (from my limited reading).

 

On the other hand, it might be interesting to have a moon that wasn't tidally locked, even if that makes things more complex. What if we have rotation (day-night cycle) of 84 terran hours and a revolution (orbit) of 14 terran days. Will the period during which the planet is "behind" the gas giant going to be problematic? Is the weather going to be constant, or will there be periods of calm followed by periods of weather more intense than we are accustomed to?

 

I found a site with the gravity formula and determined that, at .75 earth diameter, the moon would need a density of 6, as opposed to earth's 5.5, to have a gravity of ~.81. That's close enough to "earth gravity" for my purposes. I'm not sure, but it seems like such a density would be within the realm of possibility. What that does to composition - I don't know.

 

It could simply mean more available "precious metals," along with some of their cousins, and the elements that give gemstones their color (like titanium and iridium). The main issue there would be potential ground-water problems. It would probably make wells an iffy proposition in the lowlands, but higher altitude wells, and rivers would still be pretty sure water sources.

[Basil]

Re: Planet Building Help Needed

 

Based on the rest of your post' date=' which I ruthlessly snipped,[/quote']

Oh woe is me!

we can change the rotation and/or revolution periods. One reason I wanted the planet tidally locked is that I didn't want to have to work out the day-night cycles in relation to the orbit around the moon. Also' date=' most moons that we are aware of are tidally locked (from my limited reading).[/quote']

True, most are. However, since your satellite is going to be fairly far from the gas giant, and fairly massive, it could have escaped tidal locking. The simplest way to do that is to have the satellite in an orbit that is either rather eccentric, rather inclined to the gas giant's equator, or some of each.

 

On the other hand' date=' it might be interesting to have a moon that wasn't tidally locked, even if that makes things more complex. What if we have rotation (day-night cycle) of 84 terran hours and a revolution (orbit) of 14 terran days. Will the period during which the planet is "behind" the gas giant going to be problematic? Is the weather going to be constant, or will there be periods of calm followed by periods of weather more intense than we are accustomed to?[/quote']

The weather will be intense at any given time, somewhere on the satellite. However, at any given point it will vary, being the least horrible at around "solar" midday and midnight.

 

BTW, one thing to clarify is if that 84 hours and 336 hours is relative to the gas giant, the star, or the background stars. They cannot be the same relative to all three, nor even to any two.

 

I found a site with the gravity formula and determined that' date=' at .75 earth diameter, the moon would need a density of 6, as opposed to earth's 5.5, to have a gravity of ~.81. That's close enough to "earth gravity" for my purposes. I'm not sure, but it seems like such a density would be within the realm of possibility. What that does to composition - I don't know.[/quote']

OK, if that "low" a gravity is fine, then it works out better. Compostion? Well, obviously more metals and less silicates, carbonates, etc.--IOW, less "rocky" covering. You'll have a larger (in proportion to Earth) core, and a thinner mantle.

 

It could simply mean more available "precious metals' date='" along with some of their cousins, and the elements that give gemstones their color (like titanium and iridium). The main issue there would be potential ground-water problems. It would probably make wells an iffy proposition in the lowlands, but higher altitude wells, and rivers would still be pretty sure water sources.[/quote']

Actually, with a greater density comes a greater tendency for metals to sink into the core or at least the mantle. All other things being equal, you will have fewer metals ("precious" and useful) in the crust. Making this worse is the fact that, probably, due to its smaller size, your satellite will have a lesser amount of tectonic activity and thus less of the mantle's metal-bearing rocks will get to the surface.

 

Of course, this means that wells will not be a problem.

 

BTW, if the satellite is in an eccentric orbit, the tectonic problem I mentioned doesn't arise. The opposite tectonic problem, however, may; viz. Jupiter's satellite Io.

 

Another BTW; with reduced tectonic activity, the inorganic carbon cycle slows down. This has a major effect on the satellite's chance of supporting life. If you're interested, I can go into the gory details.

[Publius]

Re: Planet Building Help Needed

 

Actually' date=' with a greater density comes a greater tendancy for metals to sink into the core or at least the mantle. All other things being equal, you will have fewer metals ("precious" [i']and useful[/i]) in the crust. Making this worse is the fact that, probably, due to its smaller size, your satellite will have a lesser amount of techtonic activity and thus less of the mantle's metal-bearing rocks will get to the surface

I did not know that about the density thing. Ouch, that makes some things I have done in the past with planetary composition problematic. D'oH!

 

More ruthless snipping. Regarding Tectonics in general, I have a question (as you have much more knowledge of this material than I do): Doesn't the presence of the Gas Giant produce its own tectonic activity on the Planetary-moon. In other words, there is less tectonic activity because of the size, BUT would the presence of the Gas Goiant increase that. I seem to recall something about the Moons of Jupiter being more tectonically active because of that.

 

Second question, with the higher density, would the planet have a greater magnetic field because of the higher metal content at the core and the smaller area to cover due to the smaller radius? That would also help with some of the excess radiation from the Gas Giant would it not? And help to stimulate the tectonic activity too as I recall. AND would that increased tectonic activity tend to bring more metals closer to the surface. Just a thought.

 

Thanks a lot Basil for posting this stuff. I cannot follow all of it with my limited knowledge base, but the thread has provoked a good deal of thought. And thanks of course to Von-D Man for bringing it up in the first place.

[Basil]

Re: Planet Building Help Needed

 

I did not know that about the density thing. Ouch' date=' that makes some things I have done in the past with planetary composition problematic. D'oH![/quote']

Well, it is only a tendency. There are various events and situations that can make it less of a problem.

 

More ruthless snipping. Regarding Tectonics in general' date=' I have a question (as you have much more knowledge of this material than I do): Doesn't the presence of the Gas Giant produce its own tectonic activity on the Planetary-moon. In other words, there is less tectonic activity because of the size, BUT would the presence of the Gas Goiant increase that. I seem to recall something about the Moons of Jupiter being more tectonically active because of that.[/quote']

It is not simply the presence of Jupiter that causes its satellites to be tectonically active. There is also the resonances in their orbits, which keep their orbits from becoming perfectly circular. This non-circularity leads to tidal "flexing" of the satellites, which builds up heat in their interiors. That heat drives the tectonics.

 

A single sizable satellite around a gas giant will soon have its orbit reduced to a circle. Once that happens, not tidal flexing. And, unless the satellite has enough radioactive material, it will soon(ish) cool down to where tectonics no longer play a role.

 

Second question' date=' with the higher density, would the planet have a greater magnetic field because of the higher metal content at the core and the smaller area to cover due to the smaller radius? That would also help with some of the excess radiation from the Gas Giant would it not? And help to stimulate the tectonic activity too as I recall. AND would that increased tectonic activity tend to bring more metals closer to the surface. Just a thought.[/quote']

The higher density will definitely cause a sizable liquid-metal core, and that will cause a magnetic field. However, with a radius 0.75 that of Earth, its volume is 0.421875 that of Earth (0.422 is close enough). Thus, even with a proportionally larger core, the sheer amount of metal is much less, and the magnetic field will be weaker and extend less far than Earth's magnetic field.

 

The small surface area does not help because the shielding effect of a magnetic field depends largely on its depth (and strength), not the area covered.

 

I've never heard anything about the magnetic field contributing to tectonic activity.

 

On your last point: yes, tectonic activity will bring up metals from the mantle, which is A Good Thing®

 

Thanks a lot Basil for posting this stuff. I cannot follow all of it with my limited knowledge base' date=' but the thread has provoked a good deal of thought. And thanks of course to Von-D Man for bringing it up in the first place.[/quote']

 

You're welcome. Glad to be of help to one and all.

 

No need to applaud, just throw Rep.

[Cancer]

Re: Planet Building Help Needed

 

First ... Still pounding away on my numbers here (actually, I would characterize what I have as "a fair start"). I'm doing the elliptical orbit case, because as Basil & others have pointed out, the all-circular orbits case is trivial and boring. This will get the effects of "optical librations" into the results, which gets more interesting (and makes navigation more of a challenge). The coding isn't difficult, but keeping one's sign conventions straight is a challenge.

 

Second ... A paper that literally came out in the last week tangentially addresses some of the issues with having the terrestrial "moon" in a large orbit around the jovian planet. As was pointed out, if there are other planets in the system, then a large orbit for the "moon" will quickly get disrupted.

 

However, there are reasons to suspect that if you have a giant planet at 1 AU or so (or closer), there might not be any other planets in the system close enough to the gas giant to disrupt the orbit of the moon (in which case there's no problem). The leading hypothesis right now for how you get a gas giant inside of 5 AU or so is a "migration" of the gas giant from out where it formed (5 AU out or beyond, for a solar-mass star) to the closer solar orbit, caused by gas drag in the protosystem disk in the late stages of the system formation.

 

(It is believed you can't form gas giants in close, because those have to form out where it's cold enough for the "ices" -- water, carbon dioxide, methane, ammonia, etc. -- to freeze out into solids. That only happens at 5+ AU out in the forming disk; inside of that distance, it's too warm. To get these gas giants from where we believe they formed to where we observe them to be in extrasolar systems, it's believed that if the protosystem gas disk persists after the planets form, then gas drag -- what we commonly think of as "air friction" -- can cause the orbits of all the planets to decay or "migrate", a process that stops when the disk is expelled after the protostar's strong stellar wind starts.)

 

This "migration" by a gas giant is just about certain to destroy or eject any newly-formed terrestrial planets whose orbits started off in the range of distances through which the gas giant migrates, plus some distance more because of long-term tidal interactions.

 

The paper (abstract is here but the full paper is behind a subscription-only wall) applies some stuff worked out in other papers and applies that to the 150+ known extrasolar planet systems, and gives yes/no verdicts on whether a (still undetected) terrestrial planet could be in the habitable zones of those systems.

 

That's interesting in itself, but not directly applicable here. Indirectly, though, it does provide information on just how much clearing-out a migrated gas giant planet accomplishes. That in turn tells us something about how big an orbit (for the moon around the gas giant) we can get away with in your proposed system which has one or more other planets still in solar orbit.

[Vondy]

Re: Planet Building Help Needed

 

Sorry for the delay in posting here. I've been really busy, and will continue to be for a few weeks. I want to thank Basil and Cancer for their efforts on this. And yes, I do want the gory details. Cancel, is the "case study" you're working up taking into account my response to Basil about acceptable changes? Thanks again, Von D.

[Cancer]

Re: Planet Building Help Needed

 

Hope your High Holy Days went well.

 

What I'm writing should be able to accept sets of parameters:

• ANY TWO OF:
  
  • orbital size
    
  • orbital period
    
  • masses of the two bodies, or total mass and a mass ratio
    

  [*]orbital eccentricity

For just the two-body situation (gas giant plus terrestrial planet), that's all you need. Well, it could be that the moon orbit is not coplanar with the giant planet equator, but we don't care about that, since your navigation problem is for the moon, not the giant planet, and the nature of a locked moon means that the thing it's locked to IS in its equatorial plane.

 

When you add the third body (the Sun), it gets more complex. You have to specify two of those three orbit size/orbit period/mass things for Sun + system, which is pretty simple, and the eccentricity of the planet orbit also. The big mess comes because you can have the "moon" orbit be non-coplanar with the "planet" orbit. That adds three more angles:

• the inclination of the moon orbit plane w/r/t the planet's orbit plane
  
• the longitude of the ascending node
  
• the argument of the peripoint
  

These are better described by a good figure than words, so Wikipedia's entry on orbital elements has such a figure.

 

Minor comment: the numerical method I'm using restricts you to orbit eccentricities < 0.6. This isn't a serious problem from planets, but makes it more or less useless for comets.

 

There's a figure here which shows a lunation. The Moon's orbit has an eccentricity of 0.0554, which causes it to move closer & further from us (so the moon appears larger/smaller), and causes it to revolve at a non-uniform rate around us. Because its rotation IS uniform in rate while its revolution is not, that means the line drawn from the moon's center through its (latirude zero, longitude zero) point yaws ahead and behind of the line drawn between the Moon's center and the Earth's center ... this causes the "nodding" seen in that animated GIF.

 

A fair amount of coding and testing to be done yet.

[Cancer]

Re: Planet Building Help Needed

 

Minor update: I haven't forgotten this stuff. I've got the single-orbit piece done (which takes care of the effects of orbital eccentricity). I'm now working -- very slowly -- on the piece that gets stuff right when you have one orbit (the "moon" around the "earth") tilted w/r/to another (the "earth" around the "sun").