Planet Building Help Needed

[Vondy]

Planet Building Help

 

I'm in a world building phase for a story I'm writing (full disclosure: my wife wants me to submit it for publication, but I haven't decided). The world is not a perfect Earth Clone, so I need some help with "that science thingy." I could just hand-wave it, but I want the world to be essentially rational. I especially need help from people with some knowledge of astronomy, planetology, climatology, and tides. This is a complex and detailed endeavor and I do not feel I can pose all of my questions (and anticipate further interogative course) in one post. As a result, I will begin with the bare basics, and having hammered that out, build from there. Without further adieu, Herodom Assembled, let us begin.

 

Scope of Initial Post:

 

Star Type; Life Zone; Distance From Gas Giant To Primary; Distance Of Moon From Gas Giant; Seasons; Day Night Cycles; gravity-density-mass-size.

 

Story Requirements:

1. The biosphere must be able to support a tropical rainforrest or jungle.
   
2. The biosphere must be able to support human life.
   
3. The materials to support a bronze age culture must be present.
   
4. The surface has 83% water.
   
5. The land mass is divided between two equatorial and two polar continents
   
6. The world should have gravity in the earth range: .8 to 1.2, or so.

Planet Details:

1. The world is an "Earth-like" moon of a "Jupiter-like" gas giant.
   
2. The moon is the largest and outermost of our Jovian cousin's orbital object thingys (OOTS) (outside its gravity belts and rings).
   
3. The other OOTS are normative in size and uninhabital joyfulness.
   
4. The gas giant is within the starts life zone.
   
5. The gas giant has an almost perfect circle for an orbit.
   
6. The gas giant has almost no axial tilt.
   
7. The moon is in synchronus orbit around the gas giant (tidally locked).
   
8. The moon itself has almost no axial tilt.
   
9. The moon's path of revolution is almost in line with the gas giants equator, and is basically circular.
   
10. The moon's rotation and revolution rate is 28 (terran) days (see question below).

Immediately Relevent Assumptions to Check:

Assumption 1:

This is veeeery rough: This planet will have a "sun-side" that has, in very general terms, a 336 hour (14 terran days) day and a 336 hour (14 Terran days) night. And a planet-side that will have a period of night (facing away from the sun), followed by a period of day in which the world revolves away from the sun (fall), followed by a period of night (planetary eclipse), followed by a period of day in which the world revolves toward the sun ("spring").

 

Assumption 2:

Because the gas giant's orbit is essentially circular, the moon's seasons are based on its revolution around the Gas Giant, which takes it further from and closer to the sun.

 

Assumption 3:

The planet side will never know true night due to some level of illumination from the gas giant.

 

Assumption 5:

The moon is probably smaller than earth, but maybe as large as 75%.

 

Assumption 6:

This planet won't have much in the way of seasons, but it will have BIG WEATHER. (save weather and climate discussions for an upcoming post, please).

Initial Questions:

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?

I've been sitting here with a lamp a ball and an orange and its not so simple. I think it needs to be done for each quarter of the planet.

[mayapuppies]

Re: Planet Building Help Needed

 

I'm by no means an expert...but I did watch the National Geographic channel.

So take that for what's it worth.

 

I don't think a gas giant can form (scientifically) within the life zone of a star.

 

1. I think you can, I'm assuming that "weight" rather than "size" dictates gravity? Maybe I'm wrong on that

2. & 3. oops, you covered the gas giant thingy

7. - 9. The Gas giant is going to take up around 50%+ of the sky at any given time I imagine. I'm seeing a wierd day/night schedule. When the gas giant gets between the sun and the "planet" it's going to be a loooong night followed by a comparatively rapid set of day/night cycles, then a looooong night and so on. Pretty cool actually.

[Vondy]

Re: Planet Building Help Needed

 

I'm by no means an expert...but I did watch the National Geographic channel.

So take that for what's it worth.

 

I don't think a gas giant can form (scientifically) within the life zone of a star.

 

I was thinking the same thing, then you echoed it and I felt milisecondly depressed, then I googled "gas giant + life zone" and found THIS. Wiki said the "c" gas giant had insulation (I'm not sure how astronomers use that) similiar to venus and an orbit of 379 days. I don't know how accurate it is, but I'm holding out hope. Cancer is an astronomer, right? Maybe he can save my idea.

 

1. I think you can' date=' I'm assuming that "weight" rather than "size" dictates gravity? Maybe I'm wrong on that[/quote']

 

My understanding its a mass to size ratio, so density will be important. My understanding of this is mean, at best.

 

7. - 9. The Gas giant is going to take up around 50%+ of the sky at any given time I imagine. I'm seeing a wierd day/night schedule. When the gas giant gets between the sun and the "planet" it's going to be a loooong night followed by a comparatively rapid set of day/night cycles' date=' then a looooong night and so on. Pretty cool actually.[/quote']

 

For which side?

 

And, if you are correct about temperatures, more Olympic National Rainforrest than Amazon Jungle? If so, I may have to rethink.

[gojira]

Re: Planet Building Help Needed

 

This is pretty cool. I'd love to help out, although I'm sure Cancer will know more than me. I have various science type "life on other planets" books around here; I'll have to dig them out.

 

I'm pretty sure you can get a gas giant inside the life zone of a star, or even closer. I think you have to watch out for the gravity of the star pulling gas in from the giant. Could be a deal breaker if you want to be very scientific.

 

On thing to consider is that Earth's moon has a big effect on the climate of the Earth. First, the moon acts as a stabilizer for the Earth's rotation in the same way as an out-rigger on a boat stabilizes the boat. With out it, the Earth rotation would be much more likely to wobble, precess, and generally make things uncomfortable by pointing every which way. The interior of the Earth is not stable or evenly distributed; it's lumpy and sloshes around, so the moon's stabilizing effect is very important.

 

The moon also acts as a shield for the Earth, deflecting and absorbing meteor strikes away from the Earth. It's believed by some that life on Earth could not have developed without the moon's protective effect.

 

So I'd recommend adding one or two small moons in orbit about your planet, and a few slightly further out in orbit around the gas giant to act as meteor shields. Can't hurt.

[BlackSword]

Re: Planet Building Help Needed

 

an I get away with a planet that is roughly 75% the size of Earth' date=' but has similiar gravity? What does that do to composition?[/quote']

The planet would have to be more dense than Earth. Not a geologists so I can't answer how that would effect composition or develop of continents. Perhaps a higher percentage of heavy metals.

How far from the gas giant should the moon be? Is 28 days revolution doable based on that - or even related?[/QUOTe]

I found some equations but its going to take some time.

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?

I don't think so. The primary cause of seasons on Earth is the tilt of planet, not the distance from the sun. The long day-night periods would play a greater effect, on the far side of the Jupiter-like planet, the moon will receive almost no solar radiation and be reliant on the Jupiter planet for heat, as well as heat stored in surface and water. During the 'day', those on the Jupiter side will be in sunlight for a long period of time.

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

I know Jupiter does have a small amount of radiation, but not sure what effect it has on its moons.

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

On the sun-side of Jupiter, the night side of the planet will be bright due to reflection from the gas giant. On the night-side of Jupiter, the gas giant's planetary eclipse should be dark.

[Cancer]

Re: Planet Building Help Needed

 

Wow, this is an ambitious situation!

 

Let me start on the simplest stuff, so I'm cherry-picking through your questions and assumptions. Full treatment of some of that is gonna be huge.

 

With what is known about extrasolar planetary systems now, you are free to posit the existance of a gas giant planet at any distance from the star you want. So planet detail 4 is something that can be assumed but is subject to consistency checks.

 

All we know about satellites of jovian planets are those in the Solar System. The largest of those falls well short of Earth in mass, but let's proceed anyway.

 

The life zone gets further away from the star the more massive the star is, and it also gets broader. But the more massive the star, the more rapid the effects of stellar evolution. Stellar evolution causes stars to get more luminous while they are on the main sequence (in our solar system, the SUn now is 40% more luminous than it was 4.5 Gyr ago), which causes the life zone to move outward with time, and you need your planet to remain in that zone during the time the life on it evolves to whatever state you want for your story. For multicellular life, this took Earth 3.8 Gyr; for terrestrial megafauna, about 4.3 Gyr.

 

For the sake of speculation here, let's assume a copy of Sol. I can investigate other masses of star for you, but it won't happen for a week, I think. If we assume a copy of Sol, then the orbit distance is 1AU.

 

First thing to do is figure out what the scale of the system is. The terristrial planet orbit scale is not yet determined. The size of the terrestrial world's orbit around the jovian planet is set by the orbital period you specify (28 Earth days) and the mass of the jovian planet, via Newton's extension of Kepler's 3rd law. More mass gives you a larger orbit for the same orbital period.

 

Just some sample numbers for you. 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; the lower limit for star status -- is about 80 * 318 ~ 25,000.) Second column is the orbital size of the terrestrial planet (assumed 1 Earth mass) around the jovian planet, in units of the Earth-Moon distance (384,000 km, or 1/390 of an AU).

 

(If you want to work this out for yourself, the formula is m * p^2 = a^3, but be careful to choose a consistent set of units. m is total mass in the two planets, p is the orbit period, a is the orbit size. I chose to use the Earth mass as the unit of mass, the siderial month (OK, I got sloppy and used 28 days) as the unit of time, and with those choices the Earth-Moon distance is the unit of distance. Other unit systems are possible.)

 

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

630.0 --> 79.089 --> 0.204

1174.6 --> 97.318 --> 0.251

2189.9 --> 119.761 --> 0.309

4082.9 --> 147.389 --> 0.380

7612.3 --> 181.397 --> 0.467

14192.4 --> 223.255 --> 0.575

 

There's some trade-offs involved there to be considered. A jovian planet ranges from about 4 to about 12 times the size of earth (diameter, not mass). This diameter and the orbital size tell you the angular extent of the jovian planet as seen from the terrestrial planet. 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.

 

For reference, the Moon (and Sun) as seen from Earth have angular diameters just over half a degree (0.53 degrees). The Earth (diameter 12,760 km) seen from the Moon (diameter 0.273 Earth's) is obviously larger than that by a factor of 1/0.273, or a bit less than 2 degrees.

 

That means that because the orbit of the terrestrial planet is larger than that of the Moon by a factor larger than ratio of the diamters of jovian planet and Earth, the jovian planet as seen from the terrestrial planet is going to have a smaller angular extent than Earth as seen from the Moon. The jovian planet's albedo (how much light they reflect) isn't grossly different from Earth's: Earth's is 0.39, and the Jovians are 0.4 to 0.6. So the total amount of light reflected off the jovian planet to the terrestrial planet isn't going to be that large.

 

Now, jovian planets do emit thermal infrared radiation in excess of what they receive from the Sun, and that may provide some "seasonal" heating. I need to look up real numbers to get a handle on what that effect might be.

[Publius]

Re: Planet Building Help Needed

 

I have an excellent planet building book, but it is presently at home and I am not.

 

As far as the world around gas giant concept, that is fine. It might not be fine in a few years, but since we have lots of theories and not a lot of facts about planetary formation, it is within the boundaries of acceptablility.

 

Gas giants have been detected in the life zones of distant stars. Like a star, it is basically a ball of gas. Did they form there are did they form in the outer zone and move inward? Was the gas giant a smaler mass that broke away from the larger mass during the formation of the accretion disk and was caught in the gravitation of the stellar mass, which sucked off enough mass that the gas giant never ignited?

 

Who knows. Rival theories do exist though, and others are of course possible which could allow for the planetary arrangement that you describe.

 

As far as temperatures, remember that the gas giant is both bouncing light onto the planet and heats the planet itself (thus the planet is warmed not only by the Sun (direct from the source, indirect from the Gas Giant) but also by the Gas Giant.

[Publius]

Re: Planet Building Help Needed

 

Yeah, what Cancer said.

 

Actually, the Albedo thing I might comment on. The Albedo of the Moon is .12, so reflected light is going to be a little higher than what we get from the Moon here. Earth’s albedo is 0.37; Mars is 0.15; Jupiter, 0.52; Saturn, 0.47; Uranus, 0.51; Neptune 0.41. Pluto’s albedo varies from 0.5 to 0.7. None of these numbers however are carved in stone, earths for example fluctates by cloud cover. The Gas Giant in your story then is going to have an Albedo in the .5 range, waaaay higher than the Moon. Plus, the surface area has to be considered, it is a lot larger than the moon (unless it is quite distant, that will depend on the numbers you are using) and therefore the light which it reflects will be consequently greater.

[Cancer]

Re: Planet Building Help Needed

 

I've edited my above post too many times now ... anyway, one conclusion I draw from the discussion about angular diameters and albedos is that your Assumption 3 is true but subtler than I originally guessed. Depends on whether you consider a standard night lit by a full Moon to be "true night".

 

The planet side always has a source of light in the sky that's somewhat brighter than the Moon is. But the Moon's albedo is quite low, 0.07, so the jovian planet will be brighter by a factor of about 7 (albedo) * (5 (physical diameter relative to planet) * 3.7 (ratio of Earth physical diameter to Moon physical diameter) / 30 (distance))^2 ... that comes out to about a factor of 3 brighter, which is pretty modest.

 

BTW, something I've pointed out before, is that in a tidal-locked situation like this, the side of the terrestrial planet that's directed toward the jovian planet has a real navigation aid. The locked planets, being in what is effectively a geosynchronous arrangement, serve as something of a longitude marker in the same way as a pole star serves as a latitude marker. This makes transcontinental navigation much easier on the planetward side.

[David Johnston]

Re: Planet Building Help Needed

 

In fact extrasolar gas giants have been detected closer in than the habitable zone of the star. So there's no problem there as long as the gas giant in question is really, really big, so big that it has enough gravity to remain intact.

 

As far as the density of the planet goes, it would be a planet that had a higher metallic composition than that of Earth of course. A little too high, really. You might want to go with .9 gs which would still leave it with loads of heavy metals by comparison with Earth. Things like gold, iridium and platinum would be relatively accessible and common.

 

Any main sequence star could in theory have a gas giant in the habitable zone. However it would probably be a bit on the young side so let's call it a F-type "white star". (Note that the colour of a sun would not in fact be visible to the naked eye. They all look "white"). That gives it a biozone of 1.6 to 2.4 astronomical units. 1.8 is a safe choice. CO2 levels in the atmosphere can be manipulated to give you any kind of general climate you want.

 

Without knowing the mass of the primary it's impossible to estimate the orbital period of the satellite. However, I can tell you that since gravity increases the speed satellites can move at, and the faster a satellite moves, the farther it is away, your moon would be "somewhat farther" away from its primary than Earth's moon is. That's about all I can say.

 

The distance from the sun will be an insignificant aspect of the "seasonal" variation which happens in a 28 day cycle. By far the more important element will be getting continuously radiated with no cooling off period for days at a time. The interior of the outerside continent equatorial continent will get pretty darn unlivable for a few days every "month" unless it has an interior sea. Water acts as a heat-sink, so coastal areas and islands would probably be livable all the time but will still cool off and heat up significantly.

 

How much heat the planet gets from the gas giant itself is dependant on the size of the gas giant. A Jupiter-sized gas giant doesn't make much of a difference. A degree or so, maybe. But the planet's giantside would get most of it's warmth from solar radiation (since quite often the sun and the giant would both be in the sky at the same time) and air and water convection. The temperature variations however would be more modest than those of the farside because the sun is never hitting it full on, the way it does the farside. There would be regular eclipses of the sun on the giant side and these would cool things down, but big as the giant is in the sky the eclipses probably still wouldn't last more for than a few hours.

 

Yes, the giant side will experience a lot of moonlight much of the time of the time. However, the eclipses will be nothing but starlight.

[Vondy]

Re: Planet Building Help Needed

 

Wow' date=' this is an ambitious situation! [/quote']

 

Thanks!

 

And I applaud you. my brain started to hurt half way through your post. There's a lot for a layman to chew on.

 

Let me start on the simplest stuff' date=' so I'm cherry-picking through your questions and assumptions. Full treatment of some of that is gonna be huge.[/quote']

 

We have plenty of time.

 

With what is known about extrasolar planetary systems now' date=' you are free to posit the existance of a gas giant planet at any distance from the star you want. So planet detail 4 is something that can be assumed but is subject to consistency checks.[/quote']

 

Detail 4?

 

All we know about satellites of jovian planets are those in the Solar System. The largest of those falls well short of Earth in mass' date=' but let's proceed anyway.[/quote']

 

This was one of my concerns. I think it will work story-wise even if we go down to half Earth mass, but gravity is a sticking point.

 

The life zone gets further away from the star the more massive the star is, and it also gets broader. But the more massive the star, the more rapid the effects of stellar evolution. Stellar evolution causes stars to get more luminous while they are on the main sequence (in our solar system, the SUn now is 40% more luminous than it was 4.5 Gyr ago), which causes the life zone to move outward with time, and you need your planet to remain in that zone during the time the life on it evolves to whatever state you want for your story. For multicellular life, this took Earth 3.8 Gyr; for terrestrial megafauna, about 4.3 Gyr.

 

For the sake of speculation here, let's assume a copy of Sol. I can investigate other masses of star for you, but it won't happen for a week, I think. If we assume a copy of Sol, then the orbit distance is 1AU.

 

A general idea of the options would be good - and I can wait. I don't mind is the star is like Sol, though. The reference I found to a Gas Giant in its star's life-zone referred to a G6 star.

 

First thing to do is figure out what the scale of the system is. The terristrial planet orbit scale is not yet determined. The size of the terrestrial world's orbit around the jovian planet is set by the orbital period you specify (28 Earth days) and the mass of the jovian planet' date=' via Newton's extension of Kepler's 3rd law. More mass gives you a larger orbit for the same orbital period.[/quote']

 

First things first, then!

 

Just some sample numbers for you. 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; the lower limit for star status -- is about 80 * 318 ~ 25,000.) Second column is the orbital size of the terrestrial planet (assumed 1 Earth mass) around the jovian planet, in units of the Earth-Moon distance (384,000 km, or 1/390 of an AU).

 

(If you want to work this out for yourself, the formula is m * p^2 = a^3, but be careful to choose a consistent set of units. m is total mass in the two planets, p is the orbit period, a is the orbit size. I chose to use the Earth mass as the unit of mass, the siderial month (OK, I got sloppy and used 28 days) as the unit of time, and with those choices the Earth-Moon distance is the unit of distance. Other unit systems are possible.)

 

The math is definately something I will need help with.

 

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

630.0 --> 79.089 --> 0.204

1174.6 --> 97.318 --> 0.251

2189.9 --> 119.761 --> 0.309

4082.9 --> 147.389 --> 0.380

7612.3 --> 181.397 --> 0.467

14192.4 --> 223.255 --> 0.575

 

Column 1: is this mass relative to an earth size planet, or an astronomical measure of some sort?

Column 2: This represents miles?

Column 3: AU I understand.

 

There's some trade-offs involved there to be considered. A jovian planet ranges from about 4 to about 12 times the size of earth (diameter, not mass). This diameter and the orbital size tell you the angular extent of the jovian planet as seen from the terrestrial planet. 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.

 

For reference, the Moon (and Sun) as seen from Earth have angular diameters just over half a degree (0.53 degrees). The Earth (diameter 12,760 km) seen from the Moon (diameter 0.273 Earth's) is obviously larger than that by a factor of 1/0.273, or a bit less than 2 degrees.

 

That means that because the orbit of the terrestrial planet is larger than that of the Moon by a factor larger than ratio of the diamters of jovian planet and Earth, the jovian planet as seen from the terrestrial planet is going to have a smaller angular extent than Earth as seen from the Moon. The jovian planet's albedo (how much light they reflect) isn't grossly different from Earth's: Earth's is 0.39, and the Jovians are 0.4 to 0.6. So the total amount of light reflected off the jovian planet to the terrestrial planet isn't going to be that large.

 

In plotting out future posts I've added one for visualizing the moon's "night sky."

 

In relation to this I'll posit two sets of assumptions in a subsequent post entitled WORKING ASSUMPTIONS below that can be used to congeal our directions - and establish the potential numbers for the system.

 

Now' date=' jovian planets do emit thermal infrared radiation in excess of what they receive from the Sun, and that may provide some "seasonal" heating. I need to look up real numbers to get a handle on what that effect might be.[/quote']

 

This can be tackled along with climate, I think.

 

I really appreciate your help (and everyone else's).

[Cancer]

Re: Planet Building Help Needed

 

This Posts Big Question:

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

I've been sitting here with a lamp a ball and an orange and its not so simple. I think it needs to be done for each quarter of the planet.

 

Lamp-and-orange exercises have problems because in an astronomical situation the distances are MUCH larger than the diameters of the objects involved. That actually simplifies some situations (but it makes it near-impossible to make "realistic" demos of eclipses), but it's always going to be awkward.

 

You've specified zero axial tilt for the gas giant (detail 6) and zero or near-zero tilt for the terrestrial planet's orbit with respect to the gas giant's equator (detail 9). That makes a lot of the effects that I can imagine go away. By contrast, lots of stuff we're used to in RL comes from nonzero values for these: Earth's axial tilt is about 23 degrees, and the Moon's orbit is tilted about 5 degrees off Earth's equatorial plane.

 

What level of accuracy do you need? And what format do you want?

 

If some of your story hinges on things like earliest sunset and latest sunrise not falling on the same day as the winter solstice, then you need some pretty gruesome detail. (Those effects aren't going to happen to speak of in this situation, incidentally.)

 

The standard textbook for this kind of thing formerly was Textbook on Spherical Astronomy by Smart; it's been replaced by another (I think the same title) by Robin Green (but I haven't looked inside that book). The old Smart text was exceedingly dry and hard to read, though, and I do not at all recommend it so anyone with the least shred of math trepidation. And my memory is that it didn't have the formulas directly bearing on this. But I remember a paper in the technical literature that might....

[Cancer]

Re: Planet Building Help Needed

 

Detail 4?

Item 4 of your "planet details", "The gas giant is within the starts life zone."

Column 1: is this mass relative to an earth size planet, or an astronomical measure of some sort?

Column 2: This represents miles?

Column 3: AU I understand.

Column 1 is, as you guessed, relative to an earth-sized planet. Jupiter has a mass 318 times that of Earth, so real Jupiter would be "318" in this column.

 

Column 2 is the orbit size, in units of the size of the Moon's orbit around the Earth. "1.00" in this column would be 384,000 km.

 

In plotting out future posts I've added one for visualizing the moon's "night sky."

Believe it or not, almost precisely 30 years ago I did a term paper on exactly this! True, it was for the real Moon, not this fictitious situation, but it's directly applicable. I even remember the most important reference and have found it on-line (which was much easier than spelunking my crawlspace for the book with my old class folders).

 

I'll attack "big question" from your initial post next. I don't think that'll be too hard, as long as I use the simplest approximations (coplanar circular orbits).

[Vondy]

Re: Planet Building Help Needed

 

/snippage

 

You've specified zero axial tilt for the gas giant (detail 6) and zero or near-zero tilt for the terrestrial planet's orbit with respect to the gas giant's equator (detail 9). That makes a lot of the effects that I can imagine go away. By contrast, lots of stuff we're used to in RL comes from nonzero values for these: Earth's axial tilt is about 23 degrees, and the Moon's orbit is tilted about 5 degrees off Earth's equatorial plane.

 

Most of those zeroings were intended to simplify math, but from a storytelling perspective they may make it difficult since its easy to write in an assumption from our own experience that isn't correct.

 

What level of accuracy do you need? And what format do you want?

 

If some of your story hinges on things like earliest sunset and latest sunrise not falling on the same day as the winter solstice' date=' then you need some pretty gruesome detail. (Those effects aren't going to happen to speak of in this situation, incidentally.)[/quote']

 

I don't think we need to drill down that deep, but one of the protagonists is from a cult of navigators, so those details need to be on-target (and I noted your comment on the gas giant being an aid in that regard). I also want to make sure season, weather, climate, tide, and the positions of celestial objects from the dirtside view are correct and make sense - and that I don't "misdescribe" the sky when I'm writing.

 

There is also a cult of astrologers, so being able to map out when they can see the sky and when they can't, and the like, might crop up. And, since I'm a gamer, I'd like to be able to work out a calender, and a chart for the progression of days and nights for the sun-side and for the giant-side. I'm having a very hard time picturing the giant-side of the equation in terms of things we normally relate to like sunrise and sunset.

 

The standard textbook for this kind of thing formerly was Textbook on Spherical Astronomy by Smart; it's been replaced by another (I think the same title) by Robin Green (but I haven't looked inside that book). The old Smart text was exceedingly dry and hard to read' date=' though, and I do not at all recommend it so anyone with the least shred of math trepidation. And my memory is that it didn't have the formulas directly bearing on this. But I remember a paper in the technical literature that might....[/quote']

 

Wunderbar!

[Vondy]

Starting Point

 

LETS START HERE:

 

Using HD 73526 as our baseline working point:

 

Primary: G6 Main Sequence Yellow Dwarf (or Sol clone if problematic).

 

Gas Giant: 1.05 AU from Primary. 379 Day Orbit. Orbit basically circular. No axial tit. 9 Times the size of Earth (smaller than Jupiter), but 630 times more massive than Earth.

Our Moon: .9 the mass and size in relation to Terra. 28 Day Orbit. Orbit basically circular. However, lets give it an axial tilt similiar to Terras and an orbital eliptic similar to Luna.

 

Note: My one concern here is that it may be too close to reasonably escape any rings the gas giant has.

 

Is that enough data to work out the most basic mechanics and distances for the system?

Do we need to specify the gas giants other moons at this point?

NOTES FROM WIKI STUB AND FURTHER COMMENTS

"HD 73526 c is an extrasolar planet orbiting about 97 million miles (1.05 AU) away from its primary star. Based on its mass, this planet is likely to be a gas giant. At the distance this planet orbits from its star, which is more luminous than our Sun, HD 73526 c would receive insolation similar to Venus."

I did some checking and the temperature to insolation ratio for Venus, Earth, and Mars are really based on the density of atmosphere. Without that all three planets would be too cold to sustain life. So "Insolation like Venus" shouldn't be much of a problem.

 

Since the planet has a greater ratio of water to land than Earth it probably has thicker atmosphere (big assumption), which might make it hotter than the Earth based on its level of insolation, but we can reasonably adjust the insolation down if need be.

 

FOR ME TO RESEARCH

 

(will do searches comparing our systems gas giants to the numbers you produce - when its convenient to you):

 

Is there room between this gas giant and our moon for other moons, or would they be farther out?

Is our moon far enough out to avoid death inducing radiation belts and debris rings?

[Cancer]

Re: Planet Building Help Needed

 

OK. Minor point: the HD 73526 system has more than one known planet in it. Your specs are for "c" in the system. It has a real-life orbit eccentricity of 0.14, which I'll neglect for the moment. The other planet known in the system, (Link), "b" has a smaller orbit. I'm going to ignore that one.

 

Another minor point: that star (like many of those with extrasolar planets) seems to be more metal-rich than the Sun. For a terrestrial planet this doesn't matter much, I would guess. I'll see if I can scrape up guesses as to what that may mean for a jovian planet.

[Vondy]

Re: Planet Building Help Needed

 

OK. Minor point: the HD 73526 system has more than one known planet in it. Your specs are for "c" in the system. It has a real-life orbit eccentricity of 0.14' date=' which I'll neglect for the moment. The other planet known in the system, (Link), "b" has a smaller orbit. I'm going to ignore that one.

 

It doesn't have to take into account inconvenient details. Its a baseline rather than a replication.

 

Another minor point: that star (like many of those with extrasolar planets) seems to be more metal-rich than the Sun. For a terrestrial planet this doesn't matter much' date=' I would guess. I'll see if I can scrape up guesses as to what that may mean for a jovian planet.[/quote']

 

Very cool. I'm going to bed. And thanks again.

[Eosin]

Re: Planet Building Help Needed

 

Hey,

 

I am no scientist but you know there are a number of science minded folks who have tackled a host of the grunt work here in a book by Asimov called Writing Science Fiction and Fantasy. The particular information that you are looking for is a great article by Sci-Fi guru Paul Anderson.

 

I went through many of the same steps as yourself when I was designing The Last Dominion and this book stood out among the references for its clear, if occassionally techincal, details. More importantly, these guys have already written hundreds of stories set on other worlds and know many of the "unintended concequences" that can bite you on the backside (i.e. Zero Axial Tilt). If you can't get access to the book, I will be glad to re-read the various chapters and make comments but I think you will want it for your own use. The reprint of this book was done in 1993, so it isn't cutting edge but that doesn't diminish its relevance in my eyes. Plus, with people like Cancer running around - if he is willing to look over end product to modernize theories - you are set.

[tkdguy]

Re: Planet Building Help Needed

 

Here's a great website about world building that uses real science.

 

Loki is one of the worlds featured. It is a moon of a gas giant. The civilization that appears there is aquatic, but it may give you a couple of ideas on how to go about designing your world.

[LordGhee]

Re: Planet Building Help Needed

 

with zero tilt would not the poles be always in night and therefore very cold -100 below and that would generate intresting weather.

 

Lord Ghee

[Cancer]

Re: Planet Building Help Needed

 

Zero axial tilt also can't be maintained with real-Universe physics.

 

Just a quick note to Von D-Man ... I'm working on the numbers, but real life is rearing its ugly head. It'll take a few days.

 

Just so that it's out there, the paper which describes in technical detail how you'd do celestial navigation on the Moon is at this site, From there you can read GIF's of it page-by-page, or grab a full PDF. It does have all the glorious terseness one expects in a technical journal.

[Vondy]

Re: Planet Building Help Needed

 

with zero tilt would not the poles be always in night and therefore very cold -100 below and that would generate intresting weather.

 

Lord Ghee

 

This would depend on whether the moon's orbit around the gas giant kept it dead on to the sun. If it orbited the gas giant had no axial tilt, and the moon orbited its equator - also with no axial tilt - then the entire side of the moon facing the sun would have light, though it would be stronger at its equator. If the moon was not tidally locked, then the poles would have day-night cycles. If it was tidally locked, half the pole would be perpetual day, and the other perpetual night.

[Vondy]

Re: Planet Building Help Needed

 

Zero axial tilt also can't be maintained with real-Universe physics.

 

Do we need to alter the gas giant's tilt in this regard?

 

Just a quick note to Von D-Man ... I'm working on the numbers' date=' but real life is rearing its ugly head. It'll take a few days.[/quote']

 

All good. Rosh Hashannah starts Friday night so I won't be back until sometime on Monday. And I don't have a deadline, or anything. All in your own good time.

 

Just so that it's out there' date=' the paper which describes in technical detail how you'd do celestial navigation on the Moon is at this site, From there you can read GIF's of it page-by-page, or grab a full PDF. It does have all the glorious terseness one expects in a technical journal.

 

Cool. I'll take a look.

[Seenar]

Re: Planet Building Help Needed

 

If the orbit of the gas giant is about equal to earth's it should have about the same elemetents.

 

Changing its size will change its gravity quickly. Gavity is based on mass.

 

Mass is based on density/volumn.

 

Volumn of a sphere = V = 4/3¶r^3

 

.75% radius of earth then has a volumn that has a .75 Earth for r.

 

Equatorial Earth Radius = 6,378.135 km, so it has a volumn of roughly 3.45955+E11

 

.75 Earth radius = 4738.601

 

V Moon = V = 4/3¶(4738.601)^3 Giving you a rough Volumn of 1.4187E+11

 

This means the volmun of the moon would be 41% that of Earth.

 

Assuming densitiy of the material is constant (not a good assumption with a smaller size), we can assume that the gravity would be around 41% of Earth's.

 

If you plug in our moon's size of 3476 km radius, you will see that gives you about 1/6 a G, for the moon.

 

 

 

I would also reconmend this book Worlds Without End by John S. Lewis. It is a great great great reference for "doing it right".

[Publius]

Re: Planet Building Help Needed

 

The Book I use for this sort of thing is:

 

World-Building, Stephen L. Gillett, (Writer's Digest Books:Cinncinnati, OH) 1996

ISBN 1-58297-134-X

 

The science is probably a bit out of date, but for what it is the text is well written and has a lot of the basic formulas that you will need. I just looked it up at Amazon Here and there are a good number of them for sale on the secondary market.

 

I also have the Assimov Book Eosin mentioned earlier. It is a good book actually sections of it are great, but Gillett's is specialized for this very purpose.