Astronomy help (Dreamwalkers game)

[Lord Mhoram]

I'm thinking about the solar system in the Dreamwalkers world, and was wondering what some things would look like.

 

If there were and asteroid field say between the orbits of venus and mercury, how would it appear in the night sky? How different would it appear of the rocks were small bodies of fire themselves.

 

In the real world, gravitic pull keeps the planets bascially on the plane - what would happen if the planets were tilted every which away in thier orbits (some even perpendicular to others). If three planets all in the same orbit (around mercury's orbit maybe) roughly equdistant from each other, circled the star perpindicular to the main planets orbit, would they seem to chase each other across the sky?

 

Thanks for any help.

[Cancer]

Re: Astronomy help (Dreamwalkers game)

 

I'd have to push numbers around, but my gut reaction is that your asteroids between Mercury & Venus would need to be large -- hundred km in size and up -- in order to be seen. There's only half a dozen such objects in the real-world main asteroid belt as it is.

 

If you made those things actually self-luminous ("small bodies of fire") then it depends on how bright and large the fire is.

 

There's a page that has the relations for apparent brightness of an asteroid here. That won't help you on the "ball of fire" question, though.

 

Non-coplanar but equal-size orbits ... eventually, such bodies will collide, and the debris of the collision re-collide, and eventually you will make a nice ring around the star. They may start off in a resonant situation, but over time it'll get perturbed away and the collision will happen.

 

About the three coplanar planets ... Yes, they'd "chase each other", but three is few enough that you'd never have more than one in the sky at once.

[Lord Mhoram]

Re: Astronomy help (Dreamwalkers game)

 

 

Non-coplanar but equal-size orbits ... eventually, such bodies will collide, and the debris of the collision re-collide, and eventually you will make a nice ring around the star. They may start off in a resonant situation, but over time it'll get perturbed away and the collision will happen.

 

Thanks for the link, and you pretty much confirmed what I was figuring. Never studied astronomy, but read some books, and such.

 

As for the non coplanar - I was thinking in differing orbits - say if Mercury was perpindicular to earth, and mars and venus were at 45% from Earth, but perpindicular to each other.

 

 

Yeah I am going for a wierd fantasy Solar System. But I want to try an understand how the things really work, so I know what rules I am breaking. The PCs are going to be going all Spelljammer later.

[Cancer]

Re: Astronomy help (Dreamwalkers game)

 

Ah. Non-coplanar non-intersecting (and even opposite-sense) planetary orbits are possible and stable on a long-term basis, AFAIK. Well, longer-term. I think a retrograde planet will get its orbit perturbed into something else (possibly ejection) eventually, but it's a slower process than the collisions between non-coplanar objects I mentioned before.

[Captain Obvious]

Re: Astronomy help (Dreamwalkers game)

 

Your asteroids between Venus and Mercury would probably only be seen just before dawn and just after dusk, probably for no more than a few minutes. The dispersal of sunlight in the atmosphere would drown out a small body like that, and they'd be too close to the sun to be seen at night.

[Lord Mhoram]

Re: Astronomy help (Dreamwalkers game)

 

Your asteroids between Venus and Mercury would probably only be seen just before dawn and just after dusk' date=' probably for no more than a few minutes. The dispersal of sunlight in the atmosphere would drown out a small body like that, and they'd be too close to the sun to be seen at night.[/quote']

 

Good points. I may put it out where our Asteroid belt is, but with many of the objects are miniture suns (amoung the stones, air spheres and water objects) - which would lead them to being seen on campaign world.

[PhilFleischmann]

Re: Astronomy help (Dreamwalkers game)

 

Non-coplanar but equal-size orbits ... eventually' date=' such bodies will collide, and the debris of the collision re-collide, and eventually you will make a nice ring around the star. They may start off in a resonant situation, but over time it'll get perturbed away and the collision will happen.[/quote']

Are you sure? I'm not an expert in such things, but it seems to me that since they'll have the same period, and if they're far enough away from each other, especially when one or more of them cross the intersection point(s) of their orbits, they could each go about their merry way, indefinitely. I'm willing to take your word for it, but that's just how it intuitively seems. I can certainly see how they would collide if they start getting close to each other, but assuming they start out far enough away from each other such a process must take a very long time, wouldn't it?

 

About the three coplanar planets ... Yes, they'd "chase each other", but three is few enough that you'd never have more than one in the sky at once.

Wouldn't that depend on how they're spaced out? Or would they eventually collide with each other also, if they aren't equally spaced?

 

Of course, with magical forces in play, we can have the planets do whatever we want!

 

I'm interested in this subject, because in my FH world, I've also got some strange planetary properties. Though all of my planets are co-planar (it didn't occur to me to make them otherwise), they aren't all spherical. And some of them have multiple moons with non-co-planar orbits. And no, I don't pretend that the configuration is scientifically plausible. It's magic!

 

The planets in my mileu are the equivalent of D&D's "planes" of existance. A few of them represent possible afterlives. One is the home of the gods. And so forth. You can't actually travel through space to get between them, but you could potentially use magic to do so, or wait until you die. The main planet, is not a sphere, but a torus! Though its inhabitants aren't aware of this fact (yet). And it rotates on a skewed axis, say around 23 degrees from the perpendicular. Its one moon orbits on a 28-day cycle, and its year is 496 days long. Most of this wierdness will serve no purpose in my campaign, other than to amuse me. Unless you can suggest something...

[Cancer]

Re: Astronomy help (Dreamwalkers game)

 

Are you sure? I'm not an expert in such things, but it seems to me that since they'll have the same period, and if they're far enough away from each other, especially when one or more of them cross the intersection point(s) of their orbits, they could each go about their merry way, indefinitely. I'm willing to take your word for it, but that's just how it intuitively seems. I can certainly see how they would collide if they start getting close to each other, but assuming they start out far enough away from each other such a process must take a very long time, wouldn't it?

 

Wouldn't that depend on how they're spaced out? Or would they eventually collide with each other also, if they aren't equally spaced?

 

Well, I was mixing considerations without saying so with those two statements. Having multiple objects on more or less the same orbit is not stable in a long-term sense; over time, perturbations will cause things to bunch up and collide. That long-term sense is 10^7 years or longer in a planetary system that looks sort of like ours.

 

That said, if you did have three objects spaced around in the same orbit, and just examined what that would look like while it lasted ... you're right, I assumed that they were equally spaced (which is the longest-lived configuration).

 

The main planet, is not a sphere, but a torus! Though its inhabitants aren't aware of this fact (yet). And it rotates on a skewed axis, say around 23 degrees from the perpendicular. Its one moon orbits on a 28-day cycle, and its year is 496 days long. Most of this wierdness will serve no purpose in my campaign, other than to amuse me. Unless you can suggest something...

 

Actually, I've toyed with the idea of a toroidal world before. Making a map for such a world is actually easier than doing it for a spherical world....

[Lord Mhoram]

Re: Astronomy help (Dreamwalkers game)

 

That said, if you did have three objects spaced around in the same orbit, and just examined what that would look like while it lasted ... you're right, I assumed that they were equally spaced (which is the longest-lived configuration).

 

Yeah. That was what I was after. I like to know how things work, before I go change them.

[Cancer]

Re: Astronomy help (Dreamwalkers game)

 

As a side note: I think a human being standing (suitably protected) on the surface of Mars might, under the best possible circumstances, be able to separate Earth and Moon and see them as distinct objects, without optical aid. It's right at the hairy edge (the largest angular separation is about a quarter of a degree [half the size of the Full Moon as seen from here on Earth], but that's at inferior conjunction when the Earth-Moon system would be lost int he glare of the Sun, and you'd be looking at the dark sides of both bodies). At other times the Earth-Moon system would be further away, which means their angular separation would be smaller. I'd have to do a bunch of number-pushing, and look up some more info about the response of the eye at certain limits of performance, to answer the question. Even with just binoculars it'd be duck soup, though.

 

Similarly, the Galilean satellites of Jupiter are bright enough to be seen with the unaided eye from here on Earth, but they are so close to the much brighter planet that you can't tell they are there without optics. There are rare reports of people who say they've seen them, but that sort of thing is controversial and IIRC not believed. It doesn't take much in the way of optical aid to bring them out, though.

 

Uranus, in optimal conditions, is a threshold naked-eye object, but at that level it's more or less impossible to be certain you're seeing anything. That does take a substantial telescope to tell what you've got.

[PhilFleischmann]

Re: Astronomy help (Dreamwalkers game)

 

Well' date=' I was mixing considerations without saying so with those two statements. Having multiple objects on more or less the same orbit is not stable in a long-term sense; over time, perturbations will cause things to bunch up and collide. That long-term sense is 10^7 years or longer in a planetary system that looks sort of like ours.[/quote']

Darn. Those perturbations really perturb me! It would just be so cool if planets could do such tricks stably in the real world. But ten million years is not enough time for intelligent life to evolve to the point of being able to base an interesting role-playing game on it. Of course, with magic, you could do whatever you want in a fantasy world. But it would be kind of wierd to have a game of spellcasting eucaryotes and trilobites.

 

Actually, I've toyed with the idea of a toroidal world before. Making a map for such a world is actually easier than doing it for a spherical world....

Yes! That's part of the reason I did it that way! BTW, speaking of non-spherical worlds in fantasy, Narnia is in fact flat. It has an edge and you can fall off!

 

OK, next question: (Once you establish yourself as an expert in this field, people will start to bug you with more questions.)

 

Is there a physics reason that forces all the inner orbit planets to be small and solid, while the outer planets are gas giants? Is it feasable for a gas giant to be at say, the orbit of Mercury or Venus? And is it possible to have "gas dwarves" - gas planets the size of, say Earth or parhaps only a little bit larger? How small could a gas planet be?

 

The reason I ask, is that in my mileu, I wanted the innermost planet to be gas. Not that it really matters that much.

[Captain Obvious]

Re: Astronomy help (Dreamwalkers game)

 

Is there a physics reason that forces all the inner orbit planets to be small and solid, while the outer planets are gas giants? Is it feasable for a gas giant to be at say, the orbit of Mercury or Venus? And is it possible to have "gas dwarves" - gas planets the size of, say Earth or parhaps only a little bit larger? How small could a gas planet be?

 

The reason I ask, is that in my mileu, I wanted the innermost planet to be gas. Not that it really matters that much.

 

I'm not an expert, but What I Know Is:

 

The prevailing theory for a long time was that the inner planets are too warm to become gas giants. The heat boiled off the lighter elements early on. Astronomers have now found gas giants that are closer to their star than Earth is to the sun. I'm not sure how these "hot Jupiters" are explained, but they seem to be fairly common.

 

Gas planets as we know the term (ie primarily H and He) tend to be pretty big because they need a lot of gravity to keep those light gasses from flying out into space.

 

As a non-physicist, it wouldn't strain my suspension of disbelief to say that an Earth sized fantasy planet is entirely "air". I think you may be going for more scientific rigor than a fantasy setting requires. Some of the ideas you've thrown out are too cool to drop because they're not entirely realistic.

[Cancer]

Re: Astronomy help (Dreamwalkers game)

 

What Captain Obvious says there is a good start. Actually, planets initially form out of whatever is solid in their vicinity. Close in, that's just going to be the "refractories": metals, and high-temperature metal oxides. Once you get far enough out from the star, it's cold enough for the "ices" to condense out and become solids. It should be noted that in the standard composition of matter, there's about a factor of 10 more mass per gram of overall stuff in the ices than in the metals and metal oxides, so there's lots more matter to make initial planets out of once you get outside the "frost line" (where water, CO2, ammonia, methane, etc. start freezing out). That's important because models indicate you need a minimum of about 5 Earth masses is needed in your planet before it has enough gravity to capture and retain hydrogen and helium gas, and are about a factor of 10 more common (by mass) than the "ices": H and He make about 90% of the mass in the cloud, and their condensation points are so low they never become solids. Once the planet reaches that 5 Earth mass level, it can eat everything in its reach, including the gas. Lighter than that and the gases aren't kept by the planet.

 

So, big Jovian planets have to form well out from the central star. This makes a great deal of sense for our Solar System, but the "hot Jupiters" that are most of what we know about Out There are a problem.

 

The leading theory for how the "Hot Jupiters" got where they are is a "migration" theory. If the protoplanetary gas disk lasted a long time during the planet-forming process, then gas drag can act on all the forming planets, causing orbital decay. The disk gets dispersed before all the planets decay into the central star. Whether the terrestrial planets decay all the way into the star, or just get scattered out of the system or accreted onto the Jupiter-class planets as those barge into the neighborhood isn't well determined, but one way or other, any terrestrial planets that once were there are almost certainly gone. That doesn't exclude the possibility of satellites of those Jovian planets being present, though.

 

Why a disk would have lasted a long time in some systems but not in ours is still not well settled. Partly, that waits for better statistics on systems like ours, where the orbital periods of the big planets are more like decades than weeks (which they are in hot Jupiter systems).

[Basil]

Re: Astronomy help (Dreamwalkers game)

 

That said' date=' if you [i']did[/i] have three objects spaced around in the same orbit, and just examined what that would look like while it lasted ... you're right, I assumed that they were equally spaced (which is the longest-lived configuration).

If you have three equal masses equally spaced, with radially symetric velocity vectors, the configuation is stable. It's known as a Keplerer Rosette. Of course, the other planets in the system (if you have any) will perturb the Keplerer objects. I suspect it would fall apart, but I have no idea on what time-scale.

 

Actually' date=' I've toyed with the idea of a toroidal world before. Making a map for such a world is actually easier than doing it for a spherical world....[/quote']

Problem with a toroidal world is that, if gravity works as it does in our universe,, there is zero gravity along the inward-most "line of latitude." That is, the more-or-less "upward" gravitational pull of the much greater "far mass" balances the "downward" pull of the "near mass."

 

Rather like why the inside surface of a "hollow planet" would have zero gravity.

 

I will leave to those who know calculus the task of figuring how the surface gravity of a torus would vary with location.