where to hide a planet?

In Buckaroo Banzai they hid a planet in the 8th Dimension, but I think you're probably looking for something different.

Actually, there is no way to do it. Especially one with a mass similar to Earth's. Consider how Pluto was discovered. Percival Lowell and others looked at perturbations of Neptune and Uranus and predicted where it would have to be. Lo and behold, there it was. Something of Earth's mass would cause much larger perturbations, and since the distance scale of the inner solar system is so much smaller, have a far greater effect.

Putting it on the other side of the sun wouldn't get rid of the perturbations, and consider that the sun is only spans about a half a degree in the sky. It would be impossible, even with an identical orbit, to hide anything behind a half-degree area like that.

Sorry, but without magic, which is fine if it is consistent with your world, you can't hide an Earth-sized planet.

I was going to say exactly what this man said, about the gravitational effects of large masses. Within our own system? Not really hard-science possible.

Depends just how powerful these wizard's magic is.

They could make it out of dark matter.

Could they perhaps 'make' two planets? One to live on and the other to cancel out the perturbations caused by the first?

Also, what about not a planet, but a DeathStar like space station at one of the solar Lagrange points?

MelancholyMan said:

Also, what about not a planet, but a DeathStar like space station at one of the solar Lagrange points?

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Wouldn't work. People would constantly confuse it for a moon.

Two problems:
1) gravity
2) light

If it has around 1g it would be noticable by gravitational effect.

Any planet with earthlike conditions will be highly visible. It needs enough light for the plants (and not to freeze) which means light gets reflected which means we can see it with telescopes. Being on the other side of x doesn't matter, one of the space probes going out of the solar system (like the voyagers) would probably have picked it up. If we're looking for it we can easily send a probe to a position to see it, wherever it hides.

So, to make it invisbile the magic needs to cover both of these effects. For the gravity i should add that it could not simply not interact gravitationally (like a gravity equivalent of invisbility) because then it wouldn't stay in orbit either. But if the magic makes the whole planet invisible from a distance and holds it in orbit/provides the 1g without causing gravitic effects on the system it works.

I'd say for example a cloak around the planet so it's invisbily unless you're close, say a few lightseconds. And then make it a hollow sphere where some magical device in the middle provides 1g within, say, 20km of the sphere. This makes space travel from it very easy, and who says that magical gravity obeys the inverse square law.

GeorgeK said:

1: Somewhere between Sol and the outer edge of the Ort Cloud, not visible from Earth. If the math says it should be there, but we don't have the tech to see it, that's ok. If the math says it's absolutely laughable, I don't want it. e.g. Would it be ludicrous to have it in the same orbit as Earth but on the opposite side of the sun?

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The same orbit as Earth on the opposite side of the Sun would be ludicrous.

Somewhere far out in the Oort cloud is possible from a detection standpoint. We simply don't know what's out there well enough yet to notice the kind of perturbations it would cause.

The thing is, putting it out there creates a whole different problem: where does it get heat from? The Sun would be 5000 to 50,000 times as far away from that planet as it is from Earth. The amount of energy received from the Sun would be negligible. The habitable zone--range where liquid water can exist--for our Solar System is estimated to be about 0.95 AU to 1.3 AU.

If you put it out in the Oort cloud you're going to need a magical energy source to keep it warm. A very large magical energy source.

GeorgeK said:

2: Large enough to hold an Earth-like breatheable atmosphere and ecosystem. If it only has fresh water oceans, that's workable.

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Hold an atmosphere for how long? Are you talking thousands of years or billions of years?

Again though, to have a breathable atmosphere and liquid water, you need heat.

GeorgeK said:

3: Somewhere in the range of 0.75 to 1.50 G

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This range for gravity pretty much makes holding your atmosphere work OK. Where you go from there depends on the density of the planet. Earth has the highest density in our Solar System.

GeorgeK said:

4: You have 10,000 years to construct it without advancing technology beyond current limits.

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Current technology? Working several thousand AU out in the Oort Cloud? Do you have a magical transportation system, too?

GeorgeK said:

5: Oh yeah, wizards exist. They can affect stuff but not create out of nothing. They can mutate things by splicing genes and get stable inheritable traits but can not truly create life. They can't change time or do any interdimensional stuff.

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Can the wizards work at a distance? Or do they need to be present out where the planet is being built?

Can they transmute elements (make iron out of hydrogen, etc.)? Or do they have to work with the mix of elements they find in the Oort Cloud? They can't take much from the inner Solar System without being detected--the Asteroid Belt going missing would be kind of noticeable. But the heavier elements are going to be pretty scarce out there.

GeorgeK said:

6: What am I neglecting?

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Energy. Raw materials.

Hmmm well considering this if this is intended to be Science Fiction, and therefore can be based off of unreal science and technologies. I do not see a problem with it...Oops and Fantasy which can be based off of an unrealistic ideal/concept. But now that I reread it, we are not actually talking about writing about this possible hidden planet, so nevermind....ooops.

Maybe you should consider an inverted planet - a bubbleworld.
Such a planet could have a very low albedo, making it difficult to detect.
It could be spun for gravity, making its size variable - and thus less likely to cause orbital perturbations.
It could have an internal heat source, allowing it to be placed outside of the Sun's bio-thermal zone.

I agree, it's unlikely you could hide an Earth-sized planet in or even near the Solar System. Maybe hide it under the clouds of Jupiter, but that would (even discounting the impossible logistics) mess up the planet's breathable atmosphere, and the extra mass in Jupiter would perturb its moons' orbits.

MelancholyMan said:

...
Could they perhaps 'make' two planets? One to live on and the other to cancel out the perturbations caused by the first?

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I don't think it works like that. More masses just means more perturbations, and more complex perturbations.

Smiling Ted said:

Maybe you should consider an inverted planet - a bubbleworld.
Such a planet could have a very low albedo, making it difficult to detect.

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If the surface is anywhere near room temperature, its direct radiation could be detected. If you can lower the surface temperature to 3k, then it looks just like the cosmic background radiation and you only have to worry about the occasional occultation of stars. If the internal temperature is greater than 3k, you can't keep the surface temperature at 3k for very long.

Smiling Ted said:

It could be spun for gravity, making its size variable - and thus less likely to cause orbital perturbations.

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I dunno what "making its size variable" means, but I see where you're getting at with "less likely to cause orbital perturbations" by having a hollow shell that's much lower mass than a planet. This would be something like a mini-Ringworld.

Smiling Ted said:

It could have an internal heat source, allowing it to be placed outside of the Sun's bio-thermal zone.

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Something internally sealed would definitely be easier to heat than a typical planet with its atmosphere exposed to outer space, and also easier to hide (see above). "Current technology" for heating would directly use the heat from a nuclear ("conventional" fission) power plant.

"Never let reality get in the way of a good story" - John Kepler

GeorgeK said:

Making 2 planets if that easily solve the problems would be fine.

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Makes problems twice as big unfortunately.

What I'm trying to figure out is non magical means of maintaining the status quo after construction.

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Did you ever read Iain Banks? Just go and outright steal his orbitals. The idea is basic enough that noone can blame you for just taking it. Anyway, and orbital is a big rotating ring (providing gravity as well as day/night changes) in orbit around a sun. Since it is not massive it could easily be light enough to not be noticed, especially since it'll be small if you don't want millions of people living on it. Orbitals are the way to go when contructing living space anyway, since cunstructing a huge chunk of rock (read planet) to only really utilize the surface in the end is a real waste of effort.
As for heat, light and athmosphere that's where you'll need magic. Heat and light can be simply provided by the sun though if it's in an orbit in the habitable zone.
For athmosphere i see to possibilities: 1) have magic hold the air in. 2) make the orbital airtight. Magic is needed for 2) also, since a technology level similar to our own cannot produce materials stable enough to built such an orbital. But contrary to 1) you could explain the magic as transmuting some kind of material with extreme tensile strength which then doesn't need magic to be maintained. Depending on the way you want to construct this orbital you could either go the Iain Banks way (pretty much indestructible transparent crystal which makes up the structural elements of the orbitals) or get a bit more low-tech having only cables with near infinite strength. Works too, but is a bit more tricky construction-wise. If the specifics of the orbital aren't all that important i'd just go with the super crystals. If you don't have the info dump you don't have to be careful to avoid infodumping it.
For making it invisible i really see no alternative to constant cloaking done with magic.

I might have to go back to my original assumption of the dyson (sp?) sphere. I was hoping I could give it a traditional atmosphere, but that's sounding less and less likely or rather stretching it too far.

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Dyson is corret (like freeman). A dyson sphere being a sphere around a star though i don't quite see how it fits into you op? It being pretty damn noticable if there's a sphere blocking out the star and all.
As a not to prevent misunderstandings here: The orbitals mentioned above are not ringworlds around a star. Ringworlds as well as dyson sphere are mindblowingly big. An orbital is a ring that rotates, orbiting around the star. Basically like the station in Babylon 5, only that it is a lot more sensible to have the station be a narrow ring with a big radius, instead of a wide ring with a small radius. Bigger ring means you can have slower rotation for the same gravity and the centrifugal effects are less. And as a plus, if the width is no more than half the diameter (iirc) you can simply use a nearby sun for heat and light. You'll not get a 24h cycle though unless you rotate at 1Hz, which is not arbitrarily chosable since rotation and diameter determine gravity.

People really only know of the skies for the last century or so as far as the closer moons go and only a couple millennia as far as planets. This place will have been constructed long before that, so any gravitational effects might have run their course before people started watching the skies. If Jupiter maybe started out with 57 moons, but they were gone before people noticed, that's ok.

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That's not quite what gravitational effects are meant here. We can calculate the existence of massive objects by the pull they exert on other bodies. I.e. it's not that we see asteroid getting shunted around because someone starts moving jupiter, but that we can see suns wobble because they have massive planets. If we couldn't see venus for example, we could reconstruct its (rough) position and mass from the orbits of mercury and earth, since earth and mercury would move differently if there were no venus. The more massive and object and the less massive (and close) other visible objects are the easier it is to find of course. But nothing with the mass of a planet could from us inside our solar system. Not even a non-planet like pluto.

Could you make the planet very small and hold the atmosphere with magic? That would be interesting.

GeorgeK said:

Yes that would be possible, but it might tie up the wizards. To do something that big will require constant attention. Of course the story only features a few of them, and they could be on sabattical.

If I pick that, what would be a good size? I'd want it to be as big as possible without being noticeable mathematically. Cloaking it is not the problem.

I also don't want it so dependent upon magic that if one guy falls asleep on the job all the sheep in grid F-3 suffocate or go flying off into space.

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I have an idea you're free to steal if you find it interesting. Could the wizard's magic be amplified by regular energy? E.g., a nuclear reactor or power plant generating enough power to hold an atmosphere isn't very believable, but what about a power plant producing magic to keep an atmosphere? A wizard could channel it or something, but it'd lessen the burden. I don't know. Just something I thought of.

Well, Eris went undiscovered until a couple years ago, and it's slightly larger than Pluto. It has less mass than our own moon, though. If you can use magic or a magic generator to keep the atmosphere, though, having a larger planet with a low enough density to have unnoticeable mass might be possible. The low gravity would make getting around difficult, though. In this case, though, perhaps the people on it would merely evolve to retain far more body mass somehow to give them more weight and more easily walk around?

GeorgeK said:

Ok, then I'm using the wrong term with dyson. I was thinking an inverted planet then. (Construct an airtight egg shell the size maybe of pluto. Toss some debris on the outside to disguise it as a lump of rock and live inside with the nuclear reactor, and spin it). Ship access points at the poles.

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There are two problems with this. First, a planet like that will be highly noticeable since it will glow like a lightbulb on infrared. Though that's a minor problem since the "staying invisible" part really, seriously, requires magic. There is no technical solution to hiding from telescopes.
The second is that a hollow sphere is better than an artificial planet but still wastes lots of resources. Since centrifugal force is determined by radius and rotation, you will only have the proper gravity in a band around the equator. As you near the poles, the gravity will diminish. And gravity is perpendicular to the spin axis, not to the sphere's surface. Which is a much bigger problem than it might look like at first, since for example all rivers don't flow toward the ocean any longer but toward the equator, you have a lot of space that will be simply unusable. There's a reason i like orbitals so much. It really is the simplest and most sensible design for any kind of artificial land in space. Small stations that are built like buildings themselves would be designed differently, but any colony big enough to build building inside of it is most efficiently built as an orbital. Just imagine a big doughnut, the inner half transparent to let the light through, and people living on the flat land on the inside. Well, and the cross section should be flat not round of course, no point in having a half-circle cross section of earth underneath your feet that doesn't get used. So, more like a roll of duct tape than a doughnut.
It really is the most elegant way to get gravity and light in one go. And if it's a closed shell it's airtight as well.

Edit: This was Lhun not Maraxus. He forgot to log out before using my PC.

GeorgeK said:

Doughnut Planet...Assuming it is not compartmentalized, would there be wind because of the rotation and I guess the action of friction of the solid stuff interacting with the air?

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No. Because there is air friction the air will turn along. You don't stop or start the rotation of an orbital anyway, so that problem doesn't really come in the first place. It's a rotating ring in space, i.e. you do not have some static axis in the middle that stays still relative to the rotation. (Though you do have a spaceport in the middle that'll be connect to the ring via a number of spokes.
Theoretically you could notice the effect of different speeds on a rotating disc in the air. I.e. the inner parts of a disc move much slower than the outer parts.
However to prevent that (and other nasty consequences of the principle) you have a big ring. You want a big ring anyway, since a small ring needs to rotate very fast to create the same gravity, which means very fast night-day changes. I'm busy now but i'll calculate radius and rotation speed for a 1g 24h day orbital later today.
Slower rotating orbitals could always be used with a bit of technical help, i.e. use sunblinds when it's in the sun, and artificial lights when it's in the shade to keep a 24h rythm even though it rotates slower. For faster rotations that can work too, though there'll soon be a limit to the usefulness of such a system compared to just using artificial light all the time.

What would happen to a ball if thrown? or a bird in flight?

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Birds fly normally because their movement is relative to the air around them, which moves just as the rest of the orbital. You can actually get wind and other weather conditions if the orbital is just big enough through the normal weather mechanics.
Balls would theoreticall describe a curve. I.e. from the point of view of tha ball, when it gets thrown upwards its own rotation speed slows down and the ground moves along below it.
However this is again dependend on the size of the orbital. It's linear, i.e. if you move half the radius, your rotation speed slows down to half. I.e. if you throw the ball half a radius high, it will move sideways at about half the rotational speed of the orbital. (actually less to lazy to do the math)
Now imagine how relevant that is if your orbital has a decent size, say a thousand kilometer for a radius for example.

GeorgeK said:

Actually, that's about what I was aiming at. Also, if it matters, I'm not wedded to the 24hr day. I was figuring that this would have to be far enough out that sunlight was mostly for ambience than solar gain of any sort.

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Well, it doesn't have to be that far. Being far out will not help with the being visible like a lightbulb problem, so you can just put it in the habitable zone and use the solar energy.

Also, if there is water, forming a doughnut river at the point of maximum G, it would not have a current, right? It would be pretty much still?

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Yes. It'd be a long and thin lake. Of course, if you have a flat orbital (roll of duct tape shaped) you can just have normal lakes and rivers.
Anyway, what you need for proper rivers is proper geography. Lots of mountains, glaciers at best, places for natural rain reservoirs and significant height differences for the river to run down. That means a truly BIG orbital. Not just one with the surface area of a small country, but more like small continent. If you don't want that, you could obviously easily use some artificial means to pump water up a slope and create an artificial river.

What if there were a moon orbiting this? Woould that give the water a current?

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No, moons create tides, not currents. However a moon wouldn't orbit, since the point of an orbital is to have a lot of usable land with the least amount of mass necessary. While a moon needs a big mass to orbit around.

So, i looked up the centrifugal force equation:
Fc = mv²/r, Fc = centrifugal force, m = mass, v = speed, and r = radius.
Or to get the equation in a easier usable form:
given you want 1g,
10=(2 pi r w)²/ r
r=1/3,6 w²
So, as long as the orbital follows this relation, i.e. the radius (in meters) is equal to 1 divided by 3,6 times the square of the rotations per second the resulting centrifugal force will be 1g. Or to avoid stacking the qoutients: the radius (in meters) is equal to the square of the time a rotation takes (in seconds) divided by 3,6. (actually [2 pi]²/10 if pi isn't 3 )
And if you don't want to do all that math, just play with this:
http://www.artificial-gravity.com/sw/SpinCalc/SpinCalc.htm

Anyway. Turns i out i remembered the dimension a bit wrong (was long since i last calculated this)
With a target gravity of 1g you will always have a rotation that is much faster than 24h hours, or absurdly big radii.
The way to sole that problem is simple: mirrors. You simply have mirrors on the edge of the ring directing sunlight inwards, so you can simulate a 24h day without needing powered light.

As for a size, that depends a bit on what you want to have. A big park with a few houses? Pick any size. An environment where you can do enough farming to sustain the inhabitants? You'd need a few km of air as a buffer and to get decent climate conditions without the fine control needed for a greenhouse. So to prevent problems from coriolis force the radius should be enough so that 5km is not a significant percentage. Say, 200 or so.
Beyond that, i'd just say figure out how much area you need and adjust the orbital size accordingly. width of the ring should be significantly lower than radius, maybe 10%. Well. how big do you need the place? Say, pick a country on earth with a similar number of inhabitants and go by that. At best a european country or something with similarly high population density.