I really like Logan's answer, but here's an alternative idea: Magicians can draw on their own planet's field, but can't do anything with it.

The idea here is that everything on a given planet is so acclimated to the magical energies of that planet that they outright ignore anything which said energies try to do to them. You can gather all the local energy you want, but no matter how hard you push it into a spell, it has no effect.

The reality-based equivalent to this would probably be related to how a person can get acclimated to a constant noise or smell, to the point that they don't even notice it any more. Or more like gravity or air pressure - humans are constantly under pressure from several tons of atmosphere, but we don't feel it because we're used to it (and if it suddenly disappeared, we'd notice).

Possible bonuses with this system:

• If there's a way for one magician to tell how much energy another has currently available, then drawing local energy can be used to bluff at being more powerful than is actually true.


• There can be an "acclimation period" when physical objects/creatures from one planet are taken to another. For the next X amount of time, they can draw on the local power to affect themselves, even if they can't use it on anything else. Maybe this could explain other supernatural creatures, either summoned ("I summon a demon to terrorize my enemies, and it makes itself immensely strong and invulnerable") or permanent ("I traveled to another plane and accidentally made myself a werewolf, and now I can't change back").


• Each planet could have some unique property that it contributes to magic, and people from that planet are no good at using it (becuase they can't practice it) but are also immune to it (so when travelling, they can't be affected by it). For example, if drawing on Mars let you do elemental effects and Venus powered mental compulsions, Martians wouldn't be able to cast a fireball (unless they spent a lot of time on other planes), but could stand in the center of anyone else's without an issue, while the Venusians would be immune to mental domination.

You cannot scratch your nose with your elbow.

The astral projections of these magic users varies to some degree but is always immense, and centered on their physical form. Their own home world and other objects in the vicinity are within this form and inaccessible. Near objects can be accessed only with painful contortions. There is a set of objects within easy reach of their astral projections. Objects that are too far away are likewise unreachable.

Astral projections of non magic users might be considerably smaller, but even if somehow they learn or develop the skills to make them, these projections are of no practical use to a non magic user. But could a non magic user able to project herself team up with a magic user?

Drawing magical power affects the local magic field.

Attempting to draw magical locally creates a negative feedback loop the prevents the magic from working. By drawing the power from a remote source, the feedback effect it out of phase or otherwise diminished allowing you to get a successful power draw.

Perhaps powerful magicians can draw power locally with limited success, but the best bet is to draw from far away.

Creating a fissure in a large nearby source will create a flow of energy that is too strong to manage, and maybe dangerous. A fissure to a far-way place dissipates most of the energy along the way, or to maintain a fissure, so the resulting energy is manageable.

Maybe your hero can find a way to create micro-fissures to own planet, which limit energy to manageable level, and hence have a readily available source.

Star systems and their inhabitants share a specific resonant frequency. You can’t draw magic from something with the same frequency as you. Boom.

Feedback interference. You know how radios have feedback loops that disable them in close proximity? It's the same type of interference that makes it impossible to draw power from the planet you're on. Of course you could be more technical about it, but as far as a general explanation, this is good enough.

Additional: Mana exists in fields that exist around planets. Trying to siphon off mana from outside the field is easy, but drawing mana from inside its field is very difficult. This is the same sort of system that keeps electrons in fields around atoms. If you hit an electron with a strong force, it will simply bounce away from the nucleus before it comes into contact. However, electrons can be stolen by outside forces with a strong pull.

What immediately comes to mind when considering this question is that the the system is symmetrical - that is, it works exactly the same even if someone from planet 1 is transplanted to planet 2. It is not a subjective exclusion from the current planet. The energy of the current planet is objectively unusable. So what would make that happen? The simplest answer, for me, is that something in the transition from planet X to planet Y is necessary for it to become usable.

To find what that is, I would look at the physical layout of a solar system. What is between you and another planet that isn't between you and your own planet? This could be some object in its own orbit, or maybe the energy must be drawn through multiple planets to amplify. But my thought is that it must be drawn through the sun.

Why? Well, it's big, bright, and your planet's energy can never ever go through it on its way to you. The sun could act as a focus or an activator of another planet's energy, and that energy must be (inadvertently) drawn through the sun to be useful to a magician. Or perhaps the journey of the magical energy through the sun's radiation between planets activates it and it doesn't physically have to go through the sun, but the idea is the same. Something between the planets (the sun or some other arcane object) acts as a catalyst for magical reactions. The energy around planets is the fuel, but it needs a spark somewhere out in space that just isn't available on the planet you stand on.

The other simple observation is the sheer distance involved. I liked it Renan's inverse square law idea, but I like it better the other way. The further the planet is from you, the greater distance the energy has to build momentum. If you draw it from a paltry 4000 miles from the center of your planet (or less if it's from the atmosphere), it is equivalent to a musket-ball rolling out of the barrel. But, drawing from a Pluto analog 5 billion miles away, your magical bolt comes screaming like an abused particle doing loops somewhere in a Swiss lab.

Think of it as potential energy in a gravitational field - except the gravity is the magician pulling on the magic. The further it has to fall (be pulled), the more time the force acts upon it, applying an acceleration the whole time. Even a small (whatever small is in magical terms) acceleration over 5 billion miles produces an inordinately larger velocity than an acceleration across 4000 miles. And translating "velocity" into magical power, that's the ballgame. The furthest source of magical energy on your planet just isn't far enough for any significant, noticeable power to be built up.

It's like thermodynamics

You can't draw energy from a colder object to a hotter object, nor a colder to colder nor hotter to hotter. Only is possible from a hotter to a colder object.

Each planet has a different kind of mana (mana1, mana2, mana3...) that could be interpreted like different "kinds of heat" (heat1, heat2, heat3...).

Each planet only has its own kind of mana (they are warm in that sense) but they lack of other kinds of mana (they are colder in that manas). So mages are able to draw mana from others planets to their owns due to the principle hot -> cold.

A mage can't draw mana from its planet because he is already inside of it, he already has that kind of mana in its body so both objects have "equal temperature" or "equal mana type and amount", and you can't transfer from hot -> hot. Also, he can't spend its mana to cast spells on the world because the same principle, both things (he and Earth) share the same amount of mana.

If this isn't understandable tell me in comments and I'll try to explain it better.

Inverse square law.

Supposing you are trying to tap the energies from planet X to cast the universal problem solving spell, fireball.

Supposing planet X is at its farthest, you can cast a fireball the size of a baseball. Half a year later the planet is at its closest and now your fireballs are the size of basketballs.

Everytime you halve the distance to a planet, the amount of energy you get from it quadruples for the same fissure size.

So if by tapping a planet that is a few light minutes away you get enough energy to create a basketball sized fireball, how fast will thermodynamics turn you into plasma if you try to tap a planet that is zero inches away from you?

The local mage guild has probably gone quite out of their own way to prevent obnoxious mages from becoming walking nukes.

Electric energy requires opposite charges for current to flow.

The same thing can count for magical energy. However, unlike electricity, there are an unlimited number of different kinds of charge.

The more the sources of two charges differ, the more powerfull the release of magical energy. (Mercury/Jupiter vs Neptune/Uranus).

This allows people who know how to tap Jupiter or Pluto to be more powerfull than people who only know how to tap Mars.

You could have it that magic works from a "second level" physical factor. By second level, I mean something like the way you have the really obvious physical factor "speed" - easy to see in place everywhere about you, and to remember your own experiences - and it would be a first level factor, and then you have a second factor, well-known and experienced by many, etc., called "acceleration." Without speed, there is no factor called acceleration, so speed is the first level factor, then acceleration is a change in speed and so a second level factor.

Building on that, consider two magnets. Put North vs. South and they stick together through their magnetic force. And never do anything else. Move them separately so that their forces can interact and you generate electrical force if you have a suitable "vessel" for it (a circuit in this case). Whumped together so they are locked, there is no changing interaction and no useful electrical force. Kept separated and changing place relative to each other and it is possible for their interaction to generate a seemingly wholly different force that has interesting capabilities of its own.

So... consider the idea that your magical power factor depends on a second level factor that requires first level factors moving relative to each other. In that case, you could observe three basic outcomes:

1) Things very close, like the whumped together magnets, haven't enough relative motion to generate enough of the second level factor (magic). So the magic user would, technically, get some power from his own planet, but soooo little of it, no one ever noticed or measured that teensy bit.

2) Farther away things would obviously move relatively to their planet. Look at what even the moon does with tides. Since YOU get to decide the background, it could be OK that the magical power is significant at solar system distances. So everyone would notice the magical power, and they would realize - lol, wrongly, but hey, it's magic, not science, right... or maybe it's the scientific revolution period for magic on your world and they WILL come to realize it's not the planets and so on themselves, rising and falling in contributed strength based soley on distance, or weird properties that don't seem to exist on the populated planet, but rather the change, the relative movement - but anyway, they realize the magical power they use "comes from" the other solar bodies.

3) Distance COULD be a factor even on a near solar scale. So for us, Pluto would be noticeable nowadays, but when it runs out another billion miles in its orbit, it might not be very noticeable even when on the same side of the sun. And it could be why extra-solar things have little impact.

Naturally, there would be other things to consider in. Maybe the sun's composition is too low on the periodic table to have much effect so it doesn't swamp everything out (because anywhere habitable in the solar system might be kind of like the "magic user-his own planet-not enough change to give much magical power" thing. It could turn out some things could mask (insulate) the power or make the power futzy, like Jupiter having tons of moons and them making its contribution rise and fall rapidly and in weird ways. Just studying that could be a career. Magical items could be of interest, not really how they hold/contain the power, but more the idea that all people, even animals, actually have magic-use capability but very little. VERY little. So they don't even power up from what powers up an average magic-user. But if a magic-user concentrates the power into an object, like ramping up energy in fast discharge capacitors is used for powering lasers to stunning power levels, the average person, even animals or even plants, could then be handling a power level that even they can make work. So a +1 sword works well for me, or might be a +6 sword for someone good with magic even without the item's stored power. They would also run down...

Hence also propitious moments for some events and uses of magic. Just the right combination of planets and other astrological faves. And given they think the power is actually directly coming to them, not being generated all around them and they just use it well, worshipping various sources as gods could be a natural outcome as well.

Perhaps something like that. Simple without being awkwardly simplistic. Simple to grasp the idea when explained, haltingly in the text if a book, or game if a game, let's one happily forgive the reality of magic being so much hoo-ey, and let's one plunge right in and enjoy. But the simpleness hides your ability to slowly reveal a deep texture and outcome profile, a truly rich background to work with as author or programmer. And endless supply of operational situations. After all, look at Star Trek. We accept the idea it all works because it all seems so plausible. No one in the shows has to step to a chalkboard and teach the fans that it all starts with being able to generate and control stunning levels of energy and that transporting is actually creating a warp field around the person and moving that person like a starship does in its warp field, and stopping them just right and dropping the field... who cares? It is presented so nicely you slide right in and it's real and lends to the milieu's amazing richness and fun. Same here. Well, if I do say so myself that is.

Holistic approach - you tap into the planet "as a whole". The magical field is a holistic thing. You need to have the entire planet in your field of vision (some of it can be obscured, that is ok), and your own planet is just too close for that.

It's a case of not being able to see the forest due to all the trees.

Different potential. Like in electronics. Voltage is actually a difference between potentials. If you're standing on 10 kV and touch 0 V, there's current and it will hurt. However if you don't have a second potential, it doesn't matter what potential you're on, there's no voltage and therefore no current.

This way you can have different levels of energy flow (voltage differences) with different planets even.

The magic "energy" is pretty harmful when "raw" and freshly tapped. By letting it pass through space, most of its harmful properties disappear - some things simply dissipate and some change because of things you encounter out there, like cosmic rays and solar radiation. Only after such "filtering" is the magic rendered safe and usable.

Resistance / immunity

This explanation has a number of interesting and negative side-effects that may not suit your world.

Every planet has a magical field. Life on any given planet has evolved to develop immunity to their own planet's field. It turns out that cellular growth while within a magical field results in too many mutations. Basically, magic acts something like radiation. In order for life to develop, therefore, evolution takes it down a path of very high resistance to the magical field of the planet that life is developing on. However, an evolutionary resistance to the native magic field also means you're not very good at using it.

So, an earth wizard using earth magic can exist, but very weakly. He is so immune to his own world's magic field that he can barely interact with it at all. He can do simple cantrips but that's about it. However, this weak interaction was enough to allow wizards to stumble upon the ability to open rifts to other realms, and what a mother lode of easily manipulated magic they found there!

However, this explanation implies some potentially interesting side effects:

• Visiting another world is a hostile environment that will hurt you. Could be anything from "instant death by radiation exposure" to "you have a somewhat increased chance of developing cancer". Don't send your kids or pregnant wife to other realms. (The people may not understand exactly what's happening -- cellular damage -- but they recognize that going to another realm is "toxic" somehow.)


• Perhaps this can be mitigated by bringing your native magic field with you. Just as you open a rift to another realm to use their magic, you could travel foreign realms safely if you maintained a rift to your own world, surrounding yourself with your native field for safety. Maybe this is done by maintaining a spell or by carrying, say, a native lump of iron around with you.


• There's a potential "kryptonite" scenario here where assaulting a foreign wizard with his own planet's magic field hugely weakens him.

Have to think more about the implications but I like the idea that magic is a natural force, slightly different for each planet, which natives are necessarily, biologically immune to.

A few ideas below.

If your mage is standing on planet Y and casts Y magic, it works fine. It's just that the overall pressure/intensity/aura/field/etc. of Y magic on planet Y is so intense that Y magic spells are basically worthless. Like having a squirt gun battle under water. The guns work fine, but the effect never manifests.

It could be that once a tear is opened, the mage isn't pulling magic force from the other planet, rather the mage is pulling magic flavor/type/affinity. The magic of the planet that the mage is standing on works as the force behind the magic. The tear adds the flavor. You need both flavor and force to have a spell. Force or flavor alone is either useless or an interesting story point to be developed further - your choice.

Maybe the magic of the planet that the mage is standing on is being used at maximum capacity. Options include keeping the mage alive, powering his magic circuits, preventing him from being taken over by some kind of extradimensional entity or spirit, letting his mind and/or soul work, preventing spell backlash, powering parma magica (defense shield) - really anything that is geared towards survival and/or defense.

For a less magical and more physics-based idea, one could explain this
in terms of kinetic energy.

You’re actually drawing energy, not from the entire planet, but from the
nearest point on the given planet. For your own planet, that’s the spot
you’re standing on, which is at rest relative to you, so there’s no
kinetic energy to be used. But for any other planet, the nearest point
is moving at many kilometers per second relative to you, so there’s
plenty of energy to be had. The exact amount would vary according to how
far away the planet is and the locations of both planets in their
orbits.

Vernor Vinge’s novel The Witling uses a similar concept with
people who can teleport things. Most people can only teleport from
places they’ve been, which allows them to essentially create projectiles
by teleporting rocks from a different place on their planet. But a small
minority of people can sense matter at a vast distance, and create
vastly destructive meteors at will, making them the equivalent of
nuclear-armed states on Earth.

Building partially on Logan idea - magic is kind of like electromagnetism where each plane has potential but the only way to get a useful energy is either by connecting two celestial bodies by magic conducting rod or by field generated by accelerating magical bodies.

To a very close degree a mage does not reliably accelerate with respect to his planet. On the other hand planets are in constant accelerated motion with respect to each other so mage can retrieve magical energy from field generated by moving magical potentials.