[SPACE] Magnetospheres for Binary Planets?

[Trachmyr]

For the physicists amongst us; can a binary planet/moon system generate a magnetosphere if the barycenter lies between the two bodies but they are tide-locked to each other? Assume that the planet in question has a molten iron core. How short would the planet/moon orbital period need to be? Would the magnetosphere of the planet ‘stretch’ to cover the moon (I assume not)?

Thanks in advance!
- Trachmyr

[David Johnston2]

I'm no physicist but I don't know what the baryocenter would have to do with the magnetic fields of the respective bodies anyway. One is gravity, the other is magnetism

[Trachmyr]

I was under the impression that the magnetosphere was generated due to a planet with a molten iron core rotating on it's axis... but I could easily be wrong, thus my question.

[Sunrunners_Fire]

Originally Posted by Trachmyr

I was under the impression that the magnetosphere was generated due to a planet with a molten iron core rotating on it's axis... but I could easily be wrong, thus my question.

Luna has a magnetosphere.

In answer to your question: Absent some really weird planetary compositions, the binary pair will each have a separate magnetosphere that provide little to no protection from particles.

[lwcamp]

Originally Posted by Trachmyr

For the physicists amongst us; can a binary planet/moon system generate a magnetosphere if the barycenter lies between the two bodies but they are tide-locked to each other? Assume that the planet in question has a molten iron core. How short would the planet/moon orbital period need to be? Would the magnetosphere of the planet ‘stretch’ to cover the moon (I assume not)?

No one is really sure how strong planetary magnetic fields are generated. We know it has something to do with a molten conductive core, and probably something to do with planetary rotation, and probably something to do with convection, but the intricacies of the problem extend over such a huge distance and time scale that it is impractical to calculate or simulate, and experimentally getting a self-gravitating ball of liquid metal is a bit difficult.

So, since tide locked planets tend to have longer periods of revolution than Earth's period of rotation, you would initially expect the field to be weaker but the truth is there are too many variables and we might not know half of them. Unless the planets were very close, you would expect each to have its own magnetosphere.

Luke

[teviet]

Yeah, I started to look at the literature on this, and the impression I got is "it's complicated". These things are addressed with detailed numerical simulations rather than simple (or complicated!) theoretical formulae, so unless someone ran a simulation for your particular configuration...

One hint that I did get was that the models were very sensitive to the thermal gradient in the core, which drives the convection. Rotation then acts to organize the convective cells. My sense is that as long as the rotation is "large enough" to provide some preferred orientation, its precise value should not affect the field so much. But how much is large enough? Would a corotating binary planet have enough? I can't say for sure, but probably yes.

Some other factoids:

1. On Earth, magnetic pole reversals occur every few hundred thousand years, presumably because random convection overwhelms aligning Coriolis forces. During a pole reversal, the Earth's magnetic field is quite disorganized and about an order of magnitude weaker, but is still plenty strong enough to produce a magnetosphere.

2. Venus has no significant magnetic field. Venus is slowly rotating. This might seem suggestive, but Venus also lacks plate tectonics and thus probably has a shallower internal thermal gradient. Conventional Wisdom™ is that Venus lacks a magnetosphere because its core is not convective, rather than because it is a slow rotator. In any case, a world tidelocked to its moon will probably rotate faster than Venus.

3. Mercury has a magnetic field strong enough to produce a magnetosphere, despite its small size and slow rotation. Again, it's the core convection that's though to be key, not rotation.

So my guess is that your world's magnetic field should be based more on the core type and level of volcanic and tectonic activity, not so much on it's rotation. The field may be more disorganized or have more frequent pole reversals, which might reduce the average field strength, but not enough to eliminate the magnetosphere altogether.

As for whether there's one or two magnetospheres, that depends on how close the moon orbits. The size of the Earth's magnetosphere (standoff distance to the magnetopause) is 10 Earth radii, and it scales very weakly as the cube root of the magnetic field. Our Moon orbits at about 60 Earth radii. If your moon is much closer, or your magnetic field much larger, then the moon might orbit entirely within the magnetosphere. But a system with two magnetospheres will probably have some very interesting aurorae!

Sorry for the text wall, hope it's helpful.

TeV

[Trachmyr]

Thanks for the feedback, really appreciate it!
(Plus TeV you gave me a useful idea on how to model the planet's occassional radiation threat... pole reversals every few decades!)

[lwcamp]

Originally Posted by Trachmyr

Thanks for the feedback, really appreciate it!
(Plus TeV you gave me a useful idea on how to model the planet's occassional radiation threat... pole reversals every few decades!)

For what it is worth, even without a magnetic field life on earth would be well protected from radiation by the 10 metric tons per square meter of air over our heads. Realistically, to have a radiation threat you would need a much thiner atmosphere which probably means humans cannot survive on the surface unaided.

You might see a depletion of the ozone layer with the lack of a magnetic field and high space radiation conditions, which would let more near UV through. This would result in more sunburn and a greater risk of skin cancer and cataracts, plus it might kill the leaves of earth plants (or other plants that have not adapted to harsh UV light).

Luke