The evidence from Earth, Jupiter and Saturn determined that a planet's magnetic field should be like that of a bar magnet, with a north and south pole that runs roughly along the sphere's rotational axis.

But Uranus -- and Neptune, too, Voyager found -- is radically different.

Their magnetic fields are tipped over (the north-to-south line lies midway to the equator or even closer) and there are two north and two south poles, as if the field were produced by two bar magnets.

The reason for this, according to a new theory published on Thursday in the British journal Nature: The underlying structures of Uranus and Neptune are radically different from what was previously assumed about these cold, distant planets.

Planetary magnetic fields are created like a dynamo: beneath the planet's surface, there is an electrically conductive fluid, which is driven around by an energy source.

In the case of Earth, for instance, the planet's solid inner core is bathed in a molten iron-rich fluid, which is propelled around by the planet's rotation and by convection currents, which transfer heat from the core toward the surface.

And in Jupiter and Saturn, which are "gas giants" rather than rocky planets like Earth, the magnetic field is believed to derive from a thick layer of sub-surface hydrogen, compressed by gigantic pressures into an electric soup of protons and electrons, which revolves around a small solid core.

Where Neptune and Uranus differ, the new study says, is that even though they are gas giants, their interior structure is different from those of Jupiter and Saturn.

It suggests that these two outer planets may have only a thin layer of metallic convecting fluid.

This has a big effect on the magnetic field, limiting it to a thin "shell" just under the gassy hydrogen surface. That phenomenon may also cause the field to be tipped on its side compared to the rotational axis and also inflict the "quadrupole" effect.

The study's co-authors are Jeremy Bloxham and Sabine Stanley of Harvard University, whose computer model of the planets' interior strongly replicates the strange magnetic fields detected by Voyager 2.

Hard evidence to back it will only come from a probe to Uranus or Neptune, they admit.

No such mission is being planned for the moment, but scheduled treks by US and US-European probes to Saturn and Mercury should yield exciting new data to test the theory of how planetary magnetic fields are born, they say.

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