A new study suggests that Earth might have once had a different kind of magnetic field – one generated by oceans of magma on its surface, instead of the rotation of its core.
And that's good news, because it means more exoplanets than we thought could have a protective magnetic shield sheltering them from the harsh radiation of space, and a chance of hosting life.
According to the research, long before Earth had a skin, when its molten insides flowed on its outside and its heart was yet to harden, a magnetic cage was already beginning to bloom overhead.
An analysis of the electrochemistry of moving magma has found sufficiently sized oceans of liquid rock can generate their own magnetic fields, helping us understand not just our own planet's history, but the chances of life arising on other worlds.
Two Earth and planetary scientists from UC Berkeley went back to first principles to simulate the surface conditions of young super-Earths – huge rocky worlds with sub-surface pressures and temperatures guaranteed to keep the planets toasty.
They found the make-up of these molten crusts could give rise to an electrical conductivity large enough to form a planetary dynamo, and it would take a current of rock flowing at a speed of just 1 millimetre per second to manage it.
"This is the first detailed calculation for higher temperature and pressure conditions, and it finds that the conductivities appear to be a little bit higher, so the fluid motions you would need to make this all work are maybe a little bit less extreme," says planetary scientist Burkhard Militzer.
Our own world has a powerful dynamo churning away deep underfoot in the form of a rotating core of liquid iron and nickel swirling amid a gooey soup of lighter minerals and charged particles.
We should be super thankful for it – without it, we probably wouldn't be here.
"A magnetic field is helpful in protecting a planetary atmosphere from being blown away by the stellar winds," says co-author François Soubiran, now at the École Normale Supérieure in Lyon, France.
Not only do we need that atmosphere to keep the surface temperature constant and for life-sustaining chemical reactions, it shields the biosphere from lethal doses of radiation.
Magnetic fields also do a pretty good job of forming an umbrella that deflects high energy particles from bombarding the crust. So it's a safe bet that no magnetic field equals no life.
Knowing which planets outside of our own Solar System can generate magnetic fields might help us sort those that are likely to be sterile from the handful that just might be worth studying for biology.
What's more, categorising the different ways planets create magnetic fields opens the way to studying the geology of a planet without needing to set down on its surface.
"On Uranus and Neptune, it is assumed to be generated in the ice layers. Now we have added molten rocks to this diverse list of field-generating materials."
Just how a surface dynamo might interact with core processes is still anybody's guess, especially given we know so little about our planet's interior.
"The interaction between the liquid core magnetic field and the magma ocean is not easy to predict, but could result in a significant – or even dominant – dipolar component," the authors write.
Ideally, to form a protective bubble, a magnetic field should have a neat dipole shape, as opposed to a mess of loops like a poodle's haircut.
This could be good news for anybody hoping to include super-Earths in their list of potential alien hotspots.
Most of these insanely big planets – massive rocky bodies that fall short of Neptune's girth – tend to be pulled close to their temper-prone stars, where solar eruptions and constant heat would make short work of any atmosphere.
A sufficient dipole magnetic field would give some of them a fighting chance of holding onto precious air while shielding the surface from a scouring brush of solar activity.
Unfortunately any close proximity to a star also increases the chances such a world would be tidally locked, making its day and year more or less the same length. The team's analysis suggests a distinct dipole formation would require a relatively rapid rotation, ruling out those slower-spinning worlds.
Hunting for hints of magnetic fields from afar could help us prioritise our search for life among the stars.
This research was published in Nature Communications.