For decades, astronomers have been perplexed by planetary magnetic fields. In our own solar system, there is no rule that explains which worlds generate these magnetic sheaths: Earth, for example, has one, but its sister world — Venus — does not.

Astronomers suspect that one of the best ways to understand the mysteries of magnetism might be to study worlds orbiting other suns. By collecting a census of exoplanet magnetic fields, researchers could determine whether they are common features of other worlds. Doing so would help put our solar system in context and resolve some abiding curiosities, said Mary Knapp, an astronomer who studies exoplanets at the Massachusetts Institute of Technology’s Haystack Observatory.

“Earth versus Venus is a good example — two planets that are similar in size, fairly similar in composition, but wildly different in terms of magnetic fields,” Knapp said.

It has been a challenge to build such a census — and to even find exoplanet magnetic fields — because these fields are faint and hard to detect. But in April, two independent teams found what appears to be the signature of a magnetic field produced by a rocky planet orbiting a small, dim red dwarf star about 12 light-years away. The planet, called YZ Ceti b, is slightly smaller than Earth and likely too hot for life as we know it. Yet finding a magnetic field on a rocky world could tell us more about how magnetic fields form and how they impact a planet’s evolution — and even its suitability for life.

“We know from our solar system that magnetic fields play an important role in affecting how a planet loses or retains its atmosphere over time,” said Jackie Villadsen, an astronomer at Bucknell University and a member of one of the teams. “We’re trying to answer the question: How common are strong global magnetic fields on Earth-like planets?”

Radio Signals

In our solar system, Earth and the four giant planets — Jupiter, Saturn, Uranus and Neptune — have significant magnetic fields. Mercury has only a faint field, and Mars very likely had a more robust field in the past, which it lost for reasons that aren’t completely understood.

Planetary magnetic fields are generated by an engine called a dynamo, which is built from molten metal churning in a planet’s core. That churning produces electrical currents that drive a magnetic field. On Earth and the four gas giants, this process is strong enough to form a protective cocoon around the planet, deflecting charged particles that would otherwise blow away the planets’ atmospheres.