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?”
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.
“Magnetic fields act like a shield from radiation,” said Ayan Biswas, an astronomer at Queen’s University in Canada. “They are very important for life.”
Scientists suspect that many of the 5,000 known exoplanets have magnetic fields, but detecting them is a different matter. In the 1970s, astronomers surmised that when a planetary magnetic field interacts with the planet’s host star, it might produce an observable spike in low-frequency radio waves emitted by the star, known as auroral emissions. The timing of those spikes, as seen from Earth, would depend on a planet’s location in its orbital trek — they’re like a periodic fingerprint that indirectly reveals the planet’s presence.
Even before the first exoplanet discovery in 1992, “people thought this would be a really good way to look for exoplanets,” said Jake Turner, an astronomer at Cornell University.
The technique proved difficult; no ironclad detections of exoplanetary magnetic fields have been made before now, but there have been promising candidates.
Evgenya Shkolnik, an astrophysicist at Arizona State University, and colleagues used atmospheric data from four hot Jupiters — giant planets orbiting close to their stars — to get a hint of magnetic fields in 2019. In 2021, a team led by Turner used the Low Frequency Array (LOFAR) telescope in the Netherlands to detect a radio signal linked to a planetary magnetic field in the Tau Boötes system, 51 light-years from Earth. And later in 2021, Lotfi Ben-Jaffel at the Paris Institute of Astrophysics and colleagues detected ultraviolet emissions from a Neptune-like planet called HAT-P-11 b, 123 light-years from Earth, that were suggestive of the planet’s magnetosphere.
But none of the detections were definitive — and none were of rocky planets.
In 2017, astronomers found exactly the system they needed for the type of indirect observation they’d hypothesized about for nearly 50 years. Three rocky planets orbited the red dwarf YZ Ceti, a cosmic stone’s throw away. The system’s proximity to our own makes its planets convenient targets — especially YZ Ceti b, the innermost planet. Plus, red dwarfs typically have stronger magnetic fields than stars like our sun, which makes it easier to identify the fingerprint of an orbiting planet’s magnetic field. “This was one of the first systems discovered that meets these criteria,” Villadsen said.
Now two teams have turned up evidence of a magnetic field made by YZ Ceti b. Both teams spotted periodic bursts of radio waves that seemed to occur when YZ Ceti b reached a similar point in its two-day orbit around the star. One of the teams — Villadsen’s — detected the telltale radio fingerprint using the Very Large Array in New Mexico. “We worked out the planet would need a magnetic field strength similar to Earth’s to cause this brightness of radio waves,” Villadsen said.
The other team, which includes Biswas, posted their results shortly after. That group made similar observations of periodic radio spikes using the Giant Metrewave Radio Telescope in India. “We’re 99% sure [the signal] is coming from the planet,” Biswas said.
The results are promising, said Shkolnik, who was not involved in either study. “I wouldn’t consider it a confirmation yet, but it’s very suggestive,” she said. A more definitive detection would require more observations of the star and the periodic radio spikes. She and others are hoping that similar observations can be attempted for the TRAPPIST-1 system of seven Earth-size worlds orbiting a red dwarf 40 light-years from Earth, or even for the red dwarf Proxima Centauri, the closest star to Earth at 4.25 light-years, which hosts a (probable) rocky planet.
To the Moon
Finding exoplanetary magnetic fields is crucial for understanding how prevalent they are and how planets make magnetism. “We don’t really have an amazing understanding of how these things are generated on planets,” said Robert Kavanagh, an astronomer at the Netherlands Institute for Radio Astronomy.
In our solar system, a dynamo seems to be key. But a dynamo might not be the only way to generate a planetary magnetic field, especially in “super-Earths” — worlds that are between Earth and Neptune in mass — which are among the most common type of exoplanet spotted so far. Miki Nakajima, a planetary scientist at the University of Rochester, is investigating whether heat fluctuations within a planet could do the job inside worlds that have molten interiors but lack a solid core. “I’m interested in whether a magma ocean can produce a magnetic field,” she said, noting that “magma oceans should be pretty common in super-Earths.”
But astronomers say that new techniques are needed to transform the search from one-off detections into the type of census they’re hoping for.
One idea Knapp is working on, called GO-LoW, would use a fleet of thousands of small spacecraft to study radio waves from exoplanets. Another idea is FARSIDE, a proposed radio array from NASA that would be placed on the far side of the moon, free of radio interference from Earth. If any of these projects come to fruition, astronomers might solve these abiding mysteries — or uncover an even more puzzling trove of unearthly delights.
“Will we find Earths with Jupiter-sized fields, or Jupiters with Earth-sized fields?” Knapp said. “I don’t know, but I’d really like to find out.”
Correction: August 8, 2023.
Robert Kavanagh’s affiliation has been updated.