An exoplanet is exerting a magnetic influence on its host star in a manner opposite to what astronomers normally observe, according to researchers at the Institute of Astrophysics of Andalusia (IAA-CSIC).
In a recent paper published in Science, IAA-CSIC researchers revealed how the exoplanet GJ 436 b is affecting its host star’s behavior in a never-before-seen way. In their work, the team developed a novel method to estimate an exoplanet‘s magnetic field strength, an important factor in determining habitability.
“Until recently it was thought that it was mainly the star that influenced the planet,” said Rafael Luque, an IAA-CSIC researcher and one of the new study’s co-authors.
Luque adds that his team’s results now offer “the clearest evidence to date of something that had already been suspected: that the opposite can also happen and that a nearby planet can alter the environment of its star.”
Life and Magnetic Fields
While the most obvious ingredients for a habitable planet are water and a breathable atmosphere, other elements are less apparent to us based on our human experience. Our planet’s invisible magnetic field protects us from the solar wind and holds the atmosphere together.
While Earth’s magnetic field guided the evolution of its atmosphere, Mars provides an example of what the loss of a magnetic field can do to an otherwise potentially habitable planet. When the Martian magnetic field disappeared four billion years ago as the planet’s core rapidly cooled, the atmosphere dissipated, followed by the loss of surface water.
As shown by the fates of two cosmic neighbors like Earth and Mars, a stable magnetic field is a key element of habitability.
Observing Exoplanet Magnetic Fields
Detecting magnetic fields on distant exoplanets has proven more challenging for astronomers than other observations, such as atmospheric composition. In their new paper, the IAA-CSIC team provides the first conclusive evidence of an exoplanet’s magnetic field, observed through its impact on the host star.
“In particular, we have observed that GJ 436 b, an exoplanet similar to Neptune that orbits very close to its star, causes regular changes in the brightness and energy emitted by the star at certain wavelengths,” explained lead author Daniel Revilla, an IAA-CSIC researcher who led the study as part of his doctoral thesis.
GJ 436 b provided the team with measurable signals that allowed them to infer that the exoplanet had a magnetic field and estimate its strength. The IAA-CSIC team analyzed sixteen years of spectroscopic observations of the GJ-436 system for their research, collected by the CARMENES instrument at the Calar Alto Observatory.
“Despite its smaller size, GJ 436 b would have a magnetic field between 2.33 and 27 times more intense than that of Jupiter,” said co-author Pedro J. Beloved, a researcher at the IAA-CSIC.
A Strange Interaction
In the paper, the researchers demonstrate how, in a very close orbit, a planet can flip the normal planet-star relationship, with the planet controlling the influence of gravity, radiation, and magnetic fields between the two. As the exoplanet interacts with its host star, it injects energy into the star’s chromosphere to produce a phenomenon like a stellar aurora.
This strange interaction has only been measured periodically, occurring in eight-year cycles, with observations in 2008, 2016, and 2024. Intriguingly, these years correlate to phases of the host star’s magnetic cycle, which the researchers believe affects the intensity.
“Until now, measuring the magnetic field of an exoplanet was extremely difficult,” Devilla concluded. “This property is key to knowing if a planet can protect its atmosphere and, ultimately, if it could harbor life.”
The paper, “Constraining an Exoplanet’s Magnetic Field Using Star-Planet Interactions,” appeared in Science on June 25, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.