Study reveals the strongest evidence to date of a magnetic field on an exoplanet

Magnetic fields play a fundamental role in the habitability of planets. On Earth, the magnetic field acts as a shield against the solar wind and contributes to the evolution of its atmosphere, a key condition for the existence of life. However, detecting and measuring these magnetic fields on planets located outside the Solar System remains one of the great challenges in astronomy.

Now, a study led by the Spanish National Research Council (CSIC)—a body attached to the Ministry of Science, Innovation and Universities—with the participation of researchers from the Institute of Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE-CSIC), demonstrates for the first time, in a conclusive manner, that a planet can directly influence the behaviour of its star. This finding, published in the journal Science, provides the strongest evidence to date of the existence of a magnetic field on an exoplanet.

“In particular, we have observed that GJ 436 b, a Neptune-like exoplanet orbiting very close to its star, causes regular changes in the brightness and energy emitted by the star at certain wavelengths,” explains Daniel Revilla, a CSIC researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC), the institution leading the study.

Furthermore, by analysing how and when these variations occur in the star, the team has succeeded in estimating for the first time the intensity of the magnetic field of a planet of this type, opening a new path to study the properties and habitability of worlds beyond the Solar System.

“Magnetic fields on extrasolar planets still represent a mystery, so it is highly valuable to be able to extract information using indirect methods, such as the modulation of stellar activity with the relative position of the planet. This type of study opens the way to the systematic exploration of these kinds of signals, which could be much more common than previously thought,” states Daniele Viganò, an IEEC researcher at the ICE-CSIC.

The presence of a magnetic field can influence the evolution of a planet because, by modulating the interaction between the stellar wind and the planetary atmosphere, it conditions processes related to its habitability. Earth is an example of this. Mars, by contrast, lost its intense global magnetic field billions of years ago, which contributed to the progressive loss of its atmosphere and, with it, much of the water it harboured in the past. Knowing whether exoplanets possess magnetic fields is, therefore, a key question for evaluating their potential habitability.

In this context, the study led by the IAA-CSIC has analysed sixteen years of high-resolution spectroscopic observations of the GJ 436 system, a low-mass star around which GJ 436 b orbits. The results provide new clues about the presence of magnetic fields in worlds located beyond the Solar System.

“Until recently, it was thought that it was mainly the star that influenced the planet, but our results provide the clearest evidence to date of something that had already been suspected: that the opposite can also happen, and a nearby planet can alter its star’s environment,” notes Rafael Luque, an IAA-CSIC researcher participating in the study.

The results show that, although stars usually dominate the relationship with their planets through their gravity, radiation, and magnetic field, a planet orbiting very close to its star can also influence it. In the case of GJ 436 b, this interaction leaves observable signals that have allowed scientists to infer the existence and intensity of its magnetic field.

The interaction between the planet and the star is not observed continuously. The phenomenon has only been detected in 2008, 2016, and 2024 —three episodes separated by eight-year intervals. This periodicity coincides with the magnetic activity cycle of GJ 436, suggesting that the interaction becomes particularly intense—or easier to detect—when the star passes through certain phases of its magnetic cycle.

The comparison of these observations with theoretical models has enabled the team to estimate a property that is extremely difficult to measure on an exoplanet: the intensity of its magnetic field. “Despite its smaller size, GJ 436 b would have a magnetic field between 2.33 and 27 times more intense than that of Jupiter,” indicates Pedro J. Amado, co-author of the work and a researcher at the IAA-CSIC.

This result opens up a unique opportunity to study the magnetic fields of planets located outside the Solar System. Their analysis allows for a better understanding of how they retain their atmospheres, what their internal structure is like, and how they evolve over time.

“Until now, measuring the magnetic field of an exoplanet was extremely difficult. This property is key to knowing whether a planet can protect its atmosphere and, ultimately, whether it could become habitable,” concludes Daniel Revilla.

In addition to the  Institute of Astrophysics of Andalusia (IAA-CSIC), the Institute of Space Studies of Catalonia (IEEC) and the Institute of Space Sciences (ICE-CSIC), the study involves the Astrobiology Centre (CAB, CSIC-INTA) in Madrid, the Institute of Astrophysics of the Canary Islands (IAC), and the University of the Balearic Islands (UIB). The work also brings together researchers from the United States, Italy, Israel, Germany, and Cyprus.