Magnetic field analysis of the ultra hot Jupiter KELT-9 b

Magnetic fields play a vital role in shaping the atmospheres of planets, yet measuring them around close-in worlds remains difficult. A recent analysis of the ultra-hot Jupiter KELT-9 b sought to detect a magnetic field using the Zeeman effect in atomic lines. While a marginal signal was observed in 2025, subsequent observations failed to replicate the result, leaving the presence of the field unconfirmed.

Star system: KELT-9 | Distance from the Sun: 676 light years

Magnetic fields are fundamental in governing how planetary atmospheres circulate and escape into space, but directly measuring these fields on planets orbiting very close to their host stars has proven to be a persistent challenge. For an ultra-hot Jupiter like KELT-9 b, theoretical models suggest that an atmospheric dynamo—a process where conductive fluids generate magnetic fields—could create substantial magnetic fields ranging between 400 and 4000 Gauss.

To investigate this, measurements were conducted using the SPIRou instrument, a spectropolarimeter located at the Canada-France-Hawaii Telescope. This device searches for circular polarization in light passing through the planet's atmosphere. This polarization can be caused by the Zeeman effect, where magnetic fields split the spectral lines of atoms. By analyzing 1398 atomic lines during three separate transits—one in 2025 and two in 2026—the data was mapped against the planet's orbital velocity to identify any potential magnetic signatures.

The observations recorded in September 2025 produced a signature that aligns with the expected velocity of KELT-9 b, though the statistical probability of this being a false alarm remained significant. Crucially, two subsequent transits observed in July 2026, which possessed similar sensitivity, did not show the same signal. Given that these later observations should have easily detected a signal of the same strength as the one found in 2025, the earlier finding is currently considered marginal. It is possible that the initial signal was caused by excess noise or other spurious factors rather than a true planetary magnetic field.

Key findings
A marginal detection of a magnetic signal was recorded for KELT-9 b in 2025.
Follow-up observations in 2026 did not reproduce the signal found during the 2025 transit.
The lack of consistent results suggests the initial signal may have been caused by noise.
Current data is insufficient to confirm the presence of a magnetic field on KELT-9 b.

Scientific study

SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: tentative detection of the planet magnetic field (external link)