JWST's Search for Exomoons: Unveiling the Secrets of Kepler-167e (2026)

Could we be on the brink of discovering a moon orbiting a distant Jupiter-like planet? That's the tantalizing question at the heart of our latest research, where we dive into the search for exomoons around Kepler-167e using the James Webb Space Telescope's (JWST) NIRSpec instrument. But here's where it gets controversial: while our 60-hour time series data hints at a potential exomoon, it also exposes a critical challenge—the significant impact of long-term trends in NIRSpec observations. These trends, characterized by a gradual flux drift across six 10-hour exposures, complicate our ability to distinguish between a planet-only model and a planet-plus-moon scenario.

To tackle this, we employed a rigorous grid analysis, testing twelve different combinations of data reduction pipelines and models for these trends. And this is the part most people miss: seven out of these twelve setups pointed to a strong exomoon detection, often suggesting a moon in a Roche-skimming orbit—about 10% the size of its host planet. However, the plot thickens when we realize that the most plausible astrophysical explanation for our findings might be a syzygy-like event occurring mid-transit, which eerily resembles a spot-crossing event. Interestingly, Kepler data from years past supports the existence of a starspot large enough to mimic this behavior.

The irony? JWST's unparalleled sensitivity, while a game-changer, means our conclusions are heavily influenced by a single transit observation—a scenario where exomoons have ample leeway to explain anomalies. This raises a crucial question: Are we truly seeing an exomoon, or are we being led astray by complex systematics? To break this impasse, we urgently recommend observing the next transit in October 2027. Our study not only highlights the promise of JWST in exomoon detection but also serves as a cautionary tale about the pitfalls of long-term trends in its data. It underscores the need for a deeper understanding of these systematics and the development of robust modeling practices.