A JWST Transit of a Jupiter Analog: II. A Search for Exomoons

TL;DR

This study uses a 59.8-hour JWST/NIRSpec transit of Kepler-167e to search for exomoons around a Jupiter-analog, comparing planet-only and planet+moon models across three independent reductions and four trend-model schemes. By employing a profile likelihood approach for nuisance trend terms and a planet+moon model (LUNA) with a Bayesian evidence framework, the authors find seven of twelve fits that favor a moon, though a mid-transit syzygy-like event and potential spot-crossing degeneracies cast serious doubt on a robust exomoon detection. Anomalous GP-driven results are identified as spurious, while the most conservative analysis—custom reduction with a Gaussian-process trend and masking of the syzygy interval—yields a weak moon constraint of $R_{SP} ≈ 0.038^{+0.039}_{-0.025}$, corresponding to $R_S ≈ 0.37R_igoplus$ with large uncertainties. The work highlights exposure-long trends as a dominant systematic in JWST moon searches, emphasizes the risk of false positives from spot activity, and recommends a second transit (October 2027) to break degeneracies and establish robust exomoon constraints. It provides valuable methodological guidance for future JWST exomoon surveys, including multi-pipeline analyses, diverse trend modeling, and explicit handling of degeneracies in single-transit regimes.

Abstract

We present a search for exomoons around the Jupiter-like exoplanet Kepler-167e using a NIRSpec JWST transit. Our 60 hour time series clearly reveals the enormous impact of long-term trends in NIRSpec data, specifically a gradual flux drift occurring over each of six 10 hour exposures. We weighed the evidence for exomoons by comparing a planet-only model with a planet+moon model for a grid of twelve different analysis choices. Our grid was comprised of three different reduction pipelines and four different models for the exposure-long trends - two using linear models and two using Gaussian processes. Seven grid realizations indicate a strong exomoon detection, typically favoring a Roche-skimming orbit roughly 10% the size of the planet. We find that the only likely real astrophysical feature driving these fits is a syzygy-like event occurring almost exactly mid-transit, which is highly ambiguous with a spot-crossing event. Indeed, we show that a spot of the necessary size is compatible with the earlier Kepler data. Ironically, the fact that JWST is so superior to Kepler means that our fits are effectively driven by a single transit - a regime in which exomoons have enormous freedom to explain non-Gaussian behavior. We thus strongly urge the next transit be observed in October 2027 to break these degeneracies. Our pilot study to seek transiting exomoons with JWST reveals the profound impact exposure-long trends exert - a cautionary tale for future analyzes of this data - as well as the need for a deeper understanding of this systematic's cause and modeling best-practices.

Figures (10)

• Figure 1: Broad view of the NIRSpec time series of Kepler-167e. Top shows the light curve from the custom pipeline, overlaid in yellow with a simple planet-only model with quadratic limb darkening. Bottom zooms-in on the trends in isolation, highlighting the six distinct exposures of 10 hours each. Note the transit of Kepler-167c also occurs in the last exposure, which we mask in what follows.
• Figure 2: Overview of the customM32 results. Top panel zooms in on the light curve, $\boldsymbol{y}$ (dark points), at the scale of the exposure long trends, with the maximum likelihood estimated (MLE) of the trends-only ($\mathbf{\hat{g}}$) shown in blue (derived with the planet+moon model, $\mathcal{M}_S$). Second panel shows the data normalized by the trends model (gray points) with the moon-only component from our MLE solution in blue-dashed. Third panel shows the residuals from a planet-only model ($\mathcal{M}_P$), with a color-scaling from gray to red in proportion to their relative contribution to the $\chi^2$ improvement of the planet-moon model ($\mathcal{M}_S$). We also highlight the planetary ingress/egress with shading. Bottom panel shows the residuals of the planet-moon model ($\mathcal{M}_S$).
• Figure 3: Overview of the ExoTiC-JEDIM32 results. Same as style as Figure \ref{['fig:m32custom']}.
• Figure 4: Overview of the katahdinM32 results. Same as style as Figure \ref{['fig:m32custom']}.
• Figure 5: The four transit times observed to date of Kepler-167e. The JWST transit appears to have transited approximately an hour later than expected, given the Kepler transit times. The new ephemeris implies residual TTVs of order 10 minutes.