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The TRAPPIST-1 planetary system is a favorable target for the atmosphericcharacterization of temperate earth-sized exoplanets by means of transmissionspectroscopy with the forthcoming James Webb Space Telescope (JWST). A possibleobstacle to this technique could come from the photospheric heterogeneity ofthe host star that could affect planetary signatures in the transittransmission spectra. To constrain further this possibility, we gathered anextensive photometric data set of 25 TRAPPIST-1 transits observed in thenear-IR J band (1.2 $\mu$m) with the UKIRT and the AAT, and in the NB2090 band(2.1 $\mu$m) with the VLT during the period 2015-2018. In our analysis of thesedata, we used a special strategy aiming to ensure uniformity in ourmeasurements and robustness in our conclusions. We reach a photometricprecision of $\sim0.003$ (RMS of the residuals), and we detect no significanttemporal variations of transit depths of TRAPPIST-1 b, c, e, and g over theperiod of three years. The few transit depths measured for planets d and f hinttowards some level of variability, but more measurements will be required forconfirmation. Our depth measurements for planets b and c disagree with thestellar contamination spectra originating from the possible existence of brightspots of temperature 4500 K. We report updated transmission spectra for the sixinner planets of the system which are globally flat for planets b and g andsome structures are seen for planets c, d, e, and f.

Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared