Extended Data Fig. 1: The underestimation of uncertainties as a function of spectral binning for the L168-9b commissioning observations.

a, The observed L168-9b transmission spectrum with 1σ error bars for spectrally unbinned data (grey circles), 0.15 μm bins (black squares), 0.5 μm bins (large red circles), and a 5-12 μm broadband bin (horizontal blue shaded region). The spectrum for wavelength pairs is not shown to avoid excessive clutter. b, The median of the transit depth uncertainties are shown with blue squares, while the observed scatter in the transmission spectrum is shown with orange circles. For unbinned data, the transmission spectrum shows about 2.5 × the scatter predicted by the fits to the individual light curves. Binning pairs of wavelengths reduces the level of underestimation of the scatter in the transmission spectrum, but considerable excess noise remains. Coarser binning schemes like the constant 0.15 μm bins used in the MIRI time-series observation commissioning paper²⁹ or the 0.5 μm bins we use in this work further reduce the level of uncertainty underestimation.