Stacking transmission spectra of different exoplanets

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ID: 315910
2026
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Abstract
Abstract In many areas of astronomy, spectra of different objects are co-added or stacked to improve signal-to-noise and reveal population-level characteristics. As the number of exoplanets with measured transmission spectra grows, it becomes important to understand when stacking spectra from different exoplanets is appropriate and what stacked spectra physically represent. Stacking will be particularly valuable for long-period planets, where repeated observations of the same planet are time-consuming. Here, we show that stacked exoplanet transmission spectra can, under well-defined conditions, be represented by spectra generated from the geometric mean of each planet’s abundance ratios. We test this by comparing stacked and geometric mean spectra across grids of forward models over JWST’s NIRSpec/G395H wavelength range (2.8–5.2 μm). For two dominant species (e.g., H2O and CO2), the geometric mean accurately reflects the stacked spectrum if abundance ratios are self-similar across planets. Introducing a third species (e.g., CH4) makes temperature a critical factor, with stacking becoming inappropriate across the CO/CH4 boundary, which is the primary chemical transition considered in this work. Surface gravity exerts only a minor influence when stacking within comparable planetary regimes. We further assess the number of stacked, distinct sub-Neptunes with high-metallicity atmospheres and low-pressure, grey cloud decks required to rule out a flat spectrum at >5σ, as a function of both cloud deck pressure and per-planet spectral precision. These results provide guidance on when stacking is useful and on how to interpret stacked exoplanet spectra in the era of population studies of exoplanets.
Reference Key
openalex_W7163537339 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors James Kirk, James E. Owen
Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1035
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