Stacking transmission spectra of different exoplanets
File(s)
Published version
Author(s)
Kirk, James
Owen, James E
Type
Journal Article
Abstract
In many areas of astronomy, spectra of different objects are co-added or stacked to improve signal-to-noise ratio 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 James Webb Space Telescope’s Near Infrared Spectrograph (NIRSpec) G 395 H wavelength range (2.8–5.2µm). For two dominant species (e.g. H2 O 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 > 5 σ, 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.
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 James Webb Space Telescope’s Near Infrared Spectrograph (NIRSpec) G 395 H wavelength range (2.8–5.2µm). For two dominant species (e.g. H2 O 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 > 5 σ, 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.
Date Issued
2026-07-01
Date Acceptance
2026-05-27
Citation
Monthly Notices of the Royal Astronomical Society, 2026, 549 (4)
ISSN
0035-8711
Publisher
Oxford University Press (OUP)
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
549
Issue
4
Copyright Statement
© The Author(s) 2026. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
License URL
Publication Status
Published
Article Number
stag1035
Date Publish Online
2026-06-03