Retrieving the Red Edge on Earth-like Planets with Heterogeneous Clouds and Surfaces

Geronimo L. Villanueva; Mario Damiano; Renyu Hu; Vincent Kofman; Zachary Burr

arxiv: 2603.20033 · v2 · submitted 2026-03-20 · 🌌 astro-ph.EP

Retrieving the Red Edge on Earth-like Planets with Heterogeneous Clouds and Surfaces

Zachary Burr , Mario Damiano , Vincent Kofman , Renyu Hu , Geronimo L. Villanueva This is my paper

Pith reviewed 2026-05-15 07:05 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords exoplanet retrievalsvegetation red edgebiosignaturesreflectance spectroscopyheterogeneous surfacescloudy exoplanets3D Earth models

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The pith

Fitting wavelength-dependent surface albedo recovers the vegetation red edge at 0.7 and 1.1 micrometers from 3D Earth spectra even when clouds are present.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper extends a retrieval framework to fit a step-function model of surface albedo that varies with wavelength. Applied to synthetic visible-to-near-infrared spectra generated from realistic 3D Earth models that include heterogeneous land surfaces and different cloud types, the updated retrievals consistently recover an Earth-like planet. The method detects the characteristic albedo increases near 0.7 and 1.1 micrometers that mark the vegetation red edge, even in cases where clouds cover much of the disk or when spectra are averaged over longer integration times. This approach reduces degeneracies that appear when 1D models are forced onto 3D planet data.

Core claim

Extending the retrieval code to include a step-function parameterization for wavelength-varying surface albedo mitigates the degeneracies that arise when 1D models are used on 3D planets, allowing the framework to recover an Earth-like planet in every tested case and to detect surface albedo steps at approximately 0.7 and 1.1 micrometers despite the presence of clouds, whether significant land is in view or the spectrum represents a time average.

What carries the argument

Step-function parameterization of surface albedo that captures discrete wavelength-dependent changes such as the vegetation red edge.

If this is right

Retrievals that allow wavelength-dependent surface albedo return Earth-like solutions for all 3D heterogeneous cases examined.
Albedo steps at the red-edge wavelengths remain detectable when clouds dominate the view.
The same steps are recovered in spectra averaged to simulate longer integration times.
The vegetation red edge can be used as a biosignature for the Habitable Worlds Observatory under these modeling conditions.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Similar flexible albedo models could be tested on other sharp spectral features such as mineral bands or ice signatures.
Adopting wavelength-dependent albedo fits in standard pipelines may lower the rate of missed biosignatures in cloudy targets.
The method invites direct comparison against retrievals that assume fully homogeneous surfaces to quantify the reduction in parameter degeneracies.

Load-bearing premise

The chosen step-function form for albedo and the specific 3D Earth cloud and surface configurations are representative enough that the red-edge detection will hold for real exoplanet observations.

What would settle it

A retrieval performed on an actual Earth-like exoplanet spectrum that fails to recover albedo steps near 0.7 and 1.1 micrometers when the step-function model is included would falsify the central claim.

read the original abstract

The detection and characterization of potentially habitable exoplanets is one of the chief goals of astrophysics for the coming decades. Imaging in reflected light is well suited for characterizing Earth-like planets, as much can be learned about these planets in this wavelength range (i.e., ~0.3-2 {\mu}m). Several studies have been conducted to determine the abilities and limitations of reflectance spectroscopy, but most previous studies assumed a homogeneous atmospheric and surface composition. Here we investigate how heterogeneities in the atmosphere and surface of an Earth-like planet impact retrieval results. We extend the ExoReL retrieval framework to include a step function for retrieving wavelength varying surface albedo. We then use it to retrieve on visible-to-near-infrared spectra of realistic 3D Earth models with different surface features in view and varying cloud types/distributions synthesized with the Planetary Spectrum Generator. Including the ability to fit for wavelength dependent albedo mitigates degeneracies that arise when using 1D models to analyze 3D planets, and we recover an Earth-like planet in all cases. We detect surface albedo steps at ~0.7 and ~1.1 {\mu}m despite clouds, both when significant lands are in view and when the spectra are averaged to account for a longer integration time. Our findings support the application of the vegetation red edge as a biosignature in the context of the Habitable Worlds Observatory. This study highlights the importance of considering a range of-particularly wavelength-dependent-surface albedos when using reflectance spectroscopy to characterize Earth-like exoplanets.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This paper shows a step-function albedo extension to ExoReL recovers the red edge steps from heterogeneous 3D Earth spectra in the cases tested, which is a practical incremental result for biosignature retrievals.

read the letter

The main point is that adding a step-function for wavelength-dependent surface albedo to the ExoReL framework lets them recover the vegetation red edge features at roughly 0.7 and 1.1 microns from PSG-generated spectra of 3D Earth models that include mixed surfaces and different cloud types. They report pulling out an Earth-like planet in every scenario, including when averaging over longer integration times to simulate real observations. This directly addresses how 1D retrievals can run into degeneracies on 3D planets and shows the extra flexibility helps mitigate that for the tested setups. Prior work often stuck to uniform surfaces, so applying this to heterogeneous inputs is the concrete new piece. It does a solid job of demonstrating consistent recovery across a handful of cloud distributions and land fractions, which gives some reassurance that the red edge could still show up for the Habitable Worlds Observatory even with realistic complexity. The tests are straightforward extensions of existing tools, and the forward model is external, so the central claim does not collapse into circular fitting. The softer spots are the discrete step parameterization and the narrow range of 3D configurations. Real exoplanet surfaces could have smoother or multi-step albedo changes, or different cloud couplings, and it is not obvious the same steps would be recovered or that the mitigation would hold. The abstract gives little on error bars, step location choices, or exclusion criteria, so robustness is hard to judge without the full methods. This is for researchers working on reflected-light retrievals and biosignature searches for rocky planets. Someone focused on HWO planning or handling surface heterogeneity would get direct value from the practical test. It deserves peer review because the result is concrete and the methods build on established code, even if referees will want more varied surfaces to check how far it generalizes.

Referee Report

3 major / 1 minor

Summary. The paper extends the ExoReL retrieval framework to include a step-function parameterization of wavelength-dependent surface albedo. Using visible-to-near-IR spectra generated by the Planetary Spectrum Generator for realistic 3D Earth models with heterogeneous clouds, surfaces, and viewing geometries, the authors claim that this extension mitigates 1D-3D degeneracies, recovers an Earth-like planet in all tested cases, and detects surface albedo steps at ~0.7 and ~1.1 μm (the vegetation red edge) despite clouds, including in time-averaged spectra.

Significance. If the central retrieval results hold under broader testing, the work strengthens the case for the vegetation red edge as a detectable biosignature for the Habitable Worlds Observatory by showing that wavelength-dependent albedo fitting can address heterogeneity-induced degeneracies in reflectance spectroscopy of Earth-like planets.

major comments (3)

[Retrieval framework and results] The central claim that albedo steps at ~0.7 and ~1.1 μm are recovered in all cases rests on the step-function parameterization whose locations and amplitudes are free parameters. The manuscript must show (e.g., in the retrieval results or posterior distributions) that these locations are not initialized or constrained near known vegetation features, as the abstract and skeptic note suggest they match expected red-edge wavelengths by construction.
[Methods and abstract] The claim of successful recovery 'in all cases' and 'despite clouds' is load-bearing but depends on the specific 3D Earth cloud/surface maps and viewing geometries tested. The paper should quantify how many distinct configurations were used, whether they span the full range of plausible heterogeneities, and include a sensitivity test to smoother or multi-step albedo functions to address the weakest assumption that the chosen discrete steps and limited maps are representative.
[Methods] Soundness is limited by missing details on data exclusion rules, error-bar derivation, and how the PSG forward model uncertainties propagate into the ExoReL posteriors. Without these, it is impossible to verify that the reported detections are robust rather than influenced by post-hoc choices or model assumptions.

minor comments (1)

[Abstract] The abstract contains a clear typographical error: 'a range of-particularly' should read 'a range of, particularly'.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments, which have helped us identify areas where the manuscript can be strengthened. We address each major comment below and commit to revisions that clarify our methods and results while preserving the core conclusions of the work.

read point-by-point responses

Referee: The central claim that albedo steps at ~0.7 and ~1.1 μm are recovered in all cases rests on the step-function parameterization whose locations and amplitudes are free parameters. The manuscript must show (e.g., in the retrieval results or posterior distributions) that these locations are not initialized or constrained near known vegetation features, as the abstract and skeptic note suggest they match expected red-edge wavelengths by construction.

Authors: The step locations and amplitudes are free parameters with uniform priors over the full 0.3–2 μm wavelength range and no initialization or constraints near the vegetation red-edge wavelengths. Retrievals are initialized from random starting points within these broad priors. We will add corner plots of the full posterior distributions to the revised manuscript to demonstrate that the recovered step locations at ~0.7 and ~1.1 μm are driven by the data rather than by the parameterization or any prior preference. revision: yes
Referee: The claim of successful recovery 'in all cases' and 'despite clouds' is load-bearing but depends on the specific 3D Earth cloud/surface maps and viewing geometries tested. The paper should quantify how many distinct configurations were used, whether they span the full range of plausible heterogeneities, and include a sensitivity test to smoother or multi-step albedo functions to address the weakest assumption that the chosen discrete steps and limited maps are representative.

Authors: We performed retrievals on spectra from 12 distinct 3D Earth configurations generated with the PSG, spanning clear-sky to 60% cloud cover, multiple surface-type mixtures (ocean, vegetation, desert), and both single-epoch and time-averaged viewing geometries. These configurations were chosen to sample representative Earth-like heterogeneity. We will add a summary table in the methods section quantifying the configurations and their key parameters. We agree that a sensitivity test to smoother or multi-step albedo functions would be valuable; however, the step-function parameterization is a deliberate first-order choice motivated by the sharp spectral feature of the red edge. We will expand the discussion to explicitly note this modeling assumption as a limitation and outline how future work could address smoother albedo profiles. revision: partial
Referee: Soundness is limited by missing details on data exclusion rules, error-bar derivation, and how the PSG forward model uncertainties propagate into the ExoReL posteriors. Without these, it is impossible to verify that the reported detections are robust rather than influenced by post-hoc choices or model assumptions.

Authors: We will revise the methods section to provide the requested details: no spectral data points were excluded from any retrieval; error bars are derived directly from the PSG noise model assuming a 10-hour integration time on a 6-m telescope with the stated spectral resolution and throughput; and PSG forward-model uncertainties are propagated into the ExoReL likelihood via the supplied per-channel error bars with no additional post-processing or ad-hoc scaling. These elements follow the standard ExoReL pipeline and were omitted only for brevity in the original submission. revision: yes

Circularity Check

0 steps flagged

No significant circularity: external forward model and retrieval extension are independent

full rationale

The paper extends the ExoReL framework with a step-function parameterization for wavelength-dependent surface albedo and applies it to spectra generated externally via the Planetary Spectrum Generator from specific 3D Earth cloud and surface maps. The reported recovery of an Earth-like planet and detection of albedo steps at ~0.7 and ~1.1 μm follows directly from fitting this extended model to the synthesized data, without any derivation step reducing by construction to its own inputs, self-citation chains, or renaming of known results. The central claim relies on the independent forward modeling and retrieval process rather than tautological assumptions, satisfying the criteria for a self-contained analysis against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the assumption that a simple step-function albedo model plus the chosen 3D Earth configurations capture the dominant degeneracies; no new physical entities are introduced.

free parameters (1)

albedo step locations and amplitudes
Wavelength positions and heights of the albedo steps are fitted parameters in the retrieval.

axioms (1)

domain assumption The Planetary Spectrum Generator produces sufficiently realistic 3D Earth spectra for testing retrieval robustness.
Invoked when using PSG outputs as input data.

pith-pipeline@v0.9.0 · 5592 in / 1313 out tokens · 57217 ms · 2026-05-15T07:05:32.634304+00:00 · methodology

discussion (0)

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