Multi-epoch ultraviolet observables for breaking the radius-albedo degeneracy in directly imaged exoplanets

Direct imaging measures how bright a planet appears in reflected starlight, but brightness alone cannot tell whether the planet is large and dark or small and bright. The new analysis still awaits peer review, but it already lays out the central claim clearly.

It matters because exoplanet science has moved beyond the era of simple discovery into a period of comparative characterization. With more than five thousand confirmed planets known, the scientifically productive questions now concern atmospheric composition, internal structure, orbital history and the statistical properties of populations rather than the existence of individual worlds. A new detection or spectral measurement is most valuable when it adds a well-constrained data point to those comparative frameworks, not when it stands alone as an anecdote. This radius-albedo degeneracy limits the characterization of non-transiting exoplanets, including those targeted by the Habitable Worlds Observatory (HWO). From multi-epoch observations we construct ultraviolet observables that are independent of planetary radius: the normalized 400 nm lightcurve shape, the ultraviolet intensity and.

In a clear 360-400 nm spectral window where Rayleigh scattering is strong and bright ultraviolet surfaces are nearly colorless, these observables constrain the atmospheric column. Using the GPU accelerated vector radiative transfer model vSmartMOM, we test how uniquely these radius-free observables determine the scattering state at signal-to-noise ratios.

For the chosen six phase sequence, spectropolarimetry gives median radius consequences of 9.5%, 3.3%, and 0.45% when the phases are known. Removing polarimetry degrades the result, especially in the HWO simulations where phase coverage is restricted by inner working angle.

For solar twin systems at 6 and 12 parsecs, the required six-phase campaigns fall in the allotted range of a few hundred hours, making this a clear observing path to planetary. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy.

The broader interest lies in making the target less anecdotal and more comparable with the rest of the known planetary population. Population-level questions, such as the frequency of atmospheres around small rocky planets or the prevalence of water-rich worlds in the habitable zone, require well-characterized individual data points before statistical patterns become meaningful. Each new planet with a measured radius, mass and, ideally, atmospheric constraint is a brick in that larger structure, and the accumulation of bricks eventually allows theorists to test formation models against real distributions rather than projections.

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Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to improve independent constraints on the mass, radius, atmospheric composition and orbital dynamics of the target. Transmission spectroscopy with JWST, radial velocity campaigns with high-resolution ground-based spectrographs and phase-curve measurements from space photometry represent the observational toolkit that can move characterization from plausible to robust. That convergence of techniques is the standard the community now expects before a planetary atmosphere result is treated as confirmed. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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