Modeling the Impact of Starspot Inhomogeneity on Spectroscopic Retrievals of Directly-Imaged Planets

Stellar activity is a major complication in the detection and characterization of exoplanets by both radial velocities and transits, and the upcoming Habitable Worlds Observatory invites us to also consider its effect on direct imaging. The new analysis still awaits peer review, but it already lays out the central claim clearly.

This 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. Spectra of directly-imaged planets can vary with the activity of their host stars because the face of the star we see is not the same as the face reflected by the planet. This discrepancy could potentially result in inaccurate measurements of the planet's radius and unexpected, externally-caused variability in its contrast spectrum with the star.

To assess the scientific capabilities and requirements of HWO, it is important that the magnitude of these effects be quantified. We present results of model retrievals of Earth-like exoplanets observed with an HWO-style survey, as they would appear when affected by starspots, using the ExoVista code for.

Both Solar-type stellar activity and highly active rapid rotators are considered and compared with an idealized quiescent host star. In the quiescent case, ${\rm SNR}\approx5$ is needed to detect atmospheric water vapor at 0.9 microns and ${\rm SNR}\approx13$ at 0.74 microns.

We find that for Solar-type activity, the effect on retrievals will be negligible, but it could present problems for certain highly-active stars at limiting geometries. It also decreases the accuracy of the retrieved albedo, resulting in a value $\sim2/3$ of the true value.

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.

In light of these results, we estimate the impact that stellar variability and starspots may have on an HWO-style survey. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy.

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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