That's no moon: Exomoon survival and detection limitations

While moons are ubiquitous in the Solar System, exomoons have not yet been unambiguously confirmed despite the discov- ery of over 5, 600 exoplanets. 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. We investigated the reasons for this lack of detections by reviewing high-impact literature searches, assessing dynamical stability and detectability constraints, and comparing. We compiled a homogeneous catalogue of 24 proposed exomoon candidates, including stellar, planetary, and inferred lunar parameters.

For each case we applied Hill, Roche, and Laplace stability criteria and estimated detectability limits for current and forth- coming instruments. In addition, we tested 6038 confirmed exoplanets against the same stability and mission-specific observational filters for a given range of moon parameters.

A large fraction of the potential exomoon candidates investigated in the literature are either inconsistent with long-term stability or produce signals below Kepler sensitivity. Only 8 of the 13 candidates meet at least one of the stability criteria.

Our extended analysis highlights 27 promising targets for exomoon searches with PLATO, while moons with Solar-System-like properties remain below current thresholds. The lack of exomoon detections is likely due to a combination of the inefficient formation of stable massive moons and limited instrumental sensitivity.

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.

Other processes, such as early moon loss during planetary migration or giant impacts, may also influence moon survival. Future searches should prioritise systems that combine long-term dynamical stability with favourable observational conditions for next-generation photometric and astrometric.

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.

Source