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Open research questions in Stellar, planetary, and galactic studies

92 unresolved questions extracted from the limitations and future-work sections of 627 Stellar, planetary, and galactic studies papers in our library. Each links back to the study that raised it.

What the literature leaves open

  • A comprehensive treatment of quadruples would require modelling their full ar- chitectures and mutual interactions, which is beyond the scope of this work, particularly given that orbital parameters of the in- dividual subsystems are often incomplete or poorly constrained. A key limitation of many simplified completeness calcula- tions is the neglect of spectral line blending. While CPD−47◦2963 meets our selection criteria, its nature remains debated.

    Southern Massive Stars at High Angular Resolution (SMaSH+): Properties of hierarchical massive triples · 2026 · DOI

  • 1: Phase-folded light curves of the 6 periodic sources warranting further investigations: (a) Gaia DR3 103999471976858496; (b) Gaia DR3 930093722208184448; (c) Gaia DR3 1191504471436192512; (d) Gaia DR3 4318508939464901760; (e) Gaia DR3 6770227729752288256; (f) Gaia DR3 6844375121726139520 Table A.

    Gaia DR3 variable white dwarfs vetted by ZTF · 2026 · DOI

  • In ∼50% of cases, we could only evaluate upper limits on the accretion luminosity, either because the signal-to-noise ratio of the data was insufficient or because the measured value of the accretion luminosity was below the statistical estimate of the emission due to chromospheric activity.

    X-Shooter survey of disc accretion in Upper Scorpius. II. A lack of correlation between accretion rates and disc properties · 2026 · DOI

  • The extrapolated surface velocities of the low-frequency modes are small and thus hard to observe, but their large kinetic energies deeper in the interior could significantly impact AM transport, which has not yet been investigated.

    Revealing Mixed Modes in Compressible Hydrodynamical Simulations of Red Giant Stars · 2026 · DOI

  • Theoretically, the so-called “Cosmic Shoreline” developed for the solar system may not hold around M dwarfs unchanged due to their harsher extreme-UV (XUV) environment, possibly resulting in most M dwarf rocky planets lacking significant atmospheres—a hypothesis that remains to be statistically tested empirically through judicious target selection.

    Do Rocky Planets around M Stars Have Atmospheres? A Statistical Approach to the Cosmic Shoreline · 2026 · DOI

  • Differences between current comoving-frame implementations remain to be understood and require dedicated benchmarking efforts. More generally, empirical constraints on the cool side of the BS jump remain sparse at higher luminosities.

    The robustness of bi-stability jump predictions · 2026 · DOI

  • Because young, metal-rich globular clusters are sparsely sampled, the current data do not allow a direct test of whether NGC 6791 lies on an extrapolation of the in-situ globular-cluster AMR.

    The Absolute Age of the Open Cluster NGC 6791 and Its Implications for Galactic Archaeology and Asteroseismic Calibration · 2026 · DOI

  • (b) LAP = large-amplitude, regular variability (see text for details), LAP: = regular variability not fully established, nP = enough data to conclude that the object has no regular variability, ned = not enough data.

    OH/IR stars in the inner Galactic bulge. I. Colour, CO line, and stellar light curve characteristics · 2026 · DOI

  • Follow-up JWST observations to measure common proper motion and sample the spectral energy distribution are warranted to confirm the nature of MEAD 62 B.

    The MIRI Excesses Around Degenerates (MEAD) Survey. I. A Candidate Cold Brown Dwarf in Orbit Around the Nearby White Dwarf 2MASS J09424023−4637176 · 2026 · DOI

  • We have presented the second data release (DR2) of SPICE- RACS. Using calibrated visibilities from the third low-band epoch of the Rapid ASKAP Continuum Survey, RACS-low3, we have imaged 1596 observations of 1493 unique fields in Stokes I, Q, and U. Our observations cover a total sky area of 3.5π sr and a frequency range of 800 to 1088 MHz. From these image cubes we extract polarisation spectra towards 5.5M radio components detected in total intensity. Around these components, we measure an average rms sensitivity of ∼ 200 µJy/PSF. We adopt a position dependent PSF, with an median major axis of 14.5′′, and extrema of 11.8′′ and 75.9′′. In total intensity we are able to fit a model spectrum to 3.1M unique components, and we find our flux scale is accurate within 15 %. In linear polarisation, our use of holographic primary beam models has allowed our residual polarisation leakage to be on the order of 0.1 %. Using rotation measure (RM) synthesis we are able to measure the broad-band polarisation properties of our radio components. At a threshold of 8σ in linearly polarised signal-to-noise we detect 246509 components, making SPICE- RACS DR2 the largest polarisation catalogue ever produced by nearly an order of magnitude with an areal density of 6.7+1.8 –1.7 deg−2. We further find that a threshold of 6σ in linear polarisation can provide a Gaussian-equivalent false detection rate of 5σ. Applying such a threshold yields 342606 unique polarised components. Our derived RMs match very well existing catalogues, but our enhanced areal density has revealed a plethora of new magneto-ionic structures across the Southern sky. We find that ∼ 1 % of our polarised components exhibit Faraday complexity on average, with a weak indication of enhanced complexity towards the Galactic plane. We advise careful interpretation of our complexity metrics, however, as we find they correlate with known instrumental effects. Finally, we are able to use the time domain information to estimate the reliability of our RM catalogue. Within the centre of region each tile we estimate a lower limit of 98 %. Whereas towards the tile edges this degrades to a lower limit of 92 %. RACS was designed as a reference survey for the forthcoming deep ASKAP surveys, such as EMU and POSSUM. The latter of which will exceed the RM density achieved here by at least a factor of 5 (Vanderwoude et al., 2024). RACS has the unique advantages, however, of being both wider in sky area than planned for EMU/POSSUM (nominally 2π sr, Gaensler et al., 2025) and broader in bandwidth. Combining RACS-low3 with RACS-mid and high would increase the sampled bandwidth by a factor of ∼ 2.5, and therefore https://doi.org/10.1017/pasa.2026.10214 Published online by Cambridge University Press (a) (b) (c) Figure 24. The distributions of RM changes across time for Faraday simple components. In (a) we show the probability density of error-normalised RM differences for all pairs of repeated observations.

    The Rapid ASKAP Continuum Survey VII: Spectra and Polarisation In Cutouts of Extragalactic Sources (SPICE-RACS) Second Data Release – Unveiling the Magnetised Sky · 2026 · DOI

  • MOLUSC (Wood et al. 2021) uses rejection sampling to rule out possible companions across a range of mass ratios and orbital parameters. We apply a modified version of this code to our Keck sample to incorporate additional data types and fill in sensitivity gaps. When rapidly rotating M dwarfs in Praesepe have a companion, the separation is usually <80 AU. Wide binary companions of rapid rotators yield important clues about the role of separation in the angular momentum evolution of binary systems. Left: 90th percentile limits for companions to Keck targets. AO data alone rules out most stellar companions within 10s to 100s of AU. We combine multiple types of observations to place more stringent constraints on the presence or absence of a companion. AO alone leaves much of the potential parameter space unexplored - the companion distribution to M dwarfs peaks around 4-20 AU (Winters et al. 2019), near the inner limit of our survey. Right: 90th percentile limits when combining Keck AO, Gaia RUWE, and WIYN/Hydra RVs - this combination of data can rule out significantly more potential companions. A few RV epochs over 2-3 years clearly places strong constraints on close stellar systems; a longer survey would help remove the sensitivity gap around 1~AU. Our next steps include: ● Searching Gaia DR3 for resolved neighbors to targets without AO imaging ● Incorporating sensitivity data from RoboAO ● Modeling the distribution of binary parameters for fast and slow rotators, including both detections and sensitivity limits.

    The impact of companions on the rotation of Praesepe early M dwarfs · 2026 · DOI

  • , high cadence nightly observations to model and remove oscillations and granulation) is beyond the scope of this work, but we suggest that any such studies should make use of the integrated framework presented here. Scaling and Non-stationarity of Integrated Kernels Our integrated GP framework is currently limited by the GP scaling issue (i.

    Exposure-averaged Gaussian Processes for Combining Overlapping Datasets · 2026 · DOI

  • The paper identifies that CMFplanet-CMFstar discrepancies suggest water sequestration in planetary interiors rather than surface differentiation based on equilibrium temperature arguments, but no coupled interior-atmosphere models were used to verify water retention and outgassing under the specific thermal and compositional conditions of these hot super-Earths.

    Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs · 2026 · DOI

  • The observing strategy comparison assumes a fixed semiamplitude K = 3 m/s and fiducial RV uncertainty σRV = 2.5 m/s based on GJ 1132, the lowest-precision target. Optimization of this strategy for M dwarfs with different activity levels, rotation periods, and RV jitter profiles remains unexplored.

    Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs · 2026 · DOI

  • Stellar abundance measurements (Mg, Si, Fe, Ti) were obtained via spectral synthesis methods, but the typical CMFstar precision of ~10% may be limited by stellar spectral quality and elemental line blending. The contribution of spectroscopic measurement uncertainty to the CMFstar-CMFplanet discrepancy has not been quantitatively decomposed.

    Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs · 2026 · DOI

  • The water mass fraction (WMF) calculations assume ~1% WMF reconciles the CMFplanet-CMFstar discrepancy regardless of whether water is sequestered or fully differentiated, but no interior structure models were applied to test if this WMF is consistent with observed planetary radii and equilibrium temperatures across the sample.

    Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs · 2026 · DOI

  • The assumed CMFplanet uncertainty of 0.22 for resampling simulations is based only on the two targets GJ 357 b and L 98-59 b with ~5σ mass precisions. Validation of this uncertainty estimate across a broader range of M dwarf hosts with varying RV precision levels is needed to improve the statistical modeling.

    Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs · 2026 · DOI

  • The paper demonstrates that eight of nine target planets have smaller CMFplanet values than CMFstar, but the small sample size (n=9) precludes definitive inferences about super-Mercury occurrence rates and formation mechanisms around M dwarfs. A substantially larger sample is needed to constrain whether super-Mercuries are genuinely rare.

    Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs · 2026 · DOI

  • The study requires ten new M dwarf targets with ~5σ mass precisions to achieve statistical significance matching the 2.0σ difference between CMFplanet and CMFstar distributions. Currently, only nine planets have been measured, limiting the robustness of the tentative evidence for water-rich formation around M dwarfs.

    Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs · 2026 · DOI

  • The Saturn framework demonstrated capability on high-resolution XSHOOTER spectra at lower resolution, but the paper does not quantify how uncertainties in atmospheric parameters and individual chemical abundances scale with increasing rotation velocity and decreasing signal-to-noise ratio across the R ≈ 4000-20,000 resolution range expected from WEAVE and 4MOST surveys of tens of thousands of OB-type stars.

    Quantitative spectroscopy of single and multiple OB-type stars. Non-LTE spectrum analysis with machine learning · 2026 · DOI

  • Analysis of slowly rotating A-type and late B-type stars with non-standard abundances (Am, Ap, λ Boo stars, metal-poor blue horizontal branch stars, blue stragglers) is identified as the most difficult class to model with pre-calculated grids. The paper does not specify how neural network-based emulation could overcome the fundamental limitation that the extreme diversity of these objects may exceed the parameter space coverage achievable through training datasets.

    Quantitative spectroscopy of single and multiple OB-type stars. Non-LTE spectrum analysis with machine learning · 2026 · DOI

  • The expansion of Saturn toward A-type star analysis requires substantial extension of the non-LTE model atom database to cover the much wider range of chemical species present in cooler stellar spectra. Currently, a mixture of non-LTE and LTE spectrum synthesis is used, but the paper does not specify which specific chemical species lack adequate non-LTE treatment or provide a roadmap for prioritizing model atom development.

    Quantitative spectroscopy of single and multiple OB-type stars. Non-LTE spectrum analysis with machine learning · 2026 · DOI

  • Extension of the Saturn framework to subluminous O and B stars (log g ≈ 5-6) requires accounting for atomic diffusion effects that produce widely varying helium and metal abundances in their stable atmospheres. The paper notes first steps have been taken but does not specify how the non-LTE model atom database and neural network training datasets should be modified to handle the significant abundance variations caused by atomic diffusion in these high-gravity objects.

    Quantitative spectroscopy of single and multiple OB-type stars. Non-LTE spectrum analysis with machine learning · 2026 · DOI

  • The Saturn neural network framework was trained on spectra covering 3000-10500 Å with R ≥ 40,000, but its performance on low-resolution spectra (R ≈ 4000-20,000) from WEAVE and 4MOST facilities shows reduced accessibility to chemical species and increased uncertainties in atmospheric parameters. The specific quantitative degradation of parameter precision and elemental abundance determination as a function of spectral resolution and signal-to-noise ratio for fast-rotating OB-type stars requires systematic characterization.

    Quantitative spectroscopy of single and multiple OB-type stars. Non-LTE spectrum analysis with machine learning · 2026 · DOI

  • The SENN deep learning model was trained on hot subdwarfs with spectra from LAMOST and SDSS, but the generalization performance and systematic biases when applying this SED synthesis method to hot subdwarf samples from other surveys or with different spectral characteristics remains unvalidated and should be tested across independent datasets.

    From synthetic SEDs to stellar origins: A deep learning model for physical parameter retrieval in hot subdwarf stars · 2026 · DOI

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92 open questions have been extracted from the limitations and future-work passages of 627 Stellar, planetary, and galactic studies papers in our library. Each one below links back to the study that raised it, so you can read the original claim in context.

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