The GAPS Programme at TNG LXXIV. A reanalysis of the planetary systems TOI-1272 and TOI-1694 with HARPS-N and retraction of the planetary interpretation of TOI-1272 c

Abstract: Hot Neptunes are close-in exoplanets that occupy a sparsely populated region of parameter space known as the "hot-Neptune desert". Their presence in this extreme environment is puzzling as it implies a complex history involving intense stellar radiation, atmospheric loss, and unique migration patterns, different from Neptunes at larger orbital periods. We are running an observational programme conceived to enlarge the number of close-in Neptune-sized planets, with well-measured physical and orbital parameters, with the aim of contributing to obtaining a statistically significant sample needed to clarify what the formation and migration pathways of this class of exoplanets are. We used currently available TESS photometry, along with new (HARPS-N) and archival (HIRES) high-precision radial-velocity measurements, to review the main properties of the planetary systems TOI-1272 and TOI-1694 by means of joint-fit analyses that, in the case of TOI-1272, included Gaussian-process regressions for carefully modelling stellar activity. Our final estimates of the parameters of the two systems are consistent with previous measurements but have smaller uncertainties. We identified the radial-velocity variation of TOI-1272 found in the HIRES data as stellar activity rather than planetary in nature and, therefore, rejected the non-transiting planet TOI-1272c. This opens up the possibility that TOI-1272b's eccentricity is the result of high-eccentricity migration. The larger number of data at our disposal also allowed us to point out that both TOI-1694b and TOI-1694c move on slightly eccentric orbits. The current orbital architecture of TOI-1694 suggests a history of migration driven by both disc and dynamical interactions.

• The paper reinterprets TOI-1272 by distinguishing stellar activity from planetary signals, leading to the formal retraction of TOI-1272 c.
• HARPS-N and HIRES data enabled the robust recovery of TOI-1694 b and c, yielding refined masses, radii, and eccentricities for improved dynamical modeling.
• The study underscores the importance of multi-instrument analysis and rigorous activity diagnostics in achieving precise exoplanet characterization.

Detailed Analysis of TOI-1272 and TOI-1694 in the GAPS Programme: Refined Parameters and Retraction of TOI-1272 c

Context and Motivation

Hot Neptunes occupy the Neptune desert, a region in exoplanet parameter space defined by radii between $3$–$8.5\,R_\oplus$ and masses between $10$–$100\,M_\oplus$ on close-in orbits ($P<3$ days). The paucity of planets in this regime is attributed to extreme photoevaporation and migration histories. Nevertheless, recent detections by TESS have revealed rare desert dwellers with anomalous densities and orbital architectures, challenging canonical formation and evolution models. A statistically robust census—with precise masses, radii, and orbits—is necessary to distinguish among competing frameworks for migration (disk-driven, high-eccentricity, or planet-planet scattering) and atmospheric loss.

Observational Campaign and Methodology

Within the GAPS-Neptune and HONEI subprogrammes, a multi-instrument observational campaign was implemented targeting TOI-1272 and TOI-1694—two K dwarf systems with previously claimed multiple planets including Neptune-sized transiting components. High-resolution HARPS-N spectroscopy was combined with archival HIRES RVs and new TESS photometry, enabling joint fit analyses (RV + transit) with robust activity modeling via Gaussian Process (GP) regression. For TOI-1272, stellar activity diagnostics (FWHM, BIS, H$\alpha$, and $\log R'_{\rm HK}$) were analyzed to resolve degeneracies between activity-induced and genuine planetary RV signals.

Revision of TOI-1272: Activity Misinterpreted as Planetary Signal

HARPS-N RVs reveal only a Keplerian signal attributable to TOI-1272 b ($P=3.32$ days, $e\sim0.3$), with no resonance at the $8.7$ day period reported for TOI-1272 c. GLS periodogram analysis, alongside GP regression of activity indicators, demonstrates that the $8.5\,R_\oplus$0 day signature in HIRES data coincides with the second harmonic of stellar rotation ($8.5\,R_\oplus$1 days). The BIS is anti-correlated with RV residuals ($8.5\,R_\oplus$2), reinforcing its activity origin. Bayesian model comparison ($8.5\,R_\oplus$3) prefers a GP plus single planet model over multi-planet scenarios. Apodized periodogram analysis further confirms temporal localization of the $8.5\,R_\oplus$4 day signal to a window consistent with evolving starspots, not a coherent planetary modulation.

Key numerical result: The amplitude of the RV jitter induced by activity is $8.5\,R_\oplus$5–$8.5\,R_\oplus$6 m s$8.5\,R_\oplus$7, commensurate with the signal previously misattributed to TOI-1272 c. The revised planetary catalog omits TOI-1272 c, retracting its planetary interpretation.

TOI-1694: Recovery and Precision Enhancement of Planetary Parameters

Unlike TOI-1272, both planetary signals for TOI-1694 (b: Neptune, c: Jupiter analog) are robustly recovered in HARPS-N and HIRES RVs. No activity-related aliases are apparent. Joint fit analysis yields updated physical parameters for TOI-1694 b: $8.5\,R_\oplus$8, $8.5\,R_\oplus$9, and a notably non-zero eccentricity ($10$0), statistically favored over a circular solution. The orbital period of TOI-1694 c is refined ($10$1 days; previous error: misreported $10$2 in exoplanet archives).

Strong numerical outcome: The correct measurement of orbital and eccentricity parameters enables high-fidelity dynamical modeling.

Implications for Dynamical Histories and Migration

• TOI-1272: The absence of an external companion and substantial eccentricity ($10$3) implicate high-eccentricity migration, likely via planet-planet scattering and subsequent tidal dissipation. Compatibility with the system age and tidal $10$4 suggests the present configuration is the outcome of an inward migration followed by circularization.
• TOI-1694: The Neptune desert dweller (b) and the outer Jupiter analog (c) support a scenario of combined disk-driven (Type I and II) migration and dynamical interaction. The sustained eccentricity of TOI-1694 b ($10$5) points to ongoing dynamical excitation, potentially via Kozai-Lidov oscillations with c, given sufficient mutual inclination and appropriate hierarchy in secular precession timescales. The low equilibrium eccentricity for coplanar systems (per Mardling's model) disfavors alternate explanations, unless past planet-planet scattering and tidal damping are invoked with high $10$6.

Theoretical and Practical Implications

Precise rejection of false planet candidates is critical for occurrence rate calculations, statistical studies of migration mechanisms, and empirical calibration of atmospheric loss theories. The retraction of TOI-1272 c underscores the necessity for rigorous activity modeling and multiprobe validation (GP, apodization, activity diagnostics). The enhanced parameter precision for TOI-1272 b and TOI-1694 b/c provides constraints for theoretical models of the Neptune desert and ridge, including population synthesis approaches addressing metallicity dependence, dynamical excitation, and tidal evolution.

Future observational campaigns should prioritize:

• High-cadence RV follow-up with simultaneous activity diagnostics
• Assembly of statistically robust samples across the desert/ridge/savanna parameter space, leveraging TESS and next-generation missions
• Detailed modeling of tidal and secular dynamical evolution, incorporating mutual inclinations and architecture-specific migration pathways

Conclusion

The GAPS Programme reanalysis demonstrates that previously interpreted planetary signals may be artifacts of stellar activity, with TOI-1272 c formally rejected from the planet catalog based on convergent RV and photometric evidence. For TOI-1694, the confirmation and refinement of planetary parameters—including significant eccentricity—provide crucial empirical input for migration and dynamical evolution models. The study exemplifies rigorous multi-instrument, multi-method validation, setting a new precision standard for exoplanet characterization within transitional regimes and underscoring the complexity and diversity of planetary architectures in the Neptune desert and ridge (2606.21625).