Rethinking Exoplanet Validation Around Red Giant Stars

Context. Long-period radial-velocity signals around giant stars are commonly interpreted as planetary companions, but evolved stars can produce intrinsic RV variability on similar timescales through pulsations, surface inhomogeneities, magnetic activity, and other forms of intrinsic stellar variability. Previous studies of these systems often included photometric checks and did not find significant brightness variations matching the reported RV periods. However, the absence of a matching photometric signal does not exclude all stellar origins, because activity- or pulsation-induced RV variability may have weak, wavelength-dependent, or temporally variable photometric counterparts.

Aims. We aim to reassess the robustness of previously reported long-period planetary companions around 17 giant stars by combining published RV measurements with new observations. Our goal is to determine which systems remain consistent with a Keplerian interpretation, which require further observations, and which are not supported by the new data.

Methods. We performed a homogeneous multi-diagnostic analysis of the 17 systems. Published and new RV data were modeled
jointly and compared with the published orbital solutions. We evaluated each system using the change in log likelihood, period
agreement, RV semi-amplitude, and Mean anomaly posterior consistency, and phase stability.

Results. Only one system, HD 133086, satisfies all confirmation criteria and is classified as a likely confirmation. Eight systems
are classified as ambiguous because they satisfy only a subset of the diagnostics: HD 24064, HD 111591, HD 11755, HD 150010,
HD 112640, HD 131873, HD 12929, and HD 19615. These systems show partial agreement in likelihood, amplitude, mass, or
phase, but also exhibit significant period discrepancies, weak independent period constraints, phase tension, or prior-driven posterior agreement. The remaining eight systems are classified as likely refutations: HD 100655, HD 113996, HD 143107, HD 174205, HD 85503, HD 208527, HD 220074, and HD 158996. These targets generally show unfavorable likelihood changes, poor period recovery, inconsistent posterior morphology, or no independent support for the published Keplerian solutions.

Conclusions. By extending the temporal baseline with independent RV measurements, we find that several published long-period companion candidates around giant stars do not preserve the period, phase, amplitude, or posterior structure expected for a stable Keplerian signal. This result does not contradict the photometric and activity checks performed in the discovery studies; rather, it shows that non-detections in those diagnostics are not always sufficient to establish long-term orbital coherence. The most robustly supported case in our sample is HD 133086, while the remaining systems are either ambiguous or disfavored by one or more independent tests.