2025A&A...702A.139R

Aims. We reinvestigated the HD 208487 system to test the reality of the proposed HD 208487c world. We also searched for additional companions using applied Bayesian statistics and 15+ years of new radial velocity (RV) data from the HARPS and the PFS instruments that were taken post-discovery of HD 208487b, as part of our continued study of bright Sun-like stars within 50 pc of the Sun.
Methods. RV data were analyzed with generalized Lomb-Scargle periodograms, followed by Bayesian analysis techniques using the EMPEROR code. We also scrutinized various stellar activity indices to search for any corresponding peaks in the power spectra, correlations with the RV measurements, or significant signals from a Bayesian analysis methodology. Finally, photometric data were also checked to test for any transits or possible activity manifestations that could lead to possible false RV signals or excess noise.
Results. Our analysis points toward a candidate second planet in the system, positioned near the period of a previously proposed and subsequently challenged signal. This signal, HD 208487c, would relate to a cool Saturn world with an orbital period of 923.06_–2.76^+2.02 d and a minimum mass of M_jsini = 0.32 ± 0.01 M_j. Our analysis also led to a newly discovered candidate planet, HD 208487d, which if confirmed would be the result of a cool super-Neptune-sub-Saturn with a period of 1380.13_–8.25^+19.20 d and a minimum mass of M_jsini = 0.15 ± 0.01 M_j. Neither stellar activity indices nor photometric data show signals statistically matching these periods. We find that stellar activity is indeed affecting the RVs, and yet our joint RV+activity indicator modeling argues they are Doppler in nature. We show that the RV models are stable over long timescales, and these signals are independent of wavelength-dependent noise. The relative contributions of the data to the model were also examined.
Conclusions. We uncovered a candidate three-planet system that would consist of an inner gas giant, a central Saturn, and an outer super-Neptune-sub-Saturn. Extensive analysis of both photometric and spectroscopic data as activity proxies strongly supports the planetary system hypothesis; however, more long-term RV data would help add more statistical weight to the reality of candidate planets c and d. Assuming our model best represents reality, a dynamical analysis suggests that gravitational scattering of an initially ordered, equally spaced system in a long resonant chain of six Neptunes can explain the current architecture of HD 208487, a moderately eccentric inner massive planet with an outer nearly resonant (P_d/P_c = 1.495) gas giant and super-Neptune. More RVs may shed light on the reality of a fourth Doppler signal uncovered in the data that sits close to the 2:1 period ratio with signal of HD 208487c.