HD208487 TERRA, TERRA2, PFS and AAT RV curves

This signal, HD208487c, would relate to a cool Saturn world with an orbital period of 923.06^+2.02^_-2.76_d and a minimum mass of Mj*sini=0.32+/-0.01Mj . Our analysis also gives rise to a newly discovered candidate planet, HD208487d, which if confirmed would be the result of a cool super-Neptune/sub-Saturn with a period of 1380.13^+19.20^_-8.25_d and a minimum mass of Mj*sini=0.15+/-0.01Mj . Neither stellar activity indices nor photometric data show signals statistically matching these periods.

We find that stellar activity is indeed affecting the RVs, 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.

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, yet 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 HD208487 - a moderately-eccentric, inner massive planet with an outer nearly resonant (Pd/Pc=1.495) gas giant and super-Neptune.

More RVs may also 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 HD208487c.