HD 28471: a near-resonant compact multiplanet system with a possible cold giant planet

We present radial velocity measurements of the star HD 28471, observed by HARPS at the ESO 3.6 m telescope over a baseline of ∼19 years. We have searched for planetary companions to HD 28471 using kima, a trans-dimensional diffusive nested sampling algorithm where the number of planetary signals is explored as a free parameter. We detect a compact system of three planets, with signals in the preferred solution corresponding to orbits of P ∼ 3.16, 6.12, and 11.68 d. These planets lie firmly in the super-Earth and sub-Neptune mass regime, with (minimum) masses of 3.7, 5.7, and 4.9 M⊕, respectively. A long-period (∼1500 d) signal is also strongly detected. Assessment of activity indicator periodicities and RV correlations suggests that the three short-period signals are genuine planets, but casts doubt upon the nature of the long-period signal. The origin may be a short stellar magnetic cycle, though additional data are required to fully sample the periodicity without intervening offsets. HD 28471 b exhibits a more eccentric orbit than the other planets, which may be due to dynamical interaction, or a result of RV variation from an as-yet-undetected 4th planet interior to this compact system. The detected planets lie close to a resonant configuration, indicating that the system may retain features of its natal configuration, with convergent migration potentially responsible for evolving the planets onto such short-period orbits.