Abstract
Aims. Our new program with HARPS aims to detect mean motion resonant planetary systems around stars which were previously reported to have a single bona fide planet, often based only on sparse radial velocity data. Methods. Archival and new HARPS radial velocities for the K2V star HD 27894 were combined and fitted with a three-planet self-consistent dynamical model. The best-fit orbit was tested for long-term stability. Results. We find clear evidence that HD 27894 is hosting at least three massive planets. In addition to the already known Jovian planet with a period $P_{\rm b}$ $\approx$ 18 days we discover a Saturn-mass planet with $P_{\rm c}$ $\approx$ 36 days, likely in a 2:1 mean motion resonance with the first planet, and a cold massive planet ($\approx$ 5.3 $M_{\mathrm{Jup}}$) with a period $P_{\rm d}$ $\approx$ 5170 days on a moderately eccentric orbit ($e_{\rm d}$ = 0.39). Conclusions. HD 27894 is hosting a massive, eccentric giant planet orbiting around a tightly packed inner pair of massive planets likely involved in an asymmetric 2:1 mean motion resonance. HD 27894 may be an important milestone for probing planetary formation and evolution scenarios.
Highlights
The radial velocity (RV) technique is very successful in determining the orbital architectures of multiple extrasolar planetary systems
The archival RVs already clearly showed that HD 27984 b is not alone, and so we included the star in our ongoing HARPS program aimed at finding hidden multiple planetary systems, and obtained 16 more RV measurements in 2016
RV analysis In Fig. 1 we show the generalized Lomb-Scargle periodogram (GLS; Zechmeister & Kürster 2009) of the available HARPS data with false alarm probability (FAP) levels of 10%, 1%, and 0.1% calculated by randomly reordering 1000 bootstrap copies of the RV data (Kuerster et al 1997)
Summary
The radial velocity (RV) technique is very successful in determining the orbital architectures of multiple extrasolar planetary systems. The combined RV signal of a pair of planets in low eccentricity orbits near a 2:1 mean motion resonance (MMR) can be misinterpreted as a single planet with moderate eccentricity if the data are sparse (Anglada-Escudé et al 2010; Wittenmyer et al 2013; Kürster et al 2015). An example is the moderately hot (a = 0.125 AU, P ≈ 18 days) Jovian planet HD 27894 b (Moutou et al 2005), which was discovered based on only twenty RV measurements taken with the ESO HARPS spectrograph (La Silla Observatory, Chile, Mayor et al 2003). Kürster et al (2015) illustrated how the sparse RV data for HD 27894 can be modeled well with an additional Neptune-mass planet in an inner orbit near the 2:1 resonance with the known Jovian planet.
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