An extreme planetary system around HD 219828

Abstract

Context. With about 2000 extrasolar planets confirmed, the results show that planetary systems have a whole range of unexpected properties. This wide diversity provides fundamental clues to the processes of planet formation and evolution. Aims. We present a full investigation of the HD 219828 system, a bright metal-rich star for which a hot Neptune has previously been detected. Methods. We used a set of HARPS, SOPHIE, and ELODIE radial velocities to search for the existence of orbiting companions to HD 219828. The spectra were used to characterise the star and its chemical abundances, as well as to check for spurious, activity induced signals. A dynamical analysis is also performed to study the stability of the system and to constrain the orbital parameters and planet masses. Results. We announce the discovery of a long period (P = 13.1 yr) massive (m sin i = 15.1 M Jup ) companion (HD 219828 c) in a very eccentric orbit (e = 0.81). The same data confirms the existence of a hot Neptune, HD 219828 b, with a minimum mass of 21 M ⊕ and a period of 3.83 days. The dynamical analysis shows that the system is stable, and that the equilibrium eccentricity of planet b is close to zero. Conclusions. The HD 219828 system is extreme and unique in several aspects. First, ammong all known exoplanet systems it presents an unusually high mass ratio. We also show that systems like HD 219828, with a hot Neptune and a long-period massive companion are more frequent than similar systems with a hot Jupiter instead. This suggests that the formation of hot Neptunes follows a different path than the formation of their hot jovian counterparts. The high mass, long period, and eccentricity of HD 219828 c also make it a good target for Gaia astrometry as well as a potential target for atmospheric characterisation, using direct imaging or high-resolution spectroscopy. Astrometric observations will allow us to derive its real mass and orbital configuration. If a transit of HD 219828 b is detected, we will be able to fully characterise the system, including the relative orbital inclinations. With a clearly known mass, HD 219828 c may become a benchmark object for the range in between giant planets and brown dwarfs.

TheSpitzersearch for the transits of HARPS low-mass planets

“…Same as Fig. 1 for HD 215497 b.discovery paper (91 vs. 22 RVs) confirmed the existence of a second, more massive planet (M p sin i ∼ 15 M Jup ) planet on a very eccentric (e = 0.81) long-period (P = 13.1 yr) orbit, as recently announced bySantos et al (2016).…”

The GJ 504 system revisited

“…Additional astrometric monitoring of the companion is required to carve the distribution of relative inclinations ∆i and provide a lower limit on ϕ. Two known systems have recently been discovered with close-in lowmass planets on eccentric orbits and more massive companions on wide-orbits: HD 219828 (Santos et al 2016) and HD 4113 (Cheetham et al 2018a). These systems might then be good proxies of the architecture of the GJ 504 system prior to the putative engulfment.…”

Dynamically Hot Super-Earths from Outer Giant Planet Scattering

“…The size of the markers are proportion to M sini 1/3 . The planet systems we used in this figure including 55 Cnc (Fischer et al 2008), BD -08 2823 (Hébrard et al 2010), GJ 832 (Wittenmyer et al 2014), GJ 876 (Correia et al 2010), HD 11964 (Wright et al 2009), HD 125612 (Lo Curto et al 2010, HD 181433 (Bouchy et al 2009), HD 190360 (Courcol et al 2015, HD 215497 , HD 219828 (Santos et al 2016), HD 47186 (Bouchy et al 2009), HIP 57247 (Fischer et al 2012), Kepler-68 (Marcy et al 2014), Kepler-89 (Weiss et al 2013), and mu Ara (Pepe et al 2007). …”