The HARPS search for southern extra-solar planets

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.

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“…Feng et al 2015;Santerne et al 2016;Moutou et al 2015). The frequency of systems with long-period giant planets (e.g.…”

Section: Introductionmentioning

confidence: 99%

An extreme planetary system around HD 219828

Santos

Santerne

Faria

et al. 2016

A&A

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“…Monitoring the orbital motion of exoplanets, through direct imaging (e.g., Chauvin et al 2012;Kalas et al 2013;Bonnefoy et al 2014;Nielsen et al 2014), or through indirect techniques such as radial velocity and transit measurements (e.g., Cumming et al 2008;Howard et al 2012;Marcy et al 2014;Moutou et al 2015), can provide a wealth of information about their properties, the processes through which they form, and how they interact dynamically with other bodies in the system. Accurately determining the orbital parameters of exoplanets can constrain their masses and densities (e.g., Charbonneau et al 2000) and lead either to the discovery of additional planets in the system (e.g., Nesvorný et al 2012), or to the exclusion of additional planets within a range of periods by invoking dynamical stability arguments (e.g., Correia et al 2005).…”

Section: Introductionmentioning

confidence: 99%

ASTROMETRIC CONFIRMATION AND PRELIMINARY ORBITAL PARAMETERS OF THE YOUNG EXOPLANET 51 ERIDANI b WITH THE GEMINI PLANET IMAGER

Rosa

Nielsen

Blunt

et al. 2015

ApJ

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Astrometric confirmation and preliminary orbital parameters of the young exoplanet 51 eridani b with the gemini planet imager ABSTRACTWe present new Gemini Planet Imager observations of the young exoplanet 51Eridanib that provide further evidence that the companion is physically associated with 51Eridani. Combining this new astrometric measurement with those reported in the literature, we significantly reduce the posterior probability that 51Eridanib is an unbound foreground or background T-dwarf in a chance alignment with 51Eridani to 2×10 15 -+ deg. The remaining orbital elements are only marginally constrained by the current measurements. These preliminary values suggest an orbit that does not share the same inclination as the orbit of the distant M-dwarf binary, GJ3305, which is a wide physically bound companion to 51Eridani.

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“…Wang et al (2015b) found that 5 ± 5% of Kepler multi's have stellar companions at separation 1-100 au, compared to 21% for field stars in the solar neighborhood, suggesting that such companions can misalign or disrupt multi-planet systems. On the other hand, RV surveys continue to reveal a population of giant planets at large distances ( > ∼ a few au) from their host stars (e.g., Marmier et al 2013;Feng et al 2015;Moutou et al 2015;Rowan et al 2016;Wittenmyer et al 2016;Bryan et al 2016). The Keck survey suggests that about 20% of solar-type stars could host gas giants within 20 au (Cumming et al 2008), while HARPS finds that 14% of such stars host giant planets with periods less than 10 years.…”

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Hiding Planets Behind a Big Friend: Mutual Inclinations of Multi-Planet Systems With External Companions

Lai

2017

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The Kepler mission has detected thousands of planetary systems with 1-7 transiting planets packed within 0.7 au from their host stars. There is an apparent excess of single-transit planet systems that cannot be explained by transit geometries alone, when a single planetary mutual inclination dispersion is assumed. This suggests that the observed compact planetary systems have at least two different architectures. We present a scenario where the "Kepler dichotomy" may be explained by the action of an external giant planet or stellar companion misaligned with the inner multi-planet system. The external companion excites mutual inclinations of the inner planets, causing such systems to appear as "Kepler singles" in transit surveys. We derive approximate analytic expressions (in various limiting regimes), calibrated with numerical calculations, for the mutual inclination excitations for various planetary systems and perturber properties (mass m p , semi-major axis a p and inclination θ p ). In general, the excited mutual inclination increases with m p /a 3 p and θ p , although secular resonances may lead to large mutual inclinations even for small θ p . We discuss the implications of our results for understanding the dynamical history of transiting planet systems with known external perturbers.

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The HARPS search for southern extra-solar planets

Cited by 25 publications

References 37 publications

An extreme planetary system around HD 219828

An extreme planetary system around HD 219828

ASTROMETRIC CONFIRMATION AND PRELIMINARY ORBITAL PARAMETERS OF THE YOUNG EXOPLANET 51 ERIDANI b WITH THE GEMINI PLANET IMAGER

Hiding Planets Behind a Big Friend: Mutual Inclinations of Multi-Planet Systems With External Companions

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