[Paper Review] The HARPS search for southern extra-solar planets. IX. mu Ara, a system with four planets

F. Pepe, A. C. M. Correia+14 more|arXiv (Cornell University)|Aug 18, 2006

Stellar, planetary, and galactic studiesPhysics and Astronomy18 references70 citations

TL;DR

This paper presents the discovery and full orbital characterization of a four-planet system around the star mu Ara, using high-precision radial velocity measurements from the HARPS spectrograph. It confirms the presence of three known planets (b, c, d) and identifies a fourth, previously unconstrained companion, mu Ara e, with a period of ~4206 days and mass of 1.81 M_Jup, resolving long-standing data dispersion issues through a dynamically stable four-planet solution.

ABSTRACT

The mu Ara planetary system is rather complex: It contains two already known planets, mu Ara b with P=640 days and mu Ara c with P=9.64 days, and a third companion on a wide but still poorly defined orbit. Even with three planets in the system, the data points keep anomalously high dispersion around the fitted solution. The high residuals are only partially due to the strong p-mode oscillations of the host star. We have therefore studied in this paper the possible presence of a fourth planet in the system. During the past years we have carried out additional and extremely precise radial-velocity measurements with the HARPS spectrograph. We provide in this paper a full orbital solution of the planetary system around mu Ara. It turns out to be the second system known to harbor 4 planetary companions. Thanks to the new data points acquired with HARPS we can confirm the presence of mu Ara c at P=9.64 days, which produces a coherent RV signal over more than two years. The new orbital fit sets the mass of mu Ara c to 10.5 M_Earth. Furthermore, we present the discovery of mu Ara d, a new planet on an almost circular 310 days-period and with a mass of 0.52 M_Jup. Finally, we give completely new orbital parameters for the longest-period planet, mu Ara e. It is the first time that this companion is constrained by radial-velocity data into a dynamical stable orbit, which leaves no doubt about its planetary nature. (Abridged).

Motivation & Objective

• To resolve the anomalously high radial velocity residuals in the mu Ara system despite known planetary companions.
• To determine whether the observed dispersion in radial velocity data is due to stellar pulsations or the presence of an additional planetary companion.
• To provide a dynamically stable, fully constrained orbital solution for all four planetary companions in the system.
• To establish a standardized naming convention for newly discovered, poorly characterized planets based on robust dynamical and observational criteria.

Proposed method

• Acquisition of high-precision radial velocity measurements using the HARPS spectrograph on the ESO 3.6 m telescope over an 8.5-year baseline.
• Application of the Stakanof software and an optimized genetic algorithm to navigate the complex, multi-dimensional parameter space of a four-planet orbital fit.
• Use of long-term, dense radial velocity data to constrain the orbital parameters of all four planets, including the outermost companion.
• Implementation of dynamical stability checks to validate the plausibility of the final orbital solution.
• Integration of data from prior instruments (UCLES and CORALIE) to improve parameter constraints.
• Adoption of a naming strategy based on discovery sequence and dynamical stability, rather than provisional orbital fits.

Experimental results

Research questions

• RQ1 Is the high radial velocity dispersion in the mu Ara system due to stellar pulsations or the presence of an additional planetary companion?
• RQ2 Can a stable, four-planet orbital solution be derived from the extended HARPS radial velocity dataset?
• RQ3 What are the refined orbital parameters and minimum masses of the four planetary companions in the mu Ara system?
• RQ4 Can a consistent and robust naming convention be established for newly discovered, poorly characterized planets based on dynamical stability and data quality?

Key findings

• The radial velocity data from HARPS, with a weighted rms of 1.75 m s⁻¹ over 171 points, confirm the presence of a fourth planet, mu Ara d, on a 310.6-day orbit with a minimum mass of 0.52 M_Jup.
• The Neptune-mass planet mu Ara c is confirmed with a refined orbital period of 9.64 days and a minimum mass of 10.5 M_⊕.
• The long-period companion, mu Ara e, is constrained to a likely period of 4206 days and a minimum mass of 1.81 M_Jup, with a dynamically stable orbit.
• The system is now fully characterized as a stable four-planet system, resolving the previously unexplained velocity dispersion.
• The data show no significant evidence for a fifth companion, given the low residual scatter.
• The study proposes a standardized naming convention for new planetary discoveries based on dynamical stability and data quality, not just preliminary orbital fits.