[Paper Review] The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems

C. Lovis, D. Ségransan+12 more|arXiv (Cornell University)|Nov 23, 2010

Stellar, planetary, and galactic studies2 references132 citations

TL;DR

This paper presents the discovery of up to seven low-mass planets orbiting the solar-type star HD 10180, detected via high-precision radial velocity measurements using the HARPS spectrograph. The system hosts five Neptune-like planets (12–25 M⊕), a potential 65 M⊕ super-Earth, and a possible 1.4 M⊕ rocky planet at 0.022 AU, making it the most populous known exoplanetary system, with planets in a densely packed, dynamically stable configuration governed by secular interactions and tidal dissipation.

ABSTRACT

Context. Low-mass extrasolar planets are presently being discovered at an increased pace by radial velocity and transit surveys, opening a new window on planetary systems. Aims. We are conducting a high-precision radial velocity survey with the HARPS spectrograph which aims at characterizing the population of ice giants and super-Earths around nearby solar-type stars. This will lead to a better understanding of their formation and evolution, and yield a global picture of planetary systems from gas giants down to telluric planets. Methods. Progress has been possible in this field thanks in particular to the sub-m/s radial velocity precision achieved by HARPS. We present here new high-quality measurements from this instrument. Results. We report the discovery of a planetary system comprising at least five Neptune-like planets with minimum masses ranging from 12 to 25 M_Earth, orbiting the solar-type star HD 10180 at separations between 0.06 and 1.4 AU. A sixth radial velocity signal is present at a longer period, probably due to a 65-M_Earth object. Moreover, another body with a minimum mass as low as 1.4 M_Earth may be present at 0.02 AU from the star. This is the most populated exoplanetary system known to date. The planets are in a dense but still well-separated configuration, with significant secular interactions. Some of the orbital period ratios are fairly close to integer or half-integer values, but the system does not exhibit any mean-motion resonances. General relativity effects and tidal dissipation play an important role to stabilize the innermost planet and the system as a whole. Numerical integrations show long-term dynamical stability provided true masses are within a factor ~3 from minimum masses. We further note that several low-mass planetary systems exhibit a rather "packed" orbital architecture with little or no space left for additional planets. (Abridged)

Motivation & Objective

• To characterize the population of low-mass planets, including super-Earths and ice giants, around nearby solar-type stars using high-precision radial velocity surveys.
• To investigate the dynamical architecture and long-term stability of multi-planet systems with low-mass planets.
• To explore the role of stellar metallicity and mass in shaping planetary system formation and mass distribution.
• To determine whether such systems exhibit resonant or non-resonant orbital configurations and what this implies for formation mechanisms.
• To assess the detectability limits of radial velocity techniques for Earth-mass planets in habitable zones.

Proposed method

• High-precision radial velocity measurements were obtained using the HARPS spectrograph on the ESO 3.6-m telescope at La Silla Observatory, achieving sub-m s⁻¹ precision.
• Radial velocity data were analyzed using iterative fitting and periodogram techniques to detect multiple planetary signals with low amplitudes.
• Numerical N-body integrations were performed to assess long-term dynamical stability under various assumptions about true masses.
• Orbital architectures were examined for evidence of mean-motion resonances, secular interactions, and approximate Titius-Bode-like spacing.
• Stellar metallicity and mass were correlated with total planetary system mass to investigate formation trends.
• The influence of general relativity and tidal dissipation on the stability of the innermost planet was modeled and evaluated.

Experimental results

Research questions

• RQ1 What is the maximum number of low-mass planets that can stably orbit a solar-type star within 1–2 AU?
• RQ2 How do secular interactions and tidal dissipation contribute to the long-term stability of compact, multi-planet systems like HD 10180?
• RQ3 To what extent do orbital period ratios in low-mass planetary systems follow approximate Titius-Bode laws or integer/half-integer relationships?
• RQ4 What is the role of stellar metallicity and mass in determining the total mass and architecture of planetary systems?
• RQ5 Can radial velocity surveys detect Earth-mass planets in the habitable zone of solar-type stars, and what are the dynamical constraints on such systems?

Key findings

• The HD 10180 system hosts at least five Neptune-like planets with minimum masses between 12 and 25 M⊕, orbiting between 0.06 and 1.4 AU from the star.
• A sixth radial velocity signal at longer period suggests a 65 M⊕ object, likely a super-Earth or mini-Neptune.
• A potential 1.4 M⊕ planet is indicated at 0.022 AU, the closest known planet in the system, whose stability is maintained by tidal dissipation.
• The system exhibits a compact, nearly logarithmically spaced architecture with no mean-motion resonances, but significant secular interactions.
• Long-term numerical integrations confirm dynamical stability if true masses are within a factor of ~3 of the minimum masses derived from radial velocity data.
• The system's architecture and the presence of multiple low-mass planets correlate strongly with low stellar metallicity and low stellar mass, supporting core-accretion models of planet formation.