[Paper Review] The HARPS search for southern extra-solar planets. XVII. Super-Earth and Neptune-mass planets in multiple planet systems HD47186 and HD181433

F. Bouchy, M. Mayor+9 more|ArXiv.org|Dec 9, 2008

Stellar, planetary, and galactic studies39 references398 citations

TL;DR

This paper reports the discovery of two multiple-planet systems around solar-type stars HD 47186 and HD 181433 using radial velocity measurements from the HARPS spectrograph. It identifies a hot Neptune (22.78 M⊕) and a Saturn-mass planet (0.35 M_J) in the HD 47186 system, and a Super-Earth (7.5 M⊕) with two additional giant planets (0.64 and 0.54 M_J) in the HD 181433 system, reinforcing the prevalence of low-mass planets in multiple systems and supporting core accretion models with moderate migration.

ABSTRACT

This paper reports on the detection of two new multiple planet systems around solar-like stars HD47186 and HD181433. The first system includes a hot Neptune of 22.78 M_Earth at 4.08-days period and a Saturn of 0.35 M_Jup at 3.7-years period. The second system includes a Super-Earth of 7.5 M_Earth at 9.4-days period, a 0.64 M$_Jup at 2.6-years period as well as a third companion of 0.54 M_Jup with a period of about 6 years. These detections increase to 20 the number of close-in low-mass exoplanets (below 0.1 M_Jup) and strengthen the fact that 80% of these planets are in a multiple planetary systems.

Motivation & Objective

• To detect and characterize low-mass exoplanets in multiple-planet systems using high-precision radial velocity measurements.
• To investigate the architecture and formation pathways of close-in low-mass planets in systems with giant planets.
• To assess the role of stellar metallicity and disk mass in shaping planetary system architectures.
• To evaluate the prevalence of multiple-planet systems among close-in low-mass exoplanets.
• To test predictions of core accretion models against observed planetary mass and period distributions.

Proposed method

• High-precision radial velocity measurements were obtained using the HARPS spectrograph on the 3.6-m ESO telescope at La Silla Observatory.
• A total of 66 and 107 radial velocity measurements were collected over 4+ years for HD 47186 and HD 181433, respectively, with S/N per pixel ranging from 80–250.
• Radial velocities were derived using the HARPS pipeline, with uncertainties of 0.3–0.6 m s⁻¹ for HD 47186 and 0.4–1.0 m s⁻¹ for HD 181433.
• Orbital solutions were derived using iterative fitting to detect multiple planetary signals, including long-period companions.
• Stellar parameters including mass, temperature, metallicity, and rotation period were determined from high-resolution HARPS spectra.
• The analysis accounted for stellar activity and seismic noise through careful observing strategies and data reduction techniques.

Experimental results

Research questions

• RQ1 What is the orbital architecture of the multiple-planet systems around HD 47186 and HD 181433?
• RQ2 How do the masses and orbital periods of the detected planets compare to predictions from core accretion and gravitational instability models?
• RQ3 What fraction of close-in low-mass planets reside in multiple-planet systems, and how does this affect formation theories?
• RQ4 Is there a correlation between stellar metallicity and the presence of multiple low-mass planets?
• RQ5 What migration mechanisms could explain the current positions of the inner low-mass planets in these systems?

Key findings

• The HD 47186 system hosts a hot Neptune with a mass of 22.78 M⊕ and an orbital period of 4.08 days, along with a Saturn-mass planet (0.35 M_J) at a 3.7-year period.
• The HD 181433 system contains a Super-Earth of 7.5 M⊕ with a 9.4-day period, a 0.64 M_J planet at 2.6 years, and a third companion of 0.54 M_J with a 6-year period.
• These detections bring the total number of close-in low-mass exoplanets (below 0.1 M_J) to 20, with 80% found in multiple-planet systems.
• The observed bimodal mass distribution—giant planets and low-mass planets—aligns with predictions from core accretion models, especially with moderate disk mass and migration.
• The systems' supersolar metallicities ([Fe/H] = 0.23 and 0.33) support the core accretion model, where high-metallicity stars are more likely to form both giant and low-mass planets.
• In-situ formation is unlikely for the massive hot Neptune in HD 47186 due to insufficient material at its small semi-major axis, suggesting migration played a key role.