[Paper Review] The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets

M. Mayor, M. Marmier+12 more|arXiv (Cornell University)|Sep 12, 2011

Stellar, planetary, and galactic studies1 references342 citations

TL;DR

This study analyzes 8 years of HARPS radial velocity data to characterize super-Earths and Neptune-mass planets around solar-type stars. It reveals that over 50% of such stars host at least one low-mass planet with a period under 100 days, with a mass distribution rising sharply from 15 to 30 M⊕, and finds no preference for metal-rich hosts—contrasting with gas giants, which strongly correlate with high metallicity and exhibit higher eccentricities up to 0.9.

ABSTRACT

We report on the results of an 8-year survey carried out at the La Silla Observatory with the HARPS spectrograph to detect and characterize planets in the super-Earth and Neptune mass regime. The size of our star sample and the precision achieved with HARPS have allowed the detection of a sufficiently large number of low-mass planets to study the statistical properties of their orbital elements, the correlation of the host-star metallicity with the planet masses, as well as the occurrence rate of planetary systems around solar-type stars. A robust estimate of the frequency of systems shows that more than 50% of solar-type stars harbor at least one planet of any mass and with period up to 100 days. Different properties are observed for the population of planets less massive than about 30M-Earth compared to the population of gaseous giant planets. The mass distribution of Super-Earths and Neptune-mass planets (SEN) is strongly increasing between 30 and 15M-Earth. The SEN occurence rate does not exhibit a preference for metal rich stars. Most of the SEN planets belong to multi-planetary systems. The orbital eccentricities of the SEN planets seems limited to 0.45. At the opposite, the occurence rate of gaseous giant planets is growing with the logarithm of the period, and is strongly increasing with the host-star metallicity. About 14% of solar-type stars have a planetary companion more massive than 50M-Earth? on an orbit with a period shorter than 10 years. Orbital eccentricities of giant planets are observed up to 0.9 and beyond. The precision of HARPS-type spectrographs opens the possibility to detect planets in the habitable zone of solar-type stars. Identification of a significant number of super-Earths orbiting solar-type of the Sun vicinity is achieved by Doppler spectroscopy. 37 newly discovered planets are announced in the Appendix of this paper, among which 15 Super-Earths.

Motivation & Objective

• To determine the occurrence rate of low-mass planets (super-Earths and Neptune-mass) around solar-type stars using long-term radial velocity monitoring.
• To investigate the statistical distribution of planetary masses, orbital periods, and eccentricities in the super-Earth/Neptune (SEN) population.
• To compare the occurrence of SEN planets with that of gas giant planets (GGP) in terms of metallicity dependence and orbital properties.
• To assess the role of host star metallicity in the formation of different planetary populations.
• To evaluate the potential for detecting habitable-zone planets via Doppler spectroscopy using high-precision instruments like HARPS.

Proposed method

• Conducted an 8-year radial velocity survey using the HARPS spectrograph on the ESO 3.6 m telescope at La Silla Observatory.
• Analyzed radial velocity measurements with high precision (sub-meter-per-second level) to detect low-mass planetary signals.
• Applied statistical methods to estimate planet occurrence rates, corrected for detection biases and survey completeness.
• Used orbital solution fitting to determine planetary masses (m₂sin i), periods, and eccentricities from radial velocity curves.
• Correlated host star metallicity (Fe/H) with planetary mass and occurrence frequency to assess formation dependencies.
• Distinguished between single and multi-planetary systems, focusing on the most massive planet in multi-planet systems for comparative analysis.

Experimental results

Research questions

• RQ1 What is the occurrence rate of super-Earths and Neptune-mass planets around solar-type stars with orbital periods up to 100 days?
• RQ2 How does the mass distribution of low-mass planets vary across the 15–30 M⊕ range, and what does this imply about detection biases?
• RQ3 Is there a correlation between host star metallicity and the occurrence of super-Earths/Neptune-mass planets compared to gas giants?
• RQ4 What are the typical orbital eccentricity distributions for super-Earths/Neptunes versus gas giants, and how do they differ?
• RQ5 To what extent do multi-planetary systems dominate the population of low-mass planets, and what does this suggest about formation mechanisms?

Key findings

• More than 50% of solar-type stars host at least one planet with a mass less than 30 M⊕ and an orbital period of up to 100 days.
• The mass distribution of super-Earths and Neptune-mass planets increases sharply between 15 and 30 M⊕, indicating a higher prevalence in this range.
• The occurrence rate of super-Earths and Neptune-mass planets shows no significant preference for metal-rich host stars, with median metallicity at Fe/H ≈ -0.1 dex.
• Orbital eccentricities of super-Earths and Neptune-mass planets are limited to a maximum of 0.45, in contrast to gas giants with eccentricities up to 0.9.
• The occurrence rate of gas giant planets (m₂sin i > 50 M⊕, P < 10 years) is approximately 14% and strongly increases with host star metallicity.
• The population of low-mass planets exhibits a peak in frequency at orbital periods between 40 and 80 days, after correction for survey biases.