[Paper Review] Statistical properties of exoplanets III. Planet properties and stellar multiplicity

A. Eggenberger, S. Udry+1 more|ArXiv.org|Feb 27, 2004

Stellar, planetary, and galactic studiesPhysics and Astronomy52 references129 citations

TL;DR

This study analyzes 19 extrasolar planets in multiple star systems, finding they exhibit distinct orbital and mass properties compared to planets around single stars—particularly, short-period planets in binaries show unusually low eccentricities and higher masses. These patterns suggest gravitational interactions or migration processes shaped their systems, challenging current planet formation models.

ABSTRACT

Among the hundred or so extrasolar planets discovered to date, 19 are orbiting a component of a double or multiple star system. In this paper, we discuss the properties of these planets and compare them to the characteristics of planets orbiting isolated stars. Although the sample of planets found in multiple star systems is not large, some differences between the orbital parameters and the masses of these planets and the ones of planets orbiting single stars are emerging in the mass-period and in the eccentricity-period diagrams. As pointed out by Zucker & Mazeh (2002), the most massive short-period planets are all found in multiple star systems. We show here that the planets orbiting in multiple star systems also tend to have a very low eccentricity when their period is shorter than about 40 days. These observations seem to indicate that some kind of migration has been at work in the history of these systems. The properties of the five short-period planets orbiting in multiple star systems seem, however, difficult to explain with the current models of planet formation and evolution, at least if we want to invoke a single mechanism to account for all the characteristics of these planets.

Motivation & Objective

• Investigate differences in orbital and mass properties between exoplanets in multiple star systems and those orbiting single stars.
• Assess whether planetary characteristics in multiple systems deviate from standard planet formation models.
• Evaluate the statistical significance of observed trends in mass–period and eccentricity–period diagrams using hypergeometric distribution.
• Explore the implications of these deviations for planet formation and migration mechanisms in binary environments.
• Identify constraints on current models of giant planet formation based on observational data from multiple systems.

Proposed method

• Used the hypergeometric distribution to test statistical significance of observed planetary distributions in mass–period and eccentricity–period diagrams.
• Defined test zones in the mass–period diagram (P ≥ 100 days, M_p sin i ≥ 5 M_J) and eccentricity–period diagram (log P ≤ 1.6) to compare planet populations.
• Selected subsamples of 10 planets (mass–period) and 5 planets (eccentricity–period) from multiple systems for statistical comparison.
• Calculated probabilities of observing zero or all low-eccentricity/low-period planets in multiple systems under random sampling assumptions.
• Applied parameters: N (total population size), G (number of planets with desired traits), n (number in multiple systems), x (observed number in multiple systems).
• Used observational data from radial velocity surveys and confirmed systems with common proper motion or spectroscopic binaries.

Experimental results

Research questions

• RQ1 Are there statistically significant differences in the mass–period distribution of planets orbiting multiple stars versus isolated stars?
• RQ2 Do planets in multiple star systems exhibit distinct eccentricity–period trends compared to planets around single stars?
• RQ3 Why are the most massive short-period planets exclusively found in multiple star systems?
• RQ4 Can current planet formation models explain the low eccentricity of short-period planets in multiple systems?
• RQ5 What mechanisms might account for the observed properties of short-period planets in multiple star systems?

Key findings

• Among planets with P ≥ 100 days and M_p sin i ≥ 5 M_J, none were found in multiple star systems, with a hypergeometric probability of 3.25% under random sampling.
• In the eccentricity–period diagram (log P ≤ 1.6), all five short-period planets in multiple systems had e < 0.05, a configuration with a 3.77% probability under random sampling.
• The most massive short-period planets are exclusively found in multiple star systems, suggesting a link between binary environments and extreme planetary properties.
• Planets in multiple systems with periods < 40 days tend to have very low eccentricities, indicating possible dynamical or migration processes.
• The observed trends—low eccentricity and high mass in short-period binaries—challenge existing planet formation models, especially if a single mechanism is to explain all features.
• The data suggest that migration or gravitational interactions have played a role in shaping planetary systems in multiple star environments.