Eccentricities of planets in binary systems

Genya Takeda*, Frederic A. Rasio

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


The most puzzling property of the extrasolar planets discovered by recent radial velocity surveys is their high orbital eccentricities, which are very difficult to explain within our current theoretical paradigm for planet formation. Current data reveal that at least 25% of these planets, including some with particularly high eccentricities, are orbiting a component of a binary star system. The presence of a distant companion can cause significant secular perturbations in the orbit of a planet. At high relative inclinations, large-amplitude, periodic eccentricity perturbations can occur. These are known as "Kozai cycles" and their amplitude is purely dependent on the relative orbital inclination. Assuming that every planet host star also has a (possibly unseen, e.g., substellar) distant companion, with reasonable distributions of orbital parameters and masses, we determine the resulting eccentricity distribution of planets and compare it to observations? We find that perturbations from a binary companion always appear to produce an excess of planets with both very high (≳0.6) and very low (≲ 0.1) eccentricities. The paucity of near-circular orbits in the observed sample implies that at least one additional mechanism must be increasing eccentricities. On the other hand, the overproduction of very high eccentricities observed in our models could be combined with plausible circularization mechanisms (e.g., friction from residual gas) to create more planets with intermediate eccentricities (≃ 0.1-0.6).

Original languageEnglish (US)
Pages (from-to)239-242
Number of pages4
JournalAstrophysics and Space Science
Issue number1-4
StatePublished - Aug 2006


  • Binaries: general
  • Brown dwarfs
  • Celestial mechanics
  • Planetary systems
  • Stars: low-mass
  • Stellar dynamics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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