G.Marcy, P.Butler, D.Fischer, S.Vogt
20 Sept 2003

Most extrasolar planets reside in non-circular orbits. Of the 90 extrasolar planets that reside beyond 0.15 AU, their average orbital eccentricity is 0.32. In contrast, planets orbiting within 0.1 AU of their host star all reside in nearly circular orbits, no doubt enforced by tidal circularization. The complete list of orbital eccentricities is given in the Table of Planet Properties.

For comparison, Jupiter and the other giant planets in the Solar System have eccentricities less than 0.05.

Observed Eccentricities, viewed graphically:
  • Eccentricity vs Orbital Size
  • Orbital Shapes
  • Theories:
  • Various theories have been proposed to explain the orbital eccentricities, but none is definitive at the current time (Sept 2003). Most proposed mechanisms invoke gravitationally scattering or perturbations of planets by other planets, perhaps in resonances, or by interactions with the protoplanetary disk.



    1. Planet-Planet Interactions

  • Excitation of Orbital Eccentricities by Repeated Resonance Crossings 2003, ApJ, 584, 465, Chiang, E. I.,
  • Rasio and Ford 1996, Science, vol 274, 954.
  • Weidenschilling and Marzari, 1996, Nature, v384, p619)
  • Lin and Ida 1997
  • 2. Eccentricity Induced by Protoplanetary Disk

  • Eccentricity Evolution for Planets in Gaseous Disks, 2003, ApJ, 585, 1024, Goldreich, Peter; Sari, Re'em
  • Artymowicz 1992, PASP, 104, 769
  • Lubow and Artymowicz 1996 (see their Web presentation)
  • Cassen 1995
  • 3. Gravitational Perturbations from a Companion Star

  • Holman, Touma, Tremaine 1997, submitted to Nature
  • Mazeh et al. 1997

  • 4. Gravitational Perturbations from Passing Stars

  • G.Laughlin and F.Adams: Astrophysical Journal Letters, vol 508, p.L171
  • 5. Formation by Disk Instability

  • Boss 2000, ApJ, 536, L101.


    The occurrence of circular orbits may require special initial conditions. More common initial conditions may lead to gravitational perturbations of planes by other planets or by the protoplanetary disk, leading to orbital ellipticities or ejection. Perhaps our Solar System, with its coplanar, nearly circular orbits, represents a fortuitous unperturbed, low-entropy state for a planetary system.

    The circular orbit of Jupiter in our Solar System promotes the stability of circular orbits among the other 8 planets. If our Jupiter were in an eccentric orbit, the Earth and Mars would likely be gravitationally scattered, perhaps out of the Solar System. Thus an anthropic argument can be made for Jupiter's circular orbit, if it affects the onset or the evolution of biology on Earth. It remains a question of molecular and evolutionary biology regarding the necessity of circular orbits and the resulting nearly uniform temperatures for life.

    Eccentric orbits may occur relatively commonly in extrasolar planetary systems. The second law of thermodynamics suggests that orbits, once scrambled, will remain so. While an eccentric giant planet would certainly induce dynamical dominoes for terrestrial planets, the supposed demise of life may be a circular argument.

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