ORBITAL ECCENTRICITES

To date, all 8 extrasolar planets that reside beyond 0.2 AU from their host star orbit in elliptical orbits, having eccentricities greater than 0.1 , as shown in the Table of Planet Properties.

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

 These high orbital ellipticities may be explained by a variety of mechanisms in which planets are typically gravitationally scattered by other planets, other stars, or the protoplanetary disk out of which they formed.

The orbital ellipticities are best viewed graphically.

Eccentricity vs Orbital Size

Eccentricity vs Mass


The occurrence of circular orbits may require special initial conditions, to avoid the gravitational perturbations and to avoid the tendency of the 2nd Law of Thermodynamics to scramble the orbital ellipticities of planets. Perhaps, our Solar System, with its coplanar, nearly circular orbits represents a remarkably fortuitous low-entropy state for a planetary system.
 
 

THEORIES FOR ORBITAL ELLIPTICITIES

Four theoretical explanations have been proposed for the high eccentricities among extrasolar planets.

1. Gravitational Scattering among Giant Planets

  • Rasio and Ford 1996, Science, vol 274, 954.
  • Weidenschilling and Marzari, 1996, Nature, v384, p619)
  • Lin and Ida 1997

    • 2. Gravitational Perturbations from a Companion Star

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

    • 3. Gravitational Perturbations from Passing Stars

  • G.Laughlin and F.Adams: Astrophysical Journal Letters, vol 508, p.L171

    • 4. Gravitational Perturbations Exerted by the Protoplanetary Disk

  • Artymowicz 1992, PASP, 104, 769
  • Lubow and Artymowicz 1996 (see their Web presentation)
  • Cassen 1995

    • Protoplanetary disks having greater mass or greater longevity would naturally promote effects 1 and 3 above.

    ARE CIRCULAR ORBITS THE PLANETARY NORM ?

    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 out of the Solar System. Thus our existence, and the existence of life in the habitable zone, depends on both Jupiter and Earth being in mutually stable, circular orbits. It is probably no accident that our Solar System contains circular orbits.

    Eccentric orbits may occur relatively commonly for extrasolar planets. Just one eccentric giant planet orbiting a star can spell dynamical doom for terrestrial planets, and may bode ill for slowly evolving creatures. The claim that all planetary orbits must be like ours may well be a circular argument.