Why exoplanets have so high eccentricities

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Why exoplanets have so high eccentricities

• By Line Drube
• November 2004

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Characteristic of exoplanets

• Over 130 planets found by- Doppler Spectroscopy
  - The stars light curve
• Mass distribution 0.1 to10 Mj
• Brown dwarf desert

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Table of all planets and their semimajor axis

• All is under 5.9 AU
• Smallest orbit 0.038 AU(Mercury 0.38 AU)
• Within the snow line of 4-5 AU
• Migration

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Eccentricities

• High eccentricity small orbits
• Median eccentricity 0.28
• Plutos e 0.25
• Planet expected to have circular orbits.

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Theories for the eccentricities

• Close encounters between planets
• Resonant interactions between planets
• Interaction with the protoplanetary disk
• Interaction with a distant companion star
• Propagation of eccentricity disturbances
• Formation from protostellar cloud

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Close encounters between planets (1)

• During formation 1) Masses increase
  differential migration gt dynamical
  instability Or 2) The planets mutual perturbed
  each other gt instability
• Ejection or collision
• 1 planet far out 1 close
• Explains the migration inwards

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Close encounters between planets (2)

• Problem
• Ecc. distribution too many in close circular
  orbit, median ecc. 0.6. Equal masses
• Expected small m gt higher ecc.

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Resonant interactions between planets (1)

• Differential inward migration
• Migration caused by a torques from interactions
  between planet and disk
• Locked in orbital resonances
• Continued migration gt ecc.
• Pluto/Neptune (outwards)

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Resonant interactions between planets (2)

• Problems
• Needs extremely strong ecc. dampening.
• Have to be captured just before migration stops
• Have mostly observed single planets
• Expected low-mass planets to have higher ecc.

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Interaction with the protoplanetary disk

• Interactions at certain resonances can excite or
  dampen ecc.
• The dampening resonances are easier to saturate
  gt ecc. can grow
• Problem
• Many parameters
• Numerical 2D simulation, shows only ecc. growth
  for gt10Mj

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Interaction with a distant companion star

• Binary stars
• A weak tidal force can excited large ecc.
• Force needs to be stronger than other effect
• Problems
• Expected multi-planet system have low ecc.
• Expected high ecc. in binary system. Unseen
  companions?

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Propagation of eccentricity disturbances (1)

• During formation
• Stars passing within a couple 102 AU
• Excite outer planetesimals
• Propagate inwards as a wave
• In solar neighborhood values gt ecc 0.01-0.1
• Dense open clusters gt higher ecc.

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Propagation of eccentricity disturbances (2)

• Problems
• Works only with a long-lived extended disk
• Works only in dense open clusters
• It havent been shown if this reproduce the ecc.
  distribution.

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Formation from protostellar cloud (1)

• Protoplanetary disk vs. protostellar cloud
• Same distribution of periods and eccentricities
  as binary stars.

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Formation from protostellar cloud (2)

• Problems
• Fragmatation
• Brown dwarf desert

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Conclusion

• None of the theories can explain everything
• Likely a combination of several mechanisms
• Future
• Better statistic with more planets
• Finding smaller planets and longer periods.
• Giving new clues to the mystery.

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References

• Tremaine S., Zakamska N.L., Extrasolar Planet
  Orbits and Eccentricities by. arXiv 2003
• Tremaine S., Zakamska N.L., Excitation and
  progation of eccentricity disturbances in
  planetary system, 2004 ApJ
• Zucker S., Mazeh T., Derivation of the mass
  distribution of extrasolar planets with
  MAXLIMA., 2001 ApJ
• Stepinski T.F. and Black D.C., On orbital
  elements of extrasolar planetary candidates and
  spectroscopic binaries, 2001 AA
• Marzari F., Weidenschilling S.J., Eccentric
  Extrasolar Planets The Jumping Jupiter Model,
  2002 Icarus
• Ivanov P.B., Papaloizou J.C.B., On the tidal
  interaction of massive extrasolar planets on
  highly eccentric orbits, 2004 Mon.Not.R.Astron.So
  c
• Marcy G., Butler P., http//exoplanets.org