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Why exoplanets have so high eccentricities

Why exoplanets have so high eccentricities. By Line Drube November 2004. Characteristic of exoplanets. Over 130 planets found by - Doppler Spectroscopy - The stars light curve Mass distribution 0.1 to10 Mj Brown dwarf desert. Table of all planets and their semimajor axis.

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Why exoplanets have so high eccentricities

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  1. Why exoplanets have so high eccentricities By Line Drube November 2004

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

  3. Table of all planetsand 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

  4. Eccentricities • High eccentricity small orbits • Median eccentricity: 0.28 • Pluto’s e = 0.25 • Planet expected to have circular orbits.

  5. 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

  6. Close encounters between planets (1) • During formation 1) Masses increase & differential migration => dynamical instability Or 2) The planets mutual perturbed each other => instability • Ejection or collision • 1 planet far out & 1 close • Explains the migration inwards

  7. Close encounters between planets (2) Problem: • Ecc. distribution: too many in close circular orbit, median ecc. 0.6. Equal masses • Expected: small m => higher ecc.

  8. 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 => ecc. • Pluto/Neptune (outwards)

  9. 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.

  10. Interaction with the protoplanetary disk • Interactions at certain resonances can excite or dampen ecc. • The dampening resonances are easier to saturate => ecc. can grow Problem: • Many parameters • Numerical 2D simulation, shows only ecc. growth for >10Mj

  11. 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?

  12. 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 => ecc 0.01-0.1 • Dense open clusters => higher ecc.

  13. Propagation of eccentricity disturbances (2) Problems: • Works only with a long-lived extended disk • Works only in dense open clusters • It haven’t been shown if this reproduce the ecc. distribution.

  14. Formation from protostellar cloud (1) • Protoplanetary disk vs. protostellar cloud • Same distribution of periods and eccentricities as binary stars.

  15. Formation from protostellar cloud (2) Problems: • Fragmatation • Brown dwarf desert

  16. 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.

  17. 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 A&A • 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.Soc • Marcy G., Butler P., http://exoplanets.org

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