Download Why are so many extra-solar planets eccentric?

Why exoplanets have
so high eccentricities
By Line Drube
-November 2004
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Characteristic of exoplanets
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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
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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
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High eccentricity
small orbits
Median
eccentricity: 0.28
Pluto’s e = 0.25
Planet expected to
have circular
orbits.
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Theories for the eccentricities
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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)
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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
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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
=> higher ecc.
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Resonant interactions
between planets (1)
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Differential inward migration
Migration caused by a torques from
interactions between planet and disk
Locked in orbital resonances
Continued migration => 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 => ecc. can grow
Problem:
 Many parameters
 Numerical 2D simulation, shows
only ecc. growth for >10Mj
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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
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In solar neighborhood values => ecc 0.01-0.1
Dense open clusters => 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 haven’t been shown if this
reproduce the ecc. distribution.
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Formation from
protostellar cloud (1)
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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
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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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