types of extra-solar
planets & how they got to be that way
march 2003
the extrasolar
planets catalog
the known stars with exoplanets are distributed in distance from
their star as such
the distribution of planets' masses is here
surprise result:
at the jupiter-mass level, more planets of smaller mass
(contrary to expectations
due to selection effects)
the distribution of planets' eccentricities is here
classification of planetary
systems (Mar ‘03)
properties of extrasolar planets
|
planetary class
|
properties
|
single planet
systems (79)
|
all planets
(105)
|
hot Jupiters
in circular orbits
|
a < 0.25 au
T > 500 K
|
22%
|
18%
|
cool Jupiters
in circular orbits
(planets like ours?)
|
4 au > a > 1.0 au
130 K < T < 250 K
|
6%
|
5%
|
|
eccentric orbits
|
e > 0.1
|
72%
|
77%
|
why hot Jupiters were
unexpected
- it's too hot (that close to
the star) for significant condensation
- there's too little
solid material to build up a significant protoplanetary core
(especially quickly)
- the gas is too hot to
be held by a protoplanetary core
why are the hot Jupiters so close to their stars?
(19/105 planets have d <
0.25 au)
the majority view
- planet forms far from the
star (by condensation/accretion) & carves out a gap in the disk; perhaps other instabilities
form in the disk? (e.g., a bar? spiral density waves? both of
these are seen in spiral disk galaxies) in the disk also
- inner disk loses energy
(viscosity) and transfers angular momentum to the outer disk; the inner
disk falls onto star
- angular momentum is
transferred from planet to outer disk or to instabilities in the disk:
planet migrates toward star (migration time ~ 106 yrs)
- what stops the planets
inward migration? disappearance of the disk or disk instabilities?
other tidal effects?
- read more about the
planetary migration imperative here
the minority
view
- planet forms near
its star in a disk instability by dynamical clumping in the gas-rich
material near the star
- capable of
producing giant planets quickly at a wide variety of orbital radii,
although orbital migration may still be required to explain closest
planets and largest planets
why didn't Jupiter
migrate in?
(or why aren't all giant
planets close to their stars?)
- was Jupiter left stranded
when the disk disappeared?
- did previous
generations of jupiters spiral in?
- how did the terrestrial
planets survive?
- are we
second-generation planets?
why circular orbits
were expected
- protoplanets were
born in circular orbits
- non-circular
orbits should be circularized by frictional drag
- density wave
resonances should dampen eccentricity
- interactions
with remaining protoplanetary disk should efficiently circularize orbits
so
what about the eccentric-orbit planets?
(59/105 have e >
0.3; 81/105 have e > 0.1)
- gravitational scattering among many multiple
jupiters in the same system
- gravitational effects
of a companion (double) star
- gravitational effects
of a passing star
- gravitational effects
of a massive disk or a long-lived disk
so what are hot Jupiters made
of?
HD
209458b, the only extrasolar planet with a known atmosphere has a hot
hydrogen atmosphere evaporating from the planet
planet
supported by electron degeneracy; RP a MP1/3
but strongly influenced by
the extent to which it
has already contracted when it reached its present orbit and/or the
presence/absence of a rocky core
extrasolar planet
summary
- ~ 5% of sun-like stars have
planets of ~ Jupiter mass
- systems like ours originate
from least massive disks and/or gravitationally isolated systems
- eccentric systems originate
from most massive disks and/or gravitationally perturbed systems
- short-period planets ("hot
Jupiters") were formed at ~ few au from star and migrated in to their
present location
- (many?) planets perish during
Herbig-Haro (jet/disk phase) and the T Tauri (disk phase) & early
main-sequence phase
