A Habitable Planet Around 55 Cnc

1
A Habitable Planet Around 55 Cnc?
A review of the brilliant Predicting Planets in
Known Extra-Solar Planetary Systems III
Terrestrial Planet Formation By Raymond, Barnes
Kaib, ApJ 2006
2
Planetary Systems on the Edge (Barnes Quinn
2004))
47 UMa
HD12661


HD82943
55 Cnc


3
Packing Biases Undetected Planets
Not all systems appear packed R.V. biases
suggest Saturn or lower mass undetectable Perhaps
wide gaps harbor additional planets In 2002 five
candidates HD 168443, HD 38529, HD 74156, 55
Cnc c-d, and HD 37124 Predicting Planets I Test
Particle Simulations Survey of parameter space,
asteroid belts Predicting Planets II Saturn Mass
Companions Where might saturn-mass companions be
found Predicting Planets III Terrestrial Planet
Formation Are annuli stable enough to form
habitable planets Exotic Earths Habitable Planet
Formation During Migration A Habitable Planet
about 55 Cnc isnt crazy!
4
Predicting Planets I (Barnes Raymond 2004)
Some gaps can support an asteroid belt, others
cannot
55 Cnc
HD 168443
HD 38529
5
Predicting Planets II (Raymond Barnes 2005)
55 Cnc
For each stable system 100s of Saturns Some
stable regions grew! New planets could push
back Plot most likely orbits of
stable planets Observers havent found them
yet Note stable planets in HZ of 55
Cnc Habitable satellites?
HZ
HD 38529
6
Simulations of Terrestrial Planet Formation
Begin with 65 lunar sized bodies Spaced at least
5-10 Hill radii (an individual pair is
stable) Apply composition gradient to
protoplanets Two models SS-like disk and
depleted, post-migration disk
0.1 water
SS-like ? at 1AU 10 g/cm2 Depleted ? at 1 AU
1.5 g/cm2 Mproto a e lt 0.2 (from PPII) Known
planets Integrate for 108 years
Dry
5 water
7
55 Cnc - Heavy Disk
Habitable!
Habitable!
Too dry
Too distant
Habitable!
Too dry
Too dry
Too distant/dry
Oh well
Home
Water fraction
8
55 Cnc - continued
30 of heavy disk simulations formed a
habitable planet Results from light disk (early
migration) 5-9 embryos survive Largest is
0.13ME Not large enough for significant
stirring No water, no tectonics gt not
habitable Late migration, heavy disk, habitable
planets possible Early migration, light disk, no
habitable planets Raymond, Mandell Sigurdsson
explore formation concurrent with migration
9
HD 38529
Large planets (one is 13 MJ) High eccentricities,
but large separation 2-5 terrestrial bodies
remain after 200 Myr 0.04 ME lt M lt 0.5
ME Habitable zone is not part of stable
zone Large planetary masses -gt high collision
velocities Perfect accretion is a bad model for
this system Final terrestrial masses are upper
limits HD 38529 probably has an asteroid belt or
a saturn-mass companion
10
HD 38529
11
Planet Formation During Migration
Exotic Earths paper explores generic case One MJ
planet begins at 5 AU Gas torques push it into a
swarm of rocky protoplanets Gas giant stirs up
disk, including water-rich bodies Usually forms a
Hot Earth (but very water-rich) In giants
wake, water-rich planets may form Sometimes
leaving a habitable terrestrial planet
12
At t0, bodies have simple comp. gradient
Bodies form interior to migrating planet
Hot Earth, close-in giant planet and a HP!
13
General Results from Exotic Earths
14
Migration During Formation Implications for 55
Cnc
About 30 of migration/formation simulations form
an HP But 55 Cnc had 3 migrating
planets! Certainly the chance is smaller than 30
in 55 Cnc Three possibilities Late migration
30 chance of HP Migration during Formation
0-30 Early migration 0
15
Complications for 55 Cnc
Eccentricity of planet interior to HZ has been
substantially revised Planets interior to HZ
are in 31 mean motion resonance Resonances form
during migration This resonance weak requires
additional planets
Full system
Just resonant pair
Resonant angle
Time
Time
16
Discussion Questions
What is the net accretion during a
collision? What is the water retention
(segregation during mass loss)? What is escape
rate of H20 vs. temperature? What is the
compositional gradient in solar nebula? What is
the gradient as a function of metallicity? What
other elements should we be tracking? Are our
final eccentricities their maximum value? What is
an appropriate definition of a habitable
planet? Are these results meaningful?? In 55
Cnc, HZ is 30 of stable zone About the same in
Exotic earths In both cases 30 of simulations
produced an HP