Transcript How do “habitable” planets form?

How do “Habitable” Planets
Form?
Sean Raymond
University of Washington
Collaborators: Tom Quinn (Washington)
Jonathan Lunine (Arizona)
Habitable Zone: temperature for liquid water
HZ is function of: planet’s atmosphere, type & age of star
Habitable Planets NEED WATER!
The Paradox
of Habitable Planet Formation
• Liquid water: T > 273 K
• To form, need icy material: T < 170 K
rocky← →icy
”snow line”
The Paradox
of Habitable Planet Formation
• Liquid water: T > 273 K
• To form, need icy material: T < 170 K
rocky← →icy
”snow line”
Local building blocks of habitable planets are dry!
So where did Earth get its water?
Asteroid
Belt
• Late Veneer: Earth formed dry, accreted
water from bombardment of comets, or …
   Comets   
So where did Earth get its water?
Asteroid
Belt
• Late Veneer: Earth formed dry, accreted
water from bombardment of comets, or …
   Comets   
Some of Earth’s “building blocks” came from
past snow line, in outer Asteroid Belt: Earth
did not form entirely from local material
To guide the Habitable Planet Search
(TPF, Darwin), we need to know:
1. Are habitable planets common?
2. Can we predict the nature of extrasolar
terrestrial planets from knowledge of:
a) Giant planet mass?
b) Giant planet orbital parameters (a, e, i)?
c) Metallicity of host star?
Overview of Terrestrial Planet
Formation
1. Condensation of grains from Solar Nebula
2. Planetesimal Formation
3. Oligarchic Growth: Formation of
Protoplanets (aka “Planetary Embryos”)
4. Late-stage Accretion
Simulation Parameters
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aJUP = Giant planet’s orbital radius
eJUP = Giant planet’s orbital eccentricity
MJUP = Giant planet’s mass
tJUP = Giant planet’s time of formation
Surface density  stellar metallicity
Position of snow line
Eccentricity
Snapshots in time from 1 simulation
Semimajor Axis
Radial Migration of Protoplanets
Simulation Results
1. Stochastic Process
2. All systems form 1-4 planets inside 2 AU,
from 0.23 to 3.85 Earth masses
3. Water content: dry to 300+ oceans
(Earth has 1-10 oceans)
Trends
1. Higher eJUP  drier terrestrial planets
2. Higher MJUP  fewer, more massive
terrestrial planets
3. Higher surface density  fewer, more
massive terrestrial planets
Effects of eJUP
Habitability
• In most cases, planet forms in 0.8-1.5 AU
• In ~1/4 of cases, between 0.9-1.1 AU
• Range from dry planets to “water worlds”
with 30 times as much water as Earth
43 planets between 0.8-1.5 AU
11 planets between 0.9-1.1 AU
(1)
(2)
(3)
(4)
What might planets around
other stars look like?
(1) aJUP = 4 AU
(2) MJUP = 10 MEARTH
(3) MJUP = 1/3
(4) Solar System
Images from NASA
Conclusions
1. Most of Earth’s water was accreted during
formation from bodies past snow line
2. Terrestrial planets have a large range in
mass and water content
3. Habitable planets common in the galaxy
Conclusions
Cont’d
4. Terrestrial planets are affected by giant
planets! Can predict the nature &
habitability of extrasolar terrestrial planets
- Useful for TPF, Darwin
5. Future: develop a code to increase number
of particles by a factor of 10
Additional Information
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2004 Icarus paper, ”Making other Earths...”
http://www.astro.washington.edu/raymond
Papers by John Chambers
Talk to me!
Additional Slides
What is a “habitable” planet?
• Habitable Zone == Temperature for liquid
water on surface
– ~0.8 to 1.5 AU for Sun, Earth-like atmosphere
– varies with type of star, atmosphere of planet
• Habitable Planet: Need water!
Initial Conditions
• Assume oligarchic
growth to 3:1
resonance with Jupiter
• Surface density jumps
at snow line
• Dry inside 2 AU, 5%
water past 2.5 AU,
0.1% water in between
• Form “super embryos”
if Jupiter is at 7 AU
Simulation Parameters
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aJUP = 4, 5.2, 7 AU
eJUP = 0, 0.1, 0.2
MJUP = 10 MEARTH, 1/3, 1, 3 x real value
tJUP = 0 or 10 Myr
Surface density at 1 AU: 8-10 g/cm2
Surface density past the snow line
Simulations
• Collisions preserve mass
• Integrate for 200 Myr with serial code
called Mercury (Chambers)
– 6 day timestep
– currently limited to ~200 bodies
– 1 simulation takes 2-6 weeks on a PC
Data from our Solar System
Raymond, Quinn & Lunine 2003
Oligarchic Growth: “growth by
the few”
• Protoplanets grow
faster closer to the
Sun!
• Take approx. 10 Myr
to form at 2.5 AU
• Mass, distribution
depend on surface
density
Kokubo & Ida 2002
Distributions of Terrestrial Planets