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How do “Habitable” Planets Form?
Sean Raymond
University of Washington
Collaborators: Tom Quinn (Washington)Jonathan Lunine (Arizona)
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Habitable Zone: temperature for liquid water
HZ is function of: planet’s atmosphere, type & age of star
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Habitable Planets NEED WATER!
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The Paradox of Habitable Planet Formation
• Liquid water: T > 273 K
• To form, need icy material: T < 170 K
→icyrocky←
”snow line”
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• Liquid water: T > 273 K
• To form, need icy material: T < 170 K
Local building blocks of habitable planets are dry!
→icyrocky←
”snow line”
The Paradox of Habitable Planet Formation
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So where did Earth get its water?
• Late Veneer: Earth formed dry, accreted water from bombardment of comets, or …
Comets
Ast
eroi
d B
elt
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So where did Earth get its water?
• Late Veneer: Earth formed dry, accreted water from bombardment of comets, or …
Some of Earth’s “building blocks” came from past snow line, in outer Asteroid Belt: Earth did not form entirely from local material
Comets
Ast
eroi
d B
elt
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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?
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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
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Simulation Parameters
• 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
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Snapshots in time from 1 simulationE
ccen
tric
ity
Semimajor Axis
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Radial Migration of Protoplanets
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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)
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Trends
1. Higher eJUP drier terrestrial planets
2. Higher MJUP fewer, more massive terrestrial planets
3. Higher surface density fewer, more massive terrestrial planets
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Effects of eJUP
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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
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43 planets between 0.8-1.5 AU
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11 planets between 0.9-1.1 AU
(1)
(2)
(3) (4)
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What might planets around other stars look like?
(1) aJUP = 4 AU
Images from NASA
(4) Solar System
(2) MJUP = 10 MEARTH
(3) MJUP = 1/3
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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
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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
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• 2004 Icarus paper, ”Making other Earths...”
• http://www.astro.washington.edu/raymond
• Papers by John Chambers
• Talk to me!
Additional Information
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Additional Slides
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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!
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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
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Simulation Parameters
• 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
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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
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Data from our Solar System
Raymond, Quinn & Lunine 2003
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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
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Distributions of Terrestrial Planets