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

→icyrocky←

”snow line”

• 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

So where did Earth get its water?

• Late Veneer: Earth formed dry, accreted water from bombardment of comets, or …

Comets

Ast

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d B

elt

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

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

• 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

Snapshots in time from 1 simulationE

ccen

tric

ity

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

Images from NASA

(4) Solar System

(2) MJUP = 10 MEARTH

(3) MJUP = 1/3

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

• 2004 Icarus paper, ”Making other Earths...”

• http://www.astro.washington.edu/raymond

• Papers by John Chambers

• Talk to me!

Additional Information

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

• 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