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Planetesimal growth and Planetesimal growth and formation planet formation

Anders Johansen

Planet formation

Planetesimals

Streaming instability

Metallicity

Self-

Conclusions

Anders Johansen (Leiden University→ Lund) “ Rising: and at the Crossroads” Kavli Institute for Theoretical Physics, March–April 2010

Collaborators: Andrew Youdin, Thomas Henning, Hubert Klahr, Wlad Lyra, Mordecai-Mark Mac Low, Jeff Oishi Hydrodynamical models of planetesimal formation exhibit similar sharp dependence on

Exoplanet-metallicity connection

Planetesimal growth and planet First planet around (Gonzalez 1997; Santos et al. 2004; formation solar-type star found in Fischer & Valenti 2005) Anders Johansen 1995 (Mayor & Queloz 1995) Planet formation Today more than 400 Planetesimals exoplanets known ⇒ probability Metallicity increases sharply with Self-gravity metallicity of host star Conclusions Exoplanet-metallicity connection

Planetesimal growth and planet First planet around (Gonzalez 1997; Santos et al. 2004; formation solar-type star found in Fischer & Valenti 2005) Anders Johansen 1995 (Mayor & Queloz 1995) Planet formation Today more than 400 Planetesimals exoplanets known Streaming instability ⇒ Exoplanet probability Metallicity increases sharply with Self-gravity metallicity of host star Conclusions

Hydrodynamical models of planetesimal formation exhibit similar sharp dependence on metallicity → → → → .

Planet formation

Planetesimal Planetesimal hypothesis of Safronov 1969: growth and planet formation form in protoplanetary discs from dust grains that collide

Anders and stick together Johansen

1 Planet Dust to planetesimals formation µm → cm: contact forces during collision lead to sticking Planetesimals cm → km : ??? Streaming instability 2 Planetesimals to

Metallicity km → 1,000 km: gravity Self-gravity 3 Protoplanets to planets 6 7 Conclusions Gas giants: 10 M⊕ core accretes gas (< 10 –10 years) Terrestrial planets: protoplanets collide (107–108 years) → → → .

Planet formation

Planetesimal Planetesimal hypothesis of Safronov 1969: growth and planet formation Planets form in protoplanetary discs from dust grains that collide

Anders and stick together Johansen

1 Planet Dust to planetesimals formation µm → cm: contact forces during collision lead to sticking Planetesimals cm → km : ??? Streaming instability 2 Planetesimals to protoplanets

Metallicity km → 1,000 km: gravity Self-gravity 3 Protoplanets to planets 6 7 Conclusions Gas giants: 10 M⊕ core accretes gas (< 10 –10 years) Terrestrial planets: protoplanets collide (107–108 years)

→ → →

Planet formation

Planetesimal Planetesimal hypothesis of Safronov 1969: growth and planet formation Planets form in protoplanetary discs from dust grains that collide

Anders and stick together Johansen

1 Planet Dust to planetesimals formation µm → cm: contact forces during collision lead to sticking Planetesimals cm → km : ??? Streaming instability 2 Planetesimals to protoplanets

Metallicity km → 1,000 km: gravity Self-gravity 3 Protoplanets to planets 6 7 Conclusions Gas giants: 10 M⊕ core accretes gas (< 10 –10 years) Terrestrial planets: protoplanets collide (107–108 years)

→ → . →

Planet formation

Planetesimal Planetesimal hypothesis of Safronov 1969: growth and planet formation Planets form in protoplanetary discs from dust grains that collide

Anders and stick together Johansen

1 Planet Dust to planetesimals formation µm → cm: contact forces during collision lead to sticking Planetesimals cm → km : ??? Streaming instability 2 Planetesimals to protoplanets

Metallicity km → 1,000 km: gravity Self-gravity 3 Protoplanets to planets 6 7 Conclusions Gas giants: 10 M⊕ core accretes gas (< 10 –10 years) Terrestrial planets: protoplanets collide (107–108 years)

→ → → . Planet formation

Planetesimal Planetesimal hypothesis of Safronov 1969: growth and planet formation Planets form in protoplanetary discs from dust grains that collide

Anders and stick together Johansen

1 Planet Dust to planetesimals formation µm → cm: contact forces during collision lead to sticking Planetesimals cm → km : ??? Streaming instability 2 Planetesimals to protoplanets

Metallicity km → 1,000 km: gravity Self-gravity 3 Protoplanets to planets 6 7 Conclusions Gas giants: 10 M⊕ core accretes gas (< 10 –10 years) Terrestrial planets: protoplanets collide (107–108 years)

→ → → → . Planet formation

Planetesimal Planetesimal hypothesis of Safronov 1969: growth and planet formation Planets form in protoplanetary discs from dust grains that collide

Anders and stick together Johansen

1 Planet Dust to planetesimals formation µm → cm: contact forces during collision lead to sticking Planetesimals cm → km : ??? Streaming instability 2 Planetesimals to protoplanets

Metallicity km → 1,000 km: gravity Self-gravity 3 Protoplanets to planets 6 7 Conclusions Gas giants: 10 M⊕ core accretes gas (< 10 –10 years) Terrestrial planets: protoplanets collide (107–108 years)

→ → → → . Recipe for making planets?

Planetesimal growth and (Paszun & Dominik) planet Hydrogen and Helium (98,5%) µm formation

Anders Dust and ice (1,5%) Johansen Coagulation (dust growth) Planet formation ⇒ Planets?

Planetesimals

Streaming instability Metallicity mm Self-gravity

Conclusions

(Blum & Wurm 2008) Recipe for making planets?

Planetesimal growth and (Paszun & Dominik) planet Hydrogen and Helium (98,5%) µm formation

Anders Dust and ice (1,5%) Johansen Coagulation (dust growth) Planet formation ⇒ Planets? No

Planetesimals

Streaming instability Metallicity “Meter barrier”: mm Self-gravity Growth to mm or cm, but not larger Conclusions The problem: small dust grains stick readily with each other – sand, pebbles and rocks do not

(Blum & Wurm 2008) Overview of planets

Planetesimal growth and planet Protoplanetary discs formation

Anders Johansen

Planet Dust grains formation Gas giants and Planetesimals ice giants

Streaming instability

Metallicity

Self-gravity Pebbles

Conclusions Terrestrial planets

Dwarf planets+More than 400 exoplanets + Countless and objects + of giant planets Planetesimal formation must 1 proceed quickly 2 not rely on sticking between large solids 3 operate in a turbulent environment

Planetesimals

Planetesimal Kilometer-sized objects massive enough growth and planet to attract each other by gravity formation (two-body encounters) Anders Johansen Assembled from colliding dust grains Planet Building blocks of planets formation

Planetesimals Problems:

Streaming Pebbles, rocks and boulders: instability - drift rapidly through the disc William K. Hartmann Metallicity - have terrible sticking properties Self-gravity

Conclusions Protoplanetary discs are turbulent Planetesimals

Planetesimal Kilometer-sized objects massive enough growth and planet to attract each other by gravity formation (two-body encounters) Anders Johansen Assembled from colliding dust grains Planet Building blocks of planets formation

Planetesimals Problems:

Streaming Pebbles, rocks and boulders: instability - drift rapidly through the disc William K. Hartmann Metallicity - have terrible sticking properties Self-gravity

Conclusions Protoplanetary discs are turbulent

Planetesimal formation must 1 proceed quickly 2 not rely on sticking between large solids 3 operate in a turbulent environment Streaming instability

Planetesimal growth and Youdin & Goodman 2005: (see also Goodman & Pindor 2000) planet formation Gas orbits slightly slower than Keplerian Anders Johansen Particles lose angular momentum due to headwind

Planet Particle clumps locally reduce headwind and are fed by formation isolated particles Planetesimals

Streaming instability

Metallicity

Self-gravity Conclusions v Kep (1− η )

F F GP Clumping

Planetesimal Linear and non-linear evolution of radial drift flow of growth and planet meter-sized boulders: formation −1 −1 t=40.0 Ω t=80.0 Ω +20.0 +20.0 Anders Johansen +10.0 +10.0 ) ) r r

Planet η +0.0 +0.0 η /( /( formation z z

Planetesimals −10.0 −10.0

Streaming −20.0 −20.0 instability −20.0 −10.0 +0.0 +10.0 +20.0 −20.0 −10.0 +0.0 +10.0 +20.0 x/(ηr) x/(ηr) t=120.0 Ω−1 t=160.0 Ω−1 Metallicity +20.0 +20.0

Self-gravity +10.0 +10.0 Conclusions ) ) r r

η +0.0 +0.0 η /( /( z z

−10.0 −10.0

−20.0 −20.0 −20.0 −10.0 +0.0 +10.0 +20.0 −20.0 −10.0 +0.0 +10.0 +20.0 x/(ηr) x/(ηr) Strong clumping in non-linear state of the streaming instability (Youdin & Johansen 2007; Johansen & Youdin 2007; also Bai & Stone in preparation) Why clump?

Planetesimal growth and planet formation

Anders Johansen

Planet formation

Planetesimals

Streaming instability

Metallicity

Self-gravity

Conclusions Clumping in 3-D

Planetesimal 3-D evolution of the streaming instability: growth and planet Particle size: formation 30 cm @ 5 AU or 1 cm @ 40 AU Anders Johansen

Simulation box Disc Planet formation

Planetesimals

Streaming instability

Metallicity

Self-gravity

Conclusions

Particle clumps have up to 100 times the gas density Clumps dense enough to be gravitationally unstable But still too simplified: ⇒ no vertical gravity and no self-gravity ⇒ single-sized particles The streaming instability can provide the necessary ingredients for planetesimal formation Clumps readily contract gravitationally to form 100 km radius planetesimals

Clumping depends on metallicity in a way that matches observed correlation between host star metallicity and exoplanets

This talk

Planetesimal growth and planet ⇒ 3-D hydrodynamical simulations of particle sedimentation, formation

Anders including multiple sizes, clumping and self-gravity Johansen I will show that: Planet formation

Planetesimals

Streaming instability

Metallicity

Self-gravity

Conclusions Clumping depends on metallicity in a way that matches observed correlation between host star metallicity and exoplanets

This talk

Planetesimal growth and planet ⇒ 3-D hydrodynamical simulations of particle sedimentation, formation

Anders including multiple sizes, clumping and self-gravity Johansen I will show that: Planet formation The streaming instability can provide the necessary Planetesimals ingredients for planetesimal formation Streaming instability Clumps readily contract gravitationally to form 100 km Metallicity radius planetesimals Self-gravity

Conclusions This talk

Planetesimal growth and planet ⇒ 3-D hydrodynamical simulations of particle sedimentation, formation

Anders including multiple sizes, clumping and self-gravity Johansen I will show that: Planet formation The streaming instability can provide the necessary Planetesimals ingredients for planetesimal formation Streaming instability Clumps readily contract gravitationally to form 100 km Metallicity radius planetesimals Self-gravity

Conclusions Clumping depends on metallicity in a way that matches observed correlation between host star metallicity and exoplanets 0.040

0.035

0.030 p

Z 0.025

0.020

0.015

0.010 −0.10 −0.05 0.00 0.05 0.10 x/Hg

Sedimentation and clumping

Planetesimal Sedimentation of 10 cm rocks: growth and planet formation

Anders Gas mass Johansen decreases

Planet with time formation Solar Planetesimals

Streaming metallicity: instability puffed up Metallicity mid-plane Self-gravity layer Conclusions Clumping above Z ≈ 0.02 Sedimentation and clumping

Planetesimal Sedimentation of 10 cm rocks: growth and 0.040 planet formation

Anders Gas mass Johansen decreases 0.035

Planet with time formation Solar 0.030 Planetesimals

Streaming metallicity:

instability p

puffed up Z 0.025 Metallicity mid-plane Self-gravity layer Conclusions 0.020 Clumping above 0.015 Z ≈ 0.02

0.010 −0.10 −0.05 0.00 0.05 0.10 x/Hg Why is metallicity important?

Planetesimal growth and Gas orbits slightly slower than Keplerian planet formation Particles lose angular momentum due to headwind Anders Particle clumps locally reduce headwind and are fed by Johansen isolated particles Planet formation

Planetesimals

Streaming instability v Metallicity Kep (1− η ) Self-gravity Conclusions F F GP

Clumping relies on particles being able to accelerate the gas towards Keplerian speed Dependence on metallicity

Planetesimal growth and Particles sizes 3–12 cm at 5 AU, 1–4 cm at 10 AU planet formation Increase pebble abundance Σpar/Σgas from 0.01 to 0.03

Anders 0.1 0.1 Johansen Zp=0.01 Zp=0.02 Zp=0.03 g g H H

Planet / 0.0 0.0 / formation z z ρ ρ < p/ g>y Planetesimals −0.1 0.0 5.0 −0.1 Streaming 50 50 instability

Metallicity 40 40

Self-gravity 30 30 Conclusions orb orb T T / / t t 20 Σ Σ 20 p(x,t)/ g 0.1 10 10

0.0 0 0 −0.1 0.0 0.1 −0.1 0.0 0.1 −0.1 0.0 0.1 x/Hg x/Hg x/Hg Collapse happens much faster than the radial drift time-scale

Massive planetesimals form, with radius 100–200 km

Planetesimal formation movie

Planetesimal growth and Time is in Keplerian orbits (1 orbit ≈ 10 years) planet formation

Anders Johansen

Planet 6 formation

Planetesimals

Streaming instability

Metallicity Keplerian flow Keplerian flow

Self-gravity

Conclusions

?

Johansen, Youdin, & Mac Low (2009) Planetesimal formation movie

Planetesimal growth and Time is in Keplerian orbits (1 orbit ≈ 10 years) planet formation Collapse happens much faster than the radial drift time-scale Anders Johansen

Planet 6Massive planetesimals form, with radius 100–200 km formation

Planetesimals

Streaming instability

Metallicity Keplerian flow Keplerian flow

Self-gravity

Conclusions

?

Johansen, Youdin, & Mac Low (2009) (see John Chambers’s talk today for alternative turbulent concentration scenario)

The “clumping scenario” for planetesimal formation

Planetesimal growth and 1 planet Dust growth by coagulation to a few cm formation

Anders Johansen

Planet formation Planetesimals 2 Spontaneous clumping through streaming Streaming instability instabilities

Metallicity

Self-gravity

Conclusions

3 to 100 km radius planetesimals The “clumping scenario” for planetesimal formation

Planetesimal growth and 1 planet Dust growth by coagulation to a few cm formation

Anders Johansen

Planet formation Planetesimals 2 Spontaneous clumping through streaming Streaming instability instabilities

Metallicity

Self-gravity

Conclusions

3 Gravitational collapse to 100 km radius planetesimals

(see John Chambers’s talk today for alternative turbulent concentration scenario) From planetesimals to giant planets

Planetesimal growth and planet formation 1 Form km-scale Anders Johansen planetesimals from dust grains Planet formation 2 Planetesimals collide and Planetesimals build 10 M⊕ core Streaming instability 3 Run-away of Metallicity several hundred Self-gravity masses of gas Conclusions

(talks by David Stevenson, Jack Lissauer) 0.040

0.035

0.030 p

Z 0.025

0.020

0.015

0.010 −0.10 −0.05 0.00 0.05 0.10 x/Hg

Metallicity of host star

Z = 0.010 .020 .03 Planetesimal growth and First planet around planet (Gonzalez 1997; Santos et al. 2004; formation solar-type star found in Fischer & Valenti 2005) Anders Johansen 1995 (Mayor & Queloz 1995) Planet formation Today more than 400 Planetesimals exoplanets known Streaming instability Exoplanet probability Metallicity increases sharply with Self-gravity metallicity of host star Conclusions

⇒ Expected due to efficiency of core accretion (Ida & Lin 2004; Mordasini et al. 2009) ⇒ ... but planetesimal formation may play equally big part (Johansen, Youdin, & Mac Low 2009) Metallicity of host star

Z = 0.010 .020 .03 Planetesimal growth and 0.040 First planet around planet (Gonzalez 1997; Santos et al. 2004; formation 0.035 solar-type star found in Fischer & Valenti 2005) Anders Johansen 1995 0.030 (Mayor & Queloz 1995) Planet formation p Z 0.025 Today more than 400 Planetesimals exoplanets known Streaming 0.020 instability Exoplanet probability Metallicity 0.015 increases sharply with Self-gravity metallicity of host star 0.010 Conclusions −0.10 −0.05 0.00 0.05 0.10 x/Hg

⇒ Expected due to efficiency of core accretion (Ida & Lin 2004; Mordasini et al. 2009) ⇒ ... but planetesimal formation may play equally big part (Johansen, Youdin, & Mac Low 2009) Several modes of planet formation

Planetesimal growth and Clumping through streaming instabilities depends only on planet formation mid-plane dust-to-gas ratio (metallicity), not on absolute

Anders column density Johansen However, metallicity is not a constant of a given Planet formation protoplanetary disc

Planetesimals

Streaming instability Protoplanetary discs can obtain critical metallicity by: Metallicity 1 Self-gravity starting out with high metallicity

Conclusions ⇒ 2 photoevaporating the gas ⇒ 3 transport solids radially ⇒ . Several modes of planet formation

Planetesimal growth and Clumping through streaming instabilities depends only on planet formation mid-plane dust-to-gas ratio (metallicity), not on absolute

Anders column density Johansen However, metallicity is not a constant of a given Planet formation protoplanetary disc

Planetesimals

Streaming instability Protoplanetary discs can obtain critical metallicity by: Metallicity 1 Self-gravity starting out with high metallicity

Conclusions ⇒ born rich 2 photoevaporating the gas ⇒ 3 transport solids radially ⇒ . Several modes of planet formation

Planetesimal growth and Clumping through streaming instabilities depends only on planet formation mid-plane dust-to-gas ratio (metallicity), not on absolute

Anders column density Johansen However, metallicity is not a constant of a given Planet formation protoplanetary disc

Planetesimals

Streaming instability Protoplanetary discs can obtain critical metallicity by: Metallicity 1 Self-gravity starting out with high metallicity

Conclusions ⇒ born rich 2 photoevaporating the gas ⇒ get rich 3 transport solids radially ⇒ . Several modes of planet formation

Planetesimal growth and Clumping through streaming instabilities depends only on planet formation mid-plane dust-to-gas ratio (metallicity), not on absolute

Anders column density Johansen However, metallicity is not a constant of a given Planet formation protoplanetary disc

Planetesimals

Streaming instability Protoplanetary discs can obtain critical metallicity by: Metallicity 1 Self-gravity starting out with high metallicity

Conclusions ⇒ born rich 2 photoevaporating the gas ⇒ get rich 3 transport solids radially ⇒ restructure debt/mortgage . ⇒ Predict fewer close in planets in low metallicity systems and that low mass planets can form around low metallicity stars ⇒ Need better statistics of low metallicity systems and low mass planets

Low and high metallicity planet formation

Planetesimal growth and planet formation

Anders Johansen High metallicity systems Solar (or lower) metallicity systems

Planet Planet formation is rapid Planet formation triggered by formation photoevaporation Planetesimals Lots of time to accrete gas (Throop & Bally 2005; Alexander & Armitage 2007) Streaming Moderate mass planets instability migrate and become hot Little gas when planets form, Metallicity so gas giants rare and no Self-gravity strong migration Conclusions Low and high metallicity planet formation

Planetesimal growth and planet formation

Anders Johansen High metallicity systems Solar (or lower) metallicity systems

Planet Planet formation is rapid Planet formation triggered by formation photoevaporation Planetesimals Lots of time to accrete gas (Throop & Bally 2005; Alexander & Armitage 2007) Streaming Moderate mass planets instability migrate and become hot Little gas when planets form, Metallicity Jupiters so gas giants rare and no Self-gravity strong migration Conclusions

⇒ Predict fewer close in planets in low metallicity systems and that low mass planets can form around low metallicity stars ⇒ Need better statistics of low metallicity systems and low mass planets This is a spectacular confirmation that metallicity matters even for systems of intrinsically low metallicity

Low metallicity planets

Planetesimal growth and Santos et al. (A&A accepted): monitored 100 metal poor stars for planet formation planets. Anders Johansen ⇒ Three planets found

Planet formation ⇒ All three planets orbit the

Planetesimals most metal rich stars of

Streaming the sample instability ⇒ No hot Jupiters Metallicity

Self-gravity (a = 1.76, 1.78, 5.5 AU)

Conclusions Low metallicity planets

Planetesimal growth and Santos et al. (A&A accepted): monitored 100 metal poor stars for planet formation planets. Anders Johansen ⇒ Three planets found

Planet formation ⇒ All three planets orbit the

Planetesimals most metal rich stars of

Streaming the sample instability ⇒ No hot Jupiters Metallicity

Self-gravity (a = 1.76, 1.78, 5.5 AU)

Conclusions

This is a spectacular confirmation that metallicity matters even for systems of intrinsically low metallicity Conclusions

Planetesimal Clumping through streaming instability relevant because: growth and planet formation 1 Based on first principles hydrodynamical calculations Anders 2 Allows formation of planetesimals from pebbles and rocks Johansen 3 Efficiency depends very strongly on metallicity and Planet formation increases sharply above solar metallicity

Planetesimals 4 Can be trigged by photoevaporation, opening a new mode Streaming of planet formation around metal poor stars instability Metallicity (Johansen, Youdin, & Mac Low 2009) Self-gravity

Conclusions Collision speeds

Planetesimal growth and planet Relative speeds of particles measured in single grid cells: formation

Anders δv [m/s] δv [m/s] δv [m/s] δv [m/s] Johansen 1.00 Typical 0.75 ) v 0.1 + 0.1 0.2 + 0.1 0.3 + 0.1 0.4 + 0.1 0.50 δ (>

collision speed f Planet 0.25 formation 2–5 m/s 0.00 0.1 1.0 10.0 0.75

Planetesimals ) v 3 0.2 + 0.2 0.3 + 0.2 0.4 + 0.2 0.50 δ Only 5% of 128 (> 3 f Streaming 64 0.25 instability collisions 0.00 0.1 1.0 10.0 0.75

Metallicity ) v

faster than 10 δ 12 All 0.3 + 0.3 0.4 + 0.3 0.50 (> 10 f Self-gravity 0.1+0.1 0.25 m/s 8 0.4+0.1 NSH 0.4+0.1 0.00 Conclusions 6 0.1 1.0 10.0

v> [m/s] 0.75 δ 4 ) v

Collision < 2 0.4 + 0.4 0.50 δ (> f speed in 0 0.25 0.1 1.0 10.0 100.0 1000.0 0.00 ρ /ρ dense clumps p g 0.1 1.0 10.0 below 2 m/s Laundry list

Planetesimal growth and planet formation How do cm-sized pebbles and rocks form out of dust Anders grains? Johansen (Brauer et al. 2008; Zsom et al. 2010) Planet formation How do pebbles survive radial drift in low metallicity discs? (Takeuchi & Lin 2002; Brauer et al. 2007) Planetesimals Streaming What is the role of collisional fragmentation and instability coagulation during gravitational collapse Metallicity

Self-gravity What is the relative role of small scale turbulent

Conclusions concentrations and large scale streaming instabilities? (Cuzzi et al. 2008; John Chambers’s talk at this meeting) What is the size spectrum of newly formed planetesimals? Morbidelli et al. 2009: Asteroids were born big Core accretion and certain debris discs: Planetesimals should be small