1 / 9

Forming habitable planetson the computer

Anders Johansen

Lund University, Department of Astronomy and Theoretical Physics

2 / 9

Two protoplanetary discs

I Two ALMA images of protoplanetary discs

I HL Tau is 450 light years away, 1 million years old

I TW Hydrae is 176 light years away, 10 million years old

I Emission comes mainly from its the 1% mass in mm-sized pebbles

I Pebbles are formed by collisions between micron-sized dust grains

I Protoplanetary discs live for a few million years

(ALMA Partnership, 2015)

(Andrews et al., 2016)

3 / 9

Big questions in planet formation

Dust Pebbles

mm−cm

Planetesimals

10−1,000 km

Planets

10,000 kmµ m

Size and time

I How do pebbles gather to form km-scale planetesimals?

I How do the cores of giant planets accrete rapidly enough to accrete gas?

I How are the different planetary classes – gas giants, ice giants,super-Earths and terrestrial planets – formed?

I How are habitable planets seeded with life-essential molecules like H2O?

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Forming planetesimals through the streaming instability

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ESA

ESA

ESO

(Johansen et al., PPVI, 2014)

I Dense pebble filaments emerge through the streaming instability(Youdin & Goodman, ApJ, 2005; Johansen et al., Nature, 2007; Bai & Stone, ApJ, 2010)

I Filaments collapse by gravity to form planetesimals with sizes from 10 kmto several 100 km (Johansen et al., 2015, Science Advances; Simon et al., ApJ, 2016)

I Most mass in 100-km-scale planetesimals, as in asteroid belt

I Small bodies like comet 67P/Churyumov-Gerasimenko are piles ofprimordial pebbles, as observed for 67P (Poulet et al., MNRAS, 2016)

I Many planetesimals form as binaries, similar to those observed in theKuiper belt beyond Neptune (Noll et al., Icarus, 2008)

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Pebble accretion

I Hill radius marks the region ofgravitational influence of agrowing protoplanet

I Most planetesimals that enter theHill sphere of a protoplanet aresimply scattered – less than 0.1%are accreted

I Pebbles spiral in towards theprotoplanet due to gas friction

⇒ Very high pebble accretion rates

⇒ Possible to form solid cores of 10Earth masses before the gaseousprotoplanetary disc is accretedafter a few million years

by protoplanet

Pebble spirals towards

protoplanet due to gas friction

Planetesimal is scattered

Core growth to 10 M⊕

10−1 100 101 102

r/AU

103

104

105

106

107

108

∆t/

yr

PebblesFragments

Planetesimals

(Johansen & Lacerda, MNRAS, 2010; Ormel & Klahr, A&A, 2010; Lambrechts & Johansen, A&A, 2012)

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Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.00 MyrJ

S

U N

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.37 MyrJ

S

U N

Solid core

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.54 MyrJ

S

U N

Solid core Slow gas contraction

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.63 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.71 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.79 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.88 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.79 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.86 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.91 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.99 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

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Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 1.03 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

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Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 1.30 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

6 / 9

Growth tracks of giant planets

0 10 20 30 40r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 1.32 MyrJ

S

U N

Solid core Slow gas contraction Run−away gas accretion

I Planet formation model including the evolving protoplanetary disc,growth by pebble and gas accretion and migration(Bitsch et al., A&A, 2015; Johansen & Lambrechts, Annual Review of Earth and Planetary Sciences, 2017)

I Giant planets form solid core first (blue line), then gas envelope contractsslowly (red) and finally undergoes run-away collapse (yellow)

I Giant planets take approximately 1 Myr to form

I Protoplanets undergo substantial migration during their growth

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Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.0 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

7 / 9

Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.2 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

7 / 9

Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.4 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

7 / 9

Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.6 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

7 / 9

Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.8 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

7 / 9

Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 1.0 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

7 / 9

Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 1.2 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

7 / 9

Growth tracks of terrestrial planets (?)

0 1 2 3 4 5r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 1.4 Myr

Me

V E

Ma

Ice + rockOnly rock

I Inside water ice line at 2.5 AU particles contain no water ice

I Protoplanets overshoot the terrestrial masses and form super-Earths

I Rocky super-Earths form inside of ice line, water worlds migrate fromoutside the ice line

I In good agreement with prevalence of super-Earths around other stars

I How are terrestrial planets formed then?

8 / 9

Terrestrial planet formation with Jupiter

0 5 10 15r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.0 Myr

Me

VE

Ma

I Jupiter blocks the flow of material into the terrestrial planet zone

I Protoplanet growth quenched at Mars-mass planetary embryos

I Embryos grow to terrestrial planets after 100 Myr of mutual collisions

I The debris from one of these impacts likely created our Moon

I Another giant impact with an icy protoplanet could have delivered all theEarth’s budget of water, carbon and nitrogen

8 / 9

Terrestrial planet formation with Jupiter

0 5 10 15r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.2 Myr

Me

VE

Ma

I Jupiter blocks the flow of material into the terrestrial planet zone

I Protoplanet growth quenched at Mars-mass planetary embryos

I Embryos grow to terrestrial planets after 100 Myr of mutual collisions

I The debris from one of these impacts likely created our Moon

I Another giant impact with an icy protoplanet could have delivered all theEarth’s budget of water, carbon and nitrogen

8 / 9

Terrestrial planet formation with Jupiter

0 5 10 15r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.4 Myr

Me

VE

Ma

I Jupiter blocks the flow of material into the terrestrial planet zone

I Protoplanet growth quenched at Mars-mass planetary embryos

I Embryos grow to terrestrial planets after 100 Myr of mutual collisions

I The debris from one of these impacts likely created our Moon

I Another giant impact with an icy protoplanet could have delivered all theEarth’s budget of water, carbon and nitrogen

8 / 9

Terrestrial planet formation with Jupiter

0 5 10 15r [AU]

10−3

10−2

10−1

100

101

102

103

M [

ME]

t = 0.6 Myr

Me

VE

Ma

I Jupiter blocks the flow of material into the terrestrial planet zone

I Protoplanet growth quenched at Mars-mass planetary embryos

I Embryos grow to terrestrial planets after 100 Myr of mutual collisions

I The debris from one of these impacts likely created our Moon

I Another giant impact with an icy protoplanet could have delivered all theEarth’s budget of water, carbon and nitrogen

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Future of planet formation modeling

10−2 10−1 100 101 102

r [AU]

HR 8799HR 8799HR 8799HR 8799

Kepler 11Kepler 11Kepler 11Kepler 11Kepler 11Kepler 11

WASP 47WASP 47WASP 47WASP 47

HD 108874HD 108874

Solar SystemSolar SystemSolar SystemSolar SystemSolar SystemSolar SystemSolar SystemSolar System

I Model growth from protoplanetary disc to diverse planetary systems

I Include dust growth, planetesimal formation, planetesimal accretion,pebble accretion, gas accretion and planetary migration

I Multiple protoplanets growing and interacting gravitationally

I Self-consistent calculations of volatile delivery to habitable terrestrialplanets and super-Earths

I Formation of moon systems around giant planets and super-Earths

⇒ Understanding the necessary requirements to make planets habitable