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?
4 / 9
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)
5 / 9
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)
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 = 0.00 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
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 = 1.03 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
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 = 1.30 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
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 = 0.0 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
9 / 9
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