Forming habitable planets
on the computer
Anders Johansen
Lund University, Department of Astronomy and Theoretical Physics
1/9
Two protoplanetary discs
(Andrews et al., 2016)
(ALMA Partnership, 2015)
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
2/9
Big questions in planet formation
Size and time
Dust
µm
Pebbles
mm−cm
Planetesimals
10−1,000 km
Planets
10,000 km
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 H2 O?
3/9
Forming planetesimals through the streaming instability
ESA
9
9
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 km
to 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 of
primordial pebbles, as observed for 67P
(Poulet et al., MNRAS, 2016)
I Many planetesimals form as binaries, similar to those observed in the
Kuiper belt beyond Neptune
(Noll et al., Icarus, 2008)
4/9
Pebble accretion
Planetesimal is scattered
by protoplanet
I Hill radius marks the region of
gravitational influence of a
growing protoplanet
I Most planetesimals that enter the
Pebble spirals towards
protoplanet due to gas friction
Hill sphere of a protoplanet are
simply scattered – less than 0.1%
are accreted
I Pebbles spiral in towards the
protoplanet due to gas friction
108
107
⇒ Possible to form solid cores of 10
Earth masses before the gaseous
protoplanetary disc is accreted
after a few million years
106
∆t/yr
⇒ Very high pebble accretion rates
105
104
103
10−1
Core growth to 10 M⊕
Planetesimals
nts
gme
Fra
100
Pebbles
101
102
r/AU
(Johansen & Lacerda, MNRAS, 2010; Ormel & Klahr, A&A, 2010; Lambrechts & Johansen, A&A, 2012)
5/9
Growth tracks of giant planets
103
J
t = 0.00 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
10−3
0
10
20
r [AU]
30
40
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 contracts
slowly (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
103
J
t = 0.37 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
10
20
r [AU]
30
40
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 contracts
slowly (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
103
J
t = 0.54 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
30
40
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 contracts
slowly (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
103
J
t = 0.63 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 0.71 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 0.79 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 0.88 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 0.79 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 0.86 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 0.91 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 0.99 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 1.03 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 1.30 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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
103
J
t = 1.32 Myr
S
2
10
U
N
M [ME]
101
100
10−1
10−2
Solid core
10−3
0
Slow gas contraction
10
20
r [AU]
Run−away gas accretion
30
40
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 contracts
slowly (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 terrestrial planets (?)
103
Only rock
102
Ice + rock
t = 0.0 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside 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 (?)
103
Only rock
102
Ice + rock
t = 0.2 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside 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 (?)
103
Only rock
102
Ice + rock
t = 0.4 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside 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 (?)
103
Only rock
102
Ice + rock
t = 0.6 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside 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 (?)
103
Only rock
102
Ice + rock
t = 0.8 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside 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 (?)
103
Only rock
102
Ice + rock
t = 1.0 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside 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 (?)
103
Only rock
102
Ice + rock
t = 1.2 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside 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 (?)
103
Only rock
102
Ice + rock
t = 1.4 Myr
M [ME]
101
V
100
E
Ma
10−1
Me
10−2
10−3
0
1
2
3
4
5
r [AU]
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 from
outside the ice line
I In good agreement with prevalence of super-Earths around other stars
I How are terrestrial planets formed then?
7/9
Terrestrial planet formation with Jupiter
103
t = 0.0 Myr
102
M [ME]
101
VE
100
Ma
10−1
Me
10−2
10−3
0
5
10
15
r [AU]
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 the
Earth’s budget of water, carbon and nitrogen
8/9
Terrestrial planet formation with Jupiter
103
t = 0.2 Myr
102
M [ME]
101
VE
100
Ma
10−1
Me
10−2
10−3
0
5
10
15
r [AU]
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 the
Earth’s budget of water, carbon and nitrogen
8/9
Terrestrial planet formation with Jupiter
103
t = 0.4 Myr
102
M [ME]
101
VE
100
Ma
10−1
Me
10−2
10−3
0
5
10
15
r [AU]
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 the
Earth’s budget of water, carbon and nitrogen
8/9
Terrestrial planet formation with Jupiter
103
t = 0.6 Myr
102
M [ME]
101
VE
100
Ma
10−1
Me
10−2
10−3
0
5
10
15
r [AU]
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 the
Earth’s budget of water, carbon and nitrogen
8/9
Future of planet formation modeling
Solar System
Kepler 11
WASP 47
HD 108874
HR 8799
10−2
10−1
100
r [AU]
101
102
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 terrestrial
planets and super-Earths
I Formation of moon systems around giant planets and super-Earths
⇒ Understanding the necessary requirements to make planets habitable
9/9