Protoplanetary Disk Evolution and Planet Formation:
A Latest View from Disk Magnetohydrodynamics

Satoshi Okuzumi (奥住 聡)
Department of Earth and Planetary Sciences, Tokyo Techokuzumilab.net | satoshiokuzumi.net | Twitter @SatoshiOkuzumi
Image credits (from left to right): ALMA Partnership et al. (2015); Suzuki & Inutsuka (2014); A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO); ISAS/JAXA

How do planets form?
Is the Earth a common planet?
Where do we come from?

Credit: Bill Saxton, NRAO/AUI/NSF2

Planet Formation: From Dust to Planets
Protoplanetary diskgas accretion(99% gas, 1% dust)

Gas giants

Dust Grains(≲ µm)
sticking

sticking or

Pebbles/Aggregates(0.1 mm–10 cm?)planetesimal accretion

Planetesimals (微惑星; 1–100 km)
Image credit: NASA/JPL-Caltech

Rocky/icy planets(≳ 1000 km)

3The Planet Radius Gap

Disk Substructures

Hot Topics inPlanet Formation

Sub-Neptunes

参考：日本惑星科学会誌「遊星人」
特集「新・惑星形成論」

Super-Earths

リンク集: satoshiokuzumi.net/yuseijin
Andrews et al. (2018)

Pebbles(mm–cm)Hopp et al. (2022)

planetOrmel (2017)

Bai (2017)

ielLaminar(Wind-driven)
DiskAccretion

dPebble Accretion

B-f

Isotopic Dichotomy (同位体二分性)of the Solar System

Fulton &Petigura(2018)

Protoplanetary Disk Dynamics: Why Does it Matter?
Disk turbulence?

Possible origins
of gas/dust substructures?
Courtesy: 4D2U Project, NAOJ

Flock et al. (2011)

Disk heating

Silicates
Silicates?
Silicates + Ice

Credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)?
Bae et al. (2023) PPVII Review Chapter 12(arXiv:2210.13314)

5 /24

Protoplanetary Disk Dynamics: Why Does it Matter?

Topics covered in this talk:🌬 What drives the accretion of protoplanetary disks?
Turbulence or magnetic fields?

⛄ How does the snow line migrate in the disks?
🧭 How strong is the magnetic field in the disks?

6 /24

Accretion onto Young Stars from their Disks

Accretion rates vs. ages of young stars·−7−9−1Macc ∼ 10 –10 M⊙ yr

What is the origin ofdisk accretion?
What happens in the disks?
Hartmann et al. (2016)7 /24

Classic Model: “Viscous” Accretion Disks
Lynden-Bell & Pringle (1974)

StarDisAccretion flowHigh ΩKepler)
weivnoitcesssk (croturbulence
Turbulence

Angular momentum transpor
tby viscosity (turbulence)

Low ΩKepler

Problem
The origin and nature of disk turbulence are unknown!
→ Assume turbulent viscosity = α × sound speed × disk thickness
In many cases, α is assumed to be constant both in space and in time.
8 /24

Possible Drivers of Protoplanetary Disk Turbulence
Credit: Y. Fukuhara (Tokyo Tech)

Magnetic
field

VSI
Requiresfast coolinge.g.,
Urpin & Brandenburg (1998)
Nelson et al. (2013)

MRIRequires
suficient ionization(e.g., T ≳ 1000 K)
Armitage et al. (2015)

e.g.,
Balbus & Hawley (1991)
Bai (2011)
Flock et al. (2011)

9 /24

Are Protoplanetary Disks Really Turbulent??
Vertically thick rings

Vertically thin rings

Settling (沈殿)Dust distribution in HL Tau disk

−4α ≲ 10(ALMA Partnership et al. 2016)−2 from observed
accretion rate

α ∼ 10Gas disk

Dust ring
Pinte et al. (2016)10 /24

In Many Disks, Turbulence Is Weak or Absent !
High level of dust settling
HL Tau (Pinte et al. 2016)

Weak/absent line broadening

Oph 163131 (Villenave et al. 2022)−5α ≲ 10 !

HD 163296 (Flaherty et al. 2017)

For a review, see Rosotti et al. (2023)

However, among the examples shown here,
HL Tau and HD 163296 have high accretion rates.

Then What Drives Disk Accretion??
11 /24

Disk Accretion without Turbulence: Driven by MHD Winds
Blandford & Payne (1982); Shibata & Uchida (1986)

Back-reaction!

StarDisk Wind (Outflow)
Magnetic field

Accretionflow

turbulence
Disk (cross-section view)

For more introduction, see 森 (2021) 遊星人

12 /24

Accretion Flow Driven by MHD Disk Winds
2D Global MHD simulation with all non-ideal effects (Ohmic/Hall/ambipolar resistivities)

B-fieldline

Radial flux ρvrSurfaceAccretion
flowBai (2017)

Hd: disk scale height

13 /24

Why Surface Accretion?
Magnetic field BWindStarVU,sya
X-rAccretion
flowdezinoI

Neutral∇×B∝ J≈0Ionized

Current Layer
14 /24

What Drives Protoplanetary Disk Accretion: Summary

Viscosity (Turbulence)-driven accretion

MHD Wind-driven accretion
Surfaceaccretion?

Weak/No

15 /24

Is Wind-Driven Accretion Favorable for Planet Formation?
Dust settling

Planetesimal formation

MHD Wind-driven accretion
Surfaceaccretion?

Weak/No
Courtesy: 4D2U Project, NAOJ

Planetesimal growthSuper-Earth formation

DustEffects on planet formation yet to be
fully investigated!
Needs further understanding of theaccretion flow structure

Courtesy: 4D2U Project, NAOJ
Pinte et al. (2016); Okuzumi, in prep; Gressel et al. (2012); Ogihara & Hori (2018)16 /24

Thermal Structure ofProtoplanetaryDisks

スノーライン:ksidyrat
岩石と氷の世界の境界
nealpotoPrAccretion heating

Snow line(T∼150–170 K)

Stellar radiation

Silicates

Icy bodies

Rocky bodies
Silicates + Ice

17 /24

Internal Heating of Magnetically Accreting Protoplanetary Disks
Viscosity (Turbulence)-driven accretion

MHD Wind-driven accretion
Surfaceaccretion?

Weak/No

Assuming spatially uniform turbulence,
most of the accretion energy isreleased around the midplane.

Most of the accretion energy isreleased near the surface
or is carried away by winds.
Mori, Bai, & Okuzumi (2019)18 /24

Internal Heating of Magnetically Accreting Protoplanetary Disks
Out ow
B- eldB- eld line

X-rays, UV

Current layers(Joule heating occurs)

computationalbox

midplane• Radially local MHD simulations with Athena(Stone et al. 2008)• Includes all non-ideal MHD e ects (Ohmicresistivity, ambipolar di usion, Hall e ect)
• Ionization fraction from an ionization model
including grains

Accretion heatingindeed occurs
near surface!

ffff

ff

flfi

fi

Mori, Bai, & Okuzumi (2019)19 /24

Snow Line Migration in Magnetically Accreting Protoplanetary Disks

1 au

0.6
10 Myr

The snow line
migrated inside 1 au
in the early phase ofsolar system formation!?
(But see also Kondo, Okuzumi, et al. 2023)

Mori, Okuzumi, et al. (2021)

20 /24

Open Issue: How Strong is the B- eld in Protoplanetary Disks?
MHD Wind-driven accretion rate is afunction of magnetic field strength:

Magnetic field B·M=−·M2r

ΩKepler(r)(BzBϕ)surface
e.g. Wardle (2007)

Simulations predict|B| ∼ 0.01–1 G at midplanefor disks with observed accretion
rates (10–9…–7 Msun/yr)
For a review, see Weiss et al. (2021)

fi21 /24

Constraining the Disk B- eld Strength from Solar System Solids
Estimated B-field in the Sun’s protoplanetary disk
Meteorites (隕石):Fragments of asteroids

Weiss et al. (2021)

Magnetized (磁化した)
particles!

CAI: the oldest solids in the solar nebula

Fu et al. (2014) Sciencefi

22 /24

Constraining the Disk B- eld Strength from Solar System Solids
Estimated B-field in the Sun’s protoplanetary disk
Ryugu samples(Nakamura+23)

Weiss et al. (2021)
Weiss et al. (2021)

Magnetized magnetite (磁鉄鉱) !
(Formed in liquid water in Ryugu)

CAI: the oldest solids in the solar nebula

Nakamura et al. (2023) Science23 /24
fiAsteroidRyugu

Returned samples

Summary
Protoplanetary disks

Small bodies

Planets

Originof Life?

Starformation

ALMA

Credit: Clem & Adri Bacri-Normier(wingsforscience.com)/ESO

Disk observations

Hayabusa2

Credit: JAXA

Solar system Exploration

JWST

Credits:
IMAGE: NASA-GSFC, Adriana
M. Gutierrez (CI Lab)

Exoplanet observations

多様な観測からヒントを得ながら惑星形成の謎を解明できる時代へ
24 /24