How Planets Form
Gerard Williger
U Louisville Public Astronomy Lecture Series
11 Feb 2016
With essential support from thousands
of scientists who did the work over the
centuries!
Formation of the Solar System
How did Earth and Planets in the
Solar System form?
• Oldest known Rocks on the moon
older than 4.4 Gyr  Solar System
formed ~ 4.6 Gyr ago
Collapse of a Cloud
4.6Gyr ago gas+dust cloud collapsed by own
gravity
Triggering mechanism could be:
- shock wave from a Supernova explosion
- little gravity fluctuation in cloud of gas
Orion Nebula
Cloud Collapse & Angular Momentum
Cloud spins faster as
it collapses  from
conservation of
angular momentum
angular momentum
depends on: 1) rpm
and 2) moment of
inertia (distance of
mass from spin axis)
Angular momentum in action
Formation of Disk
If cloud spins fast enough, rotational inertia
(misnamed “centrifugal force”) counteracts
gravitational contraction
No spin perpendicular to rotation plane to stop
contraction  formation of a disk
When central region dense, hot enough to start
nuclear reactions  formation of the Sun
DiskClumps Planetesimals 
Protoplanets
Computer simulation –
view from above disk
Gas, dust start to clump
Smaller clumps form
larger clumps and
planetesimals (solid
bodies, d  1km ~100km, held together by
stickiness, not gravity)
Gravity pulled together
planetesimals 
protoplanets orbiting a
protosun (not yet doing
nuclear fusion)
Baby Solar System
Effect of strong UV light
from neighboring hot star
on a young star and
planet-forming disk.
UV light heats up gas to
10000K, which evaporates
the disk and forms a
comet-like tail
downstream.
“Pillars of Creation”
Hubble Visible
Hubble Infrared
GAS
DUST
Eagle Nebula – Star Formation Region
Formation of Sun + Planets
Protoplanets contracted and cooled 
planets
Protosun contracted  Sun
Remaining planetesimals may still be
orbiting Sun (asteroids, meteoroids and
comets)
Most of unused gas blown away by strong
solar wind
Whole process: few Myr
From
Cloud to
Star +
Planets
New Tools to See All This
Hubble Space Telescope
Infrared (IR) satellites
Atacama Large Millimeter Array
Like “looking through a dusty
old stained glass window”
Compare with Fig. 2-2 in text
Near
<-Far IR->
/mid
IR
Satellite Telescopes
See most wavelengths near-infrared (1-2 μm)
from ground, though sky glows
See some wavelengths mid-infrared (2-10 μm)
from ground, but sky glows A LOT – satellite
better than from ground (1000s of times)
We can only see a few
wavelengths of far-infrared
from ground (10-1000 μm) and
sky glows incredibly brightly –
satellite MUCH better than
from ground (millions of times)
Fig. 3-27, p. 52
Hubble Space
Telescope
Observes
UV,
optical, a
little IR
light
Mirror
Perfect
seeing – no
atmosphere!
polished
wrong –
had
spherical
Airglow in
aberration
near-IR is
but was
much lower,
fixed
too!
Fig. 3-22, p. 50
INFRARED
Spitzer Space IR
Telescope (3-170 μm,
NASA, 0.8m diameter,
launched 2003)
Herschel Space IR
Telescope (55-672 μm,
ESA, 3.5m diameter),
operated 2009-13
Fig. 3-32a, p. 54
MICROWAVE
Atacama Large Millimeter Array (ALMA)
Partners: US, Europe, Canada, Japan, Taiwan, Chile
Location: Chajnantor plateau, N Chile (elevation 5000m=16700’)
Cost: $1.5 billion (still under construction)
66 dishes, area 7240 m2, maximum separation 16km, observes 0.32-3.6
mm
I – Background Star/Planet Formation
Caselli &
Ceccarelli,
Astronomy &
Astrophysics
Review, Oct
2012
Proto-planetary/Debris Disks
Form around T Tau (low mass) or Herbig Ae/Be
star (high mass)
First debris disk observed 1984 around Vega
(IRAS)
Proto-planetary Disks: up to 25 Myr old
Dust emission strong in infrared/submm.microwaves
High resolution required (Space Telescope, ALMA)
Sizes up to 1000 AU
From young
stars in nearby
star formation
regions,
Mamajek et al
HL Tau, ALMA,
ESO public image
2009, AIP,
1158, 3
Disk structure/variety
High aspect
ratio (flat);
often
associated with
jets
Variety seen in HST
infrared images
from NICMOS
-- note dust lanes
Disk Composition: Function of Temperature
Grains /
refractories
(high
melting
points:
metals,
silicates)
Water /
volatiles
Observing Proto-planetary
Disks
C Dullemond, ITA,
Disk Types/Evolution
(Oldest)
Observing Formation: Coronography to
observe Proto-planetary Disks
Images of protoplanetary disks around stars
Observe in IR because disks are COOL & DUSTY
Coronagraphy –
KECK: Mid-IR (18
blocks light from
micron)
star so can see
Near-IR, adaptive
optics image, Keck
faint disk
Near-IR
image, Keck
Hubble (optical, occulted)
Proto-planetary disks Systems are bright in IR due
to blackbody emission from
cool dust.
HL Tau,
ALMA (mm
image)
HL Tau,
HST
Transitional Disks
•
Up to 10% develop holes, gaps
•
Maintain optically thin outer disks
•
often inner disks, too
Espaillat et al.
arXiv:1402.73
0.88mm dust emission from pre-transitional
01 disks
Forming Giant Planets – takes 1-10
Million years
– Model 1) Disk Instability – Gas
first: clumps in proto-planetary
disk video: start to collapse
gravitationally
– form giant planets with
gas+dust
– rocky core forms slowly from
impacts of rocky bodies
Forming Giant Planets – takes 1-10Myr
–Model 2) Core
Accretion – Rock
first: Rocky core of
~10 Earth masses
forms first: video
–gas comes from
proto-planetary
disk (nearby space)
Temperature drives planet formation - I
Temperature: dictates how fast gas
molecules move, likelihood of escape
from a planet’s gravity
Temperature: energy per sq. meter on a
planet depends on amount of sunlight
received (proportional to 1/d2)
Temperature: determines which
molecules stay, which go (evaporate) from
growing planet
Temperature drives planet formation - II
• Free hydrogen (H), helium (He) lightest ->
•
•
•
•
fastest to evaporate, closest to sun
Iron (Fe), nickel (Ni) last to evaporate
Outer planets – cooler -- naturally rich in light
gases (H, He, methane, ammonia)
inner planets – hotter – mostly Fe-Ni, rocky,
might take 100 Myr to form
Comets, asteroids, Kuiper Belt Objects left over
• Asteroid belt – where Jupiter’s gravity prevented
planetesimals from coming together by gravity
• Comets -- may have brought water, organic
molecules to inner planets after formation
TEMPERATURE VS. COMPOSITION
http://astro.unl.edu/classaction/animatio
ns/solar/formationtemps.html
Planets slowed down by disk
Watch the planet spiral in
NEW IDEA: PLANETS MIGRATE!
“Bullies” like Jupiter throw around
other planets in a giant cosmic dance
of “Swing Your Partner”
IMPLICATION: ORIGINAL “EARTH”
MIGHT HAVE BEEN DESTROYED!
“Grand Tack” model: Jupiter and
Saturn migrated toward Sun, due to
interactions with smaller bodies
(thrown to outer solar system)
Computer Simulations –
Alessandro Morbidelli
Nice Model
Formation of terrestrial planets:
excentricité vs. axe semi-majeur
Planetesimals asteroids
- - - - combination (e vs. a) of instability Jupiter
Planets Can Change Orbits
“Drag” from proto-planetary nebula
gas?
Gravitational interactions with each
other?
We think that Uranus & Neptune formed
closer to Sun, were flung to their
present orbits by interactions with
Jupiter, maybe Saturn
Simulations numériques – A.
Morbidelli
excentricity vs. semi-maj. axis
Nice Model
top view
Jupiter Saturn Neptune Uranus asteroide/planetesimals
After Planet
Formation/Migration
LATE HEAVY BOMBARDMENT
Many small bodies changed orbit,
impacted planets about 3.1 billion years
ago
Made many craters: video
Solar System Impacts
Mercury: Caloris Basin
Venus: retrograde rotation?
Earth: formation of Moon
Mars: northern basin, loss of mag. field?
Vesta, Mimas: giant impacts which nearly
destroyed them
Uranus: sideways rotation
Mimas