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PH709
Extrasolar Planets - 5
Professor Michael Smith
1
5 Star & Planet Formation & Theory of Exoplanets
 What is the origin of our solar system? Descartes, Kant,
Laplace: vortices, nebular hypothesis: importance of angular
momentum.
Star formation is on-going.
 In general: Gravity is fast-acting. Galaxy is old. But young
stars are still being born.
 Stars don't live forever, they must continue to be "born".
Where?
 Born in obscurity….needed infrared/millimeter/radio
wavelengths.
5.1 Sign of Youth: Gas Disks around Young Stars
Disks: During star formation, gas accretion occurs through a
geometrically thin disk that is optically thick.
The disks are cooler than the young star, and we thus see an
infrared excess superimposed on the black body stellar spectrum:
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Extrasolar Planets - 5
Professor Michael Smith
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Extrasolar Planets - 5
Professor Michael Smith
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Extrasolar Planets - 5
Professor Michael Smith
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PH709
Extrasolar Planets - 5
Professor Michael Smith
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Debris Disks
Debris disks are remnant accretion disks with little or no gas left (just
dust & rocks), outflow has stopped, the star is visible.
Theory: Gas disperses, “planetesimals” form (up to 100 km diameter
rocks), collide & stick together due to gravity forming protoplanets).
Protoplanets interact with dust disks: tidal torques cause planets to
migrate inward toward their host stars. Estimated migration time ~ 2 x
105 yrs for Earth-size planet at 5 AU.
Perturbations caused by gas giants may spawn smaller planets:
Start with a stable disk
around central star.
Jupiter-sized planet forms
& clears gap in gas disk.
Planet accretes along spiral Disk fragments into more
arms, arms become unstable. planetary mass objects.
Spiral density waves continuously produced by the gravity of
embedded or external perturber.
Debris Disks – Outer Disk
AB Aurigae outer
debris disk nearly
face on – see
structure &
condensations
(possible protoplanet formation
sites? Very far
from star) .
(Grady et al. 1999)
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Extrasolar Planets - 5
Professor Michael Smith
6
Debris: not from original nebula but from recent collisions.
After a few hundred million years, a planetary system is expected to
have assumed its final configuration and has either set the stage for
life, or will probably remain barren forever. It is difficult to probe this
era. Most of its traces have been obliterated in the solar system.
Only a minority of the nearby stars are so young.
Even for them, planets—and particularly those in the terrestrial
planet/asteroidal region—are faint and are lost in the glare of their
central stars.
However, when bodies in this zone collide, they initiate cascades of
further collisions among the debris and between it and other
members of the system, eventually grinding a significant amount of
material into dust grains distributed in a so-called debris disk.
Because the grains have larger surface area per unit mass
compared to larger bodies, they (re)radiate more energy and
therefore are more easily detected in the infrared compared to their
parent bodies.
In the absence of gas drag, a 10 m sized dust grain from the
primordial, proto–planetary nebula cannot survive longer than 1
Myr within 10 AU of a star due to a number of clearing processes,
such as sublimation, radiation pressure, Poynting-Robertson, and
stellar wind drag. Therefore, any main-sequence star older than
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Extrasolar Planets - 5
Professor Michael Smith
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10 Myr with an IR excess is a candidate to have circumstellar
material supplied through debris disk processes.
5.2 The Birth of the Solar System
The properties of the Solar System hold important clues to its origin:
•
•
•
Orbits of the planets and asteroids
Rotation of the planets and the Sun.
Composition of the planets, especially the strong distinction between
Terrestrial, Jovian, and Icy planets.
Clues from planetary motions:
•
Planets orbit in nearly the same plane. Planetary orbital angular
momentum is close to direction of Sun’s spin angular momentum (within
7o)

Planet orbits are nearly circular.

Planets & Asteroids orbit in the same direction.
•
Rotation axes of the planets tends to align with the sense of their
orbits, with exceptions. 3 of 4 terrestrial planets and 3 of 4 giant
planets have obliquities (angle between spin and orbital angular
momentum) < 30o; but Uranus is tipped at 98o

Sun rotates in the same direction in the same sense.

Jovian moon systems mimic the Solar System.
Clues from planet composition:
Inner Planets & Asteroids:

Small rocky bodies

Few ices or volatiles
Major facts in favour of a nebula hypothesis:
 Coplanar orbits of the planets
 All planets have prograde revolution (orbits)
PH709
Extrasolar Planets - 5
Professor Michael Smith
8
 The revolution of rings and natural moons are all prograde
(some moons of the outer planets are not prograde, but
these are believed to be captured satellites)
 All planets except Venus and Uranus have prograde
rotation
 The sun contains all the mass
 The planets (especially Jupiter and Saturn) contain most
of the angular momentum in the solar system
 Abundance gradient. Inner solar system is poor in light
volatile gases such as H, He, but rich in Fe & Ni. Outer
solar system is rich in volatiles H, He, etc. Abundances
similar to that of the sun
 Small, dense, iron and silicate rich planets in the inner 2
AU. Slow rotors, few or no moons, no rings, differentiated
(molten interiors)
 Large, low density, gaseous planets rich in H, He and
volatile elements at >= 5 AU
 Jovian Planets:

Deep Hydrogen & Helium atmospheres rich in volatiles.
 Large ice & rock cores
 Rapid rotors, many moons, all have ring systems

. Outer solar system moons & icy bodies: Small ice & rock
mixtures with frozen volatiles.
PH709
Extrasolar Planets - 5
Professor Michael Smith
9
giant planets (Jupiter,
Saturn)
•
composed mostly of H and He
but enriched in metals and
appear to have rock-ice core
comprising 10-20 Earth masses
intermediate or “ice”
planets (Uranus, Neptune)
•
rock-ice core comprising most
of mass surrounded by a gas
envelope ; 5-20% H and He
terrestrial planets
(Mercury, Venus, Earth,
Mars)
•
composed of rocky, refractory
(high condensation
temperature) material

Formation of the Sun: back to the Primordial Solar Nebula
Stage 1: All stars form out of interstellar gas clouds:

Large cold cloud of H2 molecules and dust gravitationally collapses
and fragments.
Rotating fragments collapse further:

Rapid collapse along the poles, but centrifugal forces slow the
collapse along the equator.

Result is collapse into a spinning disk

Central core collapses into a rotating proto-Sun surrounded by a
rotating "Solar Nebula"
Primordial Solar Nebula
The rotating solar nebula is composed of

~75% Hydrogen & 25% Helium

Traces of metals and dust grains
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Extrasolar Planets - 5
Professor Michael Smith
10
Stage 2. Starts out at ~2000 K, then cools:


As it cools, various elements condense out of the gas into
solid form as grains or ices.
Which materials condense out when depends on their
"condensation temperature".
Condensation Temperatures
Temp (K)
Elements
>2000 K
Condensate
All elements are gaseous
1600 K
Al, Ti, Ca
Mineral Oxides
1400 K
Iron & Nickel
Metallic Grains – Refractory, Rocky
1300 K
Silicon
Silicate Grains - Rocky
300 K
Carbon,
Oxygen
Carbonaceous grains -Volatiles
300-100 K
Hydrogen,
Nitrogen
Ices (H2O, CO2, NH3, CH4)
The "Frost Line"
Rock & Metals can form anywhere it is cooler than about 1300 K.
Carbon grains & ices can only form where the gas is cooler than 300 K.
Inner Solar System:

Too hot for ices & carbon grains.
Outer Solar System:

Carbon grains & ices form beyond the "frost line".
The location of the "frost line" is also a matter of some debate
but current thinking holds that it is probably about 4 AU . A
great deal depends on how much solar radiation can penetrate
deep into the outer parts of the primordial Solar Nebula.
Stage 3: From Grains to Planetesimals
PH709
Extrasolar Planets - 5
Professor Michael Smith
11
Grains that have low-velocity collisions can stick together,
forming bigger grains.


Beyond the "frost line", get additional growth by
condensing ices onto the grains.
Grow to where their mutual gravitation assists in the
aggregation process, accelerating the growth rate. Can
form kilometre-sized planetesimals after a few 1000 years
of initial growth.
Stage 4: From Planetesimals to Planets

Aggregation of planetesimals into planets
Terrestrial vs. Jovian planet formation.
Terrestrial Planets
Only rocky planetesimals inside the frost line:



Collisions between planetesimals form small rocky bodies.
It is hotter closer to the Sun, so the proto-planets cannot
capture H and He gas.
Solar wind is also dispersing the solar nebula from the
inside out, removing H & He.
Result:

Form rocky terrestrial planets with few ices.
Jovian Planets
The addition of ices to the mix greatly augments the masses of
the planetesimals
These collide to form large rock and ice cores:.

Jupiter & Saturn: 10-15 MEarth rock/ice cores.
PH709
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Extrasolar Planets - 5
Professor Michael Smith
12
Uranus & Neptune: 1-2 MEarth rock/ice cores.
As a consequence of their larger masses & colder temperatures:


Can accrete H & He gas from the solar nebula.
Planets with the biggest cores grow rapidly in size,
increasing the amount of gas accretion.
Result:

Form large Jovian planets with massive rock & ice cores
and heavy H and He atmospheres
Moons & Asteroids
Some of the gas attracted to the proto-Jovians forms a
rotating disk of material:

Get mini solar nebula around the Jovians

Rocky/icy moons form in these disks.

Later moons added by asteroid/comet capture.
Asteroids:


Gravity of the proto-Jupiter keeps the planetesimals in
the main belt stirred up.
Never get to aggregate into a larger bodies.
Icy Bodies & Comets
Outer reaches are the coldest, but also the thinnest parts of
the Solar Nebula:

Ices condense very quickly onto rocky cores.

Stay small because of a lack of material.
Gravity of the proto-Neptune also plays a role:

Assisted the formation of Pluto-sized bodies in 3:2
resonance orbits (Pluto and Plutinos)
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Extrasolar Planets - 5
Professor Michael Smith
13
Disperses the rest into the Kuiper Belt to become Kuiper
Belt Objects.
Comets and other Trans-Neptunian objects are the leftover icy
planetesimals from the formation of the Solar System.
Stage 5: Mopping up...
The entire planetary assembly process probably took about 100
Million years.
•
•
Followed by a 1 Billion year period during which the planets
were subjected to heavy bombardment by the remaining
rocky & icy pieces leftover from planet formation.
Solid planetary and satellite surfaces are heavily cratered;
cratering rate must have been far greater in first 109 yr of
solar system history than it is now (“late heavy
bombardment”)
Light from the Sun dispersed the remaining gas in the Solar
Nebula gas into the interstellar medium.
Planetary motions reflect the history of their formation.
Planets share the same sense of rotation, but have been
perturbed from perfect alignment by strong collisions during
formation.
The Sun "remembers" this original rotation. Rotates in the
same direction with its axis aligned with the plane of the Solar
System.
.
5.3 Now Exoplanets …….
Two obvious differences:
Existence of planets at small orbital radii, where our previous theory
suggested formation was very difficult.
PH709
Extrasolar Planets - 5
Professor Michael Smith
14
Substantial eccentricity of many of the orbits. No clear answers to either
of these surprises, but lots of ideas...