Formation of Planetary Systems

Question 1

Structure of the Solar System

Answer 1

• four terrestrial planets; Mercury, Venus, Earth and Mars
• asteroid belt
• four gas-giant / Jovian planets; Jupiter, Saturn. Uranus, Neptune
• Kuiper belt objects

Question 2

Pluto

Answer 2

• pluto doesn’t fit in
• inner planets are small and rocky, outer planets are gas giants
• in a small icy world on an elliptical orbit inclined by 17’ to the ecliptic
• now thought to be an remnant from the planet building phase of the Solar System’s early history, a planetesimal

Question 3

The Terrestrial Planets of the Solar System

Answer 3

• Mercury, Venus, Earth and Mars share many features
• small compared with the huge planets in the outer solar system
• they have rocky surfaces surrounded by relatively thin atmosphere

Question 4

The Jovian Planets of the Solar System

Answer 4

• Jupiter, Saturn, Uranus and Neptune are the giant planets
• much bigger, more massive and less dense than the inner terrestrial planets
• internal structure is very different than the inner four planets

Question 5

Jovian Planets

Structure

Answer 5

• inner core of rock and ice
• mantle of water, ammonia, methane and ices
• surrounded by hydrogen, helium and methane gas atmosphere

Question 6

Asteroids

Answer 6

• rocky bodies
• many thousands are known
• most orbit the sun near the ecliptic plane at distances 2-3.5au, the asteroid belt
• largest is Ceres which orbits ~2.8au and has a diameter ~1000km

Question 7

Kuiper Belt

Answer 7

• collection of cometary nuclei located roughly in the plane of the ecliptic
• located beyond the orbit of Neptune, >30au from the sun
• source of short period comets

Question 8

Oort Cloud

Answer 8

• giant shell of icy bodies surrounding the solar system
• an approximately spherically symmetric cloud of cometary nuclei with orbital radii between ~3000-10000au
• source of all long-period comets

Question 9

Comets

Answer 9

• icy bodies
• ancient remains of the formation of the solar system
• ‘pristine material’

Question 10

Trojan Asteroids

Answer 10

-pockets of asteroids found near Jupiter’s orbit where the gravitational fields of the sun and Jupiter cancel out

Question 11

The Solar System Formation Scenario

Cloud

Answer 11

• the molecular cloud from which the Solar System formed accumulated from remnants of one or more stars that went supernova billions of years ago
• the cloud contained 2-3M☉ in mass and was ~10000au in size
• the massive loosely bound cloud of dust and gas had a small but non-negligible rate of rotation

Question 12

The Solar System Formation Scenario

Disk

Answer 12

• the cloud collapsed inwards under gravity, possible triggered by a nearby supernova
• conservation of angular momentum coupled with magnetic fields leads to a flattened disk

Question 13

The Solar System Formation Scenario

MMSN

Answer 13

• a useful piece of information when considering the formation of our solar system is the minimum amount of mass that is required to build all the bodies orbiting the sun
• this minimum mass solar nebula (MMSN) contains roughly a few dozen times the mass of Jupiter
• the matter will be distributed in the original disk around the young sun

Question 14

The Solar System Formation Scenario

Snow Line

Answer 14

• the composition of the material in the disk changes as a function of distance from the star
• this is where the concept of the snow line comes in
• at distances further away, ice coatings on dust grains increase the mass of solids available for building planetesimals

Question 15

Snow / Ice Line

Answer 15

• very close to the star, material in the disk is very hot
• the snow line marks the transition between bare dust grains and icy dust grains
• the position of the snow line depends on the mass of the star
• usually within a few au of the parent star

Question 16

Icy Dust Grains

Answer 16

• molecules can collide with dust grains in cold, dense environments
• the dust grain becomes coated by an icy mantle of water and other molecules e.g. CO in solid form
• this coating of ice increases the ‘stickiness’ of the dust grains helping to grow larger grains / bodies
• it also provides an environment for complex chemistry

Question 17

Mass Distribution of the Protoplanetary Disk

Answer 17

-Fsnow is the solid mass enhancement due to freeze out, sticking, of water onto the dust grains beyond the snow line
Fsnow = 1, r=rsnow

Question 18

Total Mass in the Minimum Mass Disk

Answer 18

-integrate the surface density of the gas over the surface area of the disk

Question 19

Jupiter Mass

Answer 19

Mj = 0.001M☉

Question 20

Do we know how dust grains combine to form larger bodies?

Answer 20

-the mechanical and chemical processes related to grain agglomeration are poorly understood

Question 21

Condensation and Growth of Solid Bodies

van der Waals

Answer 21

• loosely packed fractal structures that are held together by van der Waals forces may be formed
• have some observational information from interplanetary dust particles (IDPs)

Question 22

Condensation and Growth of Solid Bodies

Macroscopic Bodies

Answer 22

• when dust grains condense, the vertical component of the star’s gravity causes the dust to sediment out towards the mid-plane of the disk
• current models suggest that the bulk of solid material was able to agglomerate into bodies of macroscopic size within <10^4yr at 1au
• most of the bodies are confined to a thin region in the mid-plane

Question 23

What are the two main hypotheses on how bodies grow from ~cm to ~km-sized objects?

Answer 23

• if the nebula is quiescent, the dust and small particles settle into a layer thin enough to be gravitationally unstable to clumping and planetesimals are formed, the plaetesimals formed have sizes of the order of ~1km
• if the nebula is turbulent, growth continues via simple two body collisions, the growth of solid bodies from mm to km size must occur very quickly but the related physics is poorly understood
• molecular forces can lead to ~km sized planetesimals by coagulation

Question 24

How are bodies >1km formed?

Answer 24

-when size > 1km, gravity takes over and mutual gravitational perturbations become important

Question 25

Do sub-cm sized grains follow a Keplarian velocity profile?

Answer 25

• motion is coupled with the gas
• the gas is partially supported against stellar gravity by a pressure gradient in the radial direction: the gas orbits the star at slightly less than the Keplarian velocity
• for circular orbits, the effective gravity must be balanced by centrifugal acceleration
• considering that the pressure is much smaller than gravity, one finds that the gas orbits at ~0.5% slower than Keplarian velocity

Question 26

Drifting Dust Grains

Answer 26

• due to the balance of pressure and gravity
• large particles encounter a head wind which removes angular momentum and causes them to spiral inwards towards the star
• small grains drift less
• a meter-sized body at 1au would approach the sun in ~100yr

Question 27

What are the consequences of the differences in velocities between different sized bodies in the disk?

Answer 27

• small, sub-cm, sized grains can be swept up by larger bodies
• gas drag on the meter-sized planetesimals induces considerable radial motions
• the material that survives to form planets must complete the transition from cm to km size rather quickly unless the material is confined to a thin dust-dominated sub-disk in which the gas is dragged along at the same Keplarian velocity

Question 28

Solar System Formation

Dust

Answer 28

• dust settles gravitationally to the mid-plane
• dust composition changes with distance to the proto-Sun (snow-lines)
• this is because of the different condensation temperatures for material in the disk:
• -high Tc; silicates, oxides
• -low Tc; molecules e.g. H2O, CO2, NH3
• close to the sun only high Tc materials, further away there are both
• H and He are mostly in gas form
• T∝R^(-0.4)

Question 29

Solar System Formation

Grain Growth

Answer 29

• occurs as the disk becomes thinner and thinner
• collisions and sticking: dust agglomerates, creates meteor-type bodies
• collisions and gravitational attraction: planetesimals , creates km-sized bodies

Question 30

Formation of Terrestrial Planets

Answer 30

• runaway coagulation of planetesimals to Earth masses
• takes less than 100Myr
• giant impacts e.g. Earth-Moon system
• ends with depletion of available material
• steady, slow accretion of remaining gas if present

Question 31

Formation of Jovian Planets

Answer 31

• form in the outer disk, where there are lower temperatures: slower moving grains
• also, increase in mass of solids available (as ice, past the snow line)
• allows more rapid core growth
• when core mass >10M⊕, gravitational accretion of the gaseous envelope: this is a runaway process
• this creates a ‘gas giant’ planet
• accretion stops only when there is no more material available
• combination of accretion and tidal forces create a gap in the disk

Question 32

Formation of the Solar System

Timeline

Answer 32

• formed 4.568Gyr ago, age of the oldest known solids in the Solar System
• Mars formed ~13Myr later
• Earth formed ~30-40Myr later
• leading theory for formatino of the moon is that about 100Myr after birth of SS, the proto-Earth was hit by a Mars-sized object: the heavy cores of both formed the new earth and the light silicate crusts formed the moon
• the Jovian planets must have formed in less than 10Myr, the lifetime of the gaseous protoplanetary disk