Formation of Planetary Systems Flashcards
1
Q
Structure of the Solar System
A
- four terrestrial planets; Mercury, Venus, Earth and Mars
- asteroid belt
- four gas-giant / Jovian planets; Jupiter, Saturn. Uranus, Neptune
- Kuiper belt objects
2
Q
Pluto
A
- 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
3
Q
The Terrestrial Planets of the Solar System
A
- 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
4
Q
The Jovian Planets of the Solar System
A
- 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
5
Q
Jovian Planets
Structure
A
- inner core of rock and ice
- mantle of water, ammonia, methane and ices
- surrounded by hydrogen, helium and methane gas atmosphere
6
Q
Asteroids
A
- 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
7
Q
Kuiper Belt
A
- 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
8
Q
Oort Cloud
A
- 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
9
Q
Comets
A
- icy bodies
- ancient remains of the formation of the solar system
- ‘pristine material’
10
Q
Trojan Asteroids
A
-pockets of asteroids found near Jupiter’s orbit where the gravitational fields of the sun and Jupiter cancel out
11
Q
The Solar System Formation Scenario
Cloud
A
- 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
12
Q
The Solar System Formation Scenario
Disk
A
- 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
13
Q
The Solar System Formation Scenario
MMSN
A
- 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
14
Q
The Solar System Formation Scenario
Snow Line
A
- 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
15
Q
Snow / Ice Line
A
- 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
16
Q
Icy Dust Grains
A
- 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
17
Q
Mass Distribution of the Protoplanetary Disk
A
-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
18
Q
Total Mass in the Minimum Mass Disk
A
-integrate the surface density of the gas over the surface area of the disk
19
Q
Jupiter Mass
A
Mj = 0.001M☉
20
Q
Do we know how dust grains combine to form larger bodies?
A
-the mechanical and chemical processes related to grain agglomeration are poorly understood
21
Q
Condensation and Growth of Solid Bodies
van der Waals
A
- 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)
22
Q
Condensation and Growth of Solid Bodies
Macroscopic Bodies
A
- 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
23
Q
What are the two main hypotheses on how bodies grow from ~cm to ~km-sized objects?
A
- 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
24
Q
How are bodies >1km formed?
A
-when size > 1km, gravity takes over and mutual gravitational perturbations become important
25
Do sub-cm sized grains follow a Keplarian velocity profile?
- 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
26
Drifting Dust Grains
- 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
27
What are the consequences of the differences in velocities between different sized bodies in the disk?
- 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
28
Solar System Formation
| Dust
- 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)
29
Solar System Formation
| Grain Growth
- 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
30
Formation of Terrestrial Planets
- 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
31
Formation of Jovian Planets
- 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
32
Formation of the Solar System
| Timeline
- 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
