The Formation of Our Solar System (Chapter 8)

Stellar and Planet Formation – the origin of stellar systems - Current theories of the formation of planets are based mainly on our solar system

All the planets orbit the Sun in the same plane

Rocky planets are close to the Sun

Gas giants are far from the Sun

All planets have relatively small eccentricity and inclination (explain)

However planets around other stars (exoplanets, extra-solar planets, in stellar systems) teach us differently: giants gas planets can be found close to their parent stars Exoplanets can have large eccentricity and inclination

New Theories of planet formation are being considered now A Stellar system = a star with its planets The Solar System = our Sun and the planets (Mercury, Venus, etc..) Here we will consider the formation of the Solar System only.

Solar Composition The composition of the planets, other stars, the inter-stellar medium (ISM), distant galaxies, … are all

usually expressed in units of Solar Composition.

The solar composition is the composition of our Sun (by mass): Hydrogen – 71% Helium – 27% All the other elements – 2 % (the other elements are sometimes referred to as “heavy elements” or even “metals”). For Example, Jupiter and its atmosphere have Solar composition, the same composition as the Sun.

The Origin of the elements - The Universe formed 13.7 billion yrs ago (the “Big Bang”) during which mainly Hydrogen and

Helium were formed, with tiny traces of Li and Be. - All the other elements were formed inside stars and are ejected into space by various processed

such as e.g. supernova explosions (when a massive star dies, it goes supernova) - Elements are basically “recycled” into new stars - Thermonuclear explosions (like “atomic bombs”) are at work inside stars and generate heavier

elements using Hydrogen (4H He), Helium (triple alpha process),… we will cover this later.

“At the beginning” stars that formed had only Hydrogen and Helium. A first generation of stars produced heavy elements (metals) That first generation of stars then ejected heavy elements into the insterstellar medium (ISM) New stars formed out of the “enriched” inter-stellar medium (enriched with heavy elements) As time passes, the interstellar medium has more and more “metals” As we look in our own Galaxy (the Milky Way) we see that the younger stars have more “metals” than the older stars As we look at Galaxies further away, it takes millions of years for the light to come to us, so the image of the galaxies that we see is millions of years younger For Galaxies billions of years away, we see their images when they were billions of years younger The distant galaxies have less metals than the nearby ones, they are younger Rocky planets and rocky cores of planets could not form at the very beginning after the Big Bang as there were no “metals” around.

Abundance of Elements in our Neighborhood – Solar abundances - Our solar system is 4.56 billion yrs old

- In our neighborhood in the galaxy most stars and inter-stellar gas have solar abundances (see

Fig.8-4)

The Nebular Hypothesis The Solar System formed out of a rotating cloud (nebula) of gas and dust This hypothesis was first proposed by Immanuel Kant and Pierre Simon de Laplace independently. The initial collapse was due to self-gravity, the gas cloud collapsed due to its own gravitational pull on itself. The mass of the initial collapsing cloud depends on its density and temperature. Out of a very large cloud several smaller clouds can collapse each independently to form stars

Flatening due to rotation & Angular Momentum Conservation The forces at work: Gravity, Centrifugal force and pressure forces.

Out of the collapsing cloud two main components form: the protosun and the protoplanetary disk.

The protosun: as the bulk of the material collapses its pressure and temperature increases but its gravity

increases too, compressing the gas even further.

After about 10 millions yrs the center of the protosun reaches a density of 100 times that of water and a

temperature of several million Kelvin. At this point nuclear reactions (fusion) start and increases the

temperature and pressure such that the collapse is stopped. A star is born.

The protoplanetary disk: - conservation of angular momentum rotation speed increases

until centrifugal force balances gravity (Keplerian rotation) in the radial direction

Review of conservation of angular momentum: radius x velocity x mass = constant in time

R1 x V1 x M (at time 31) = R2 x V2 x M (at time #2) for an element of mass M in the disk or a

“ring” of matter

R1 x V1 = R2 x V2 (since M is on both sides of the “=” is can be removed)

As the radius decreases (due to infall) we have R2 < R1 and therefore for the angular

momentum to be conserved (R1 x V1 = R2 x V2) V must increase V2 > V1

As material falls inwards it increases its rotational speed

As the rotational velocity increases, eventually it is large enough for the centrifugal force to balance the

gravity and stops the inward motion to smaller radii Keplerian orbit for the gas in the disk

Pressure forces in the gas balance gravity in the vertical direction

Redraw graph with Gr, =Fc, Gz=Fp etc…

Fg=GMm/R^2 = Fcent = mV^2/R V = sqrt( GM/R )

Pressure Forces = Total (Pressure x Area)

The protoplanetary disk is lso known as an ACCRETION DISK

The dust is not subject to the pressure as it is much heavier and “sinks” to the middle of the plane of the

protoplanetary disk gravitational sedimentation of the dust to the disk mid-plane

The disk is made of gas: Hydrogen and Helium And ice particles of H2O, NH3, CH4, CO2, SO2 … icy particles And dust particles made of Fe, Si, Mg, S, Al, Ca, Ni, … The velocity increases with radius, therefore rings of adjacent material do not have the same speed and

there is a shear between rings some kind of friction between the rings, so material slowly looses its

energy and moves inwards till it is “accreted” by the “protosun”.

Very small dust particles are carried away by the gas Very large particles move on their own and “feel the drag” of the gas (like e.g. a car moving feels the

wind).

These disks are seen in interstellar clouds:

Two kinds of disks are found:

- T Tauri disks: the accretion rate is slow and the disk is rather cold

- FU Ori disks: the accretion rate is high and the disk is hotter

The same protoplanetary disk can go through phases of low accretion rate (T Tauri) and phases of high

accretion rate (FU Ori).

Stars and planets also form for example in the Pleiades:

Accretion disks have also Jets, they are “ejecting” material

Terrestrial Planet Formation: Due to the contraction of the protosun, the temperature reaches 2000K in the inner disk and 50K in the

outer disk the icy particles evaporate in the inner disk and the dust is mainly made of “rocky”

material.

The collapse of the gas creates shocks in the gas that compresses the dust particles and heat them to create `1mm size chondrules Chondrules aggregate and stick together due to electric forces These form large fluffy agglomerates of chondrules, found nowadays in meteorites:

Chondrules inside a meteorite

This meteorite is 4cm, or less than 2 inch

There are mainly Iron (Fe) chondrules… within this Carbon rich meteorite. These meteorites are rich in “olivine” (Mg Fe)2 SO4 *magnesium iron silicate+ These objects keep on growing in size up to 1km in size – the planetesimals When they reach 1km in size they start attracting each other due to their gravity Planetesimals look like oddly shaped km-size potatoes like asteroids or comet nuclei:

The planetesimals grow by gravitational attraction up to 1000km in size protoplanets

This is Ceres, possibly what protoplanets looked like

about 1/10 the size of the Earth…

The protoplanets grow further into the terrestrial planets.

Jovian Planet Formation

Two theories or more… 1) The core accretion model, similar to terrestrial planet formation, but the heavy planetesimals

form the core of the giant planets which then accrete the gas (H + He)

The core of the giant planets has to accrete the gas fairly quickly before it is accreted by the

protosun, and the growth from chondrules into cores of protoplanets has to work in the presence of

the gas (the chondrules and dust grow within the gas disk).

2) The Gas Disk Instability model self gravity of the gas in the disk triggers planet formation

This shows the density of

the disk, yellow is high density and blue is low density (red is intermediate). Blobs of material can form

to form the beginning of a giant planet. The spiral seen is a high density spiral due to the self-gravity of

the disk and/or the tidal force of the protoplanet on the disk.

3) Alternative theories: gas + dust in vortices

Dust aggregates into fluffy “stuff” (“dusty bunnies”) and these are then crushes into denser objects

as they collect in the center of vortices in the disk.

The vortices are like giant hurricanes, like the Giant Red Spot of Jupiter (Great Red Spot), and these

vortices roll in the local shear.

Vortices in disk:

The vortices are spinning but are also rotating around the central star (protosun for example)

Vortices can merge together in the disk:

If you add dust to the disk:

The dust concentrates in the vortices and

vortices are therefore ideal to help trigger the formation of the Jovian planets.