Formation of the Solar System Chapter 8

To understand the formation of the solar system one has to apply concepts such as:

• Conservation of angular momentum

• Conservation of energy

The theory of the formation of the solar system need to explain:

• The pattern of the sense of rotation of planets

• The plane of the orbits of planets

• The sense of rotation of satellites

• The different composition of terrestrial, Jovian and dwarf planets

Some of the patterns we can find in the solar system: • All planets orbit the Sun counterclockwise (as seen from the north pole)

• Nearly all the planetary orbits lie in a plane

• Almost all planets rotate in the same direction with their axes perpendicular to the orbital plane

• Most satellites revolve around planets in the same direction that the planet rotates on its axis.

But there are some exceptions

• Venus rotates backwards (Rotational axis tilted close to 189 degrees)

• Uranus rotates on its side (Rotational axis tilted close to 90 degrees)

• Most small moons do not share the orbital plane of the planet

• Triton (Satellite of Neptune) orbit the planet in opposite sense

• Earth is the only terrestrial planet with a large moon

A model for the formation of the solar system has to account for:

• Different composition of planets (rocky, gaseous, icy)

• Existence of many asteroids and comets

Angular Momentum

• Objects rotating around a point have angular momentum.

• Simplest case ( a small sphere orbiting a larger mass)

L = m x v x r

L :angular momentum

m: mass of small sphere

v: velocity of the small sphere

r :separation between the small sphere and the larger object

• Conservation of angular momentum if r changes, v must change (ice skaters)

But the value of L remains constant

The nebular hypothesis (and the rejected collision theory)

• The idea that the solar system was born from the collapse of a cloud of dust and gas for proposed by Immanuel Kant (1755) and by Pierre Simon Laplace 40 years later.

• During the first part of the 20th century, some proposed that the solar system was the result of a near collision of the Sun with another star. Planets formed from debris of the collision. But we know now that collision (or near collisions) between two stars are very, very rare.

• Considering that collision are rare, the proposed idea of the collision may explain a unique event on how our planetary system formed but not how other planetary systems formed.

• During the rest of the 20th century, new ideas and theories about the formation of stars (and possible planets) made this collision theory obsolete and was discarded

• In 1995, the first exoplanet (planets orbiting other stars) was discovered

• Many more planets have been found so far in the solar neighborhood ( close to 1000 confirmed and more than 2000 possible ones). It is clear now that formation of planets is not a rare event.

• Any theory about the formation of planetary system must explain the formation of planets, not as a single unique and rare event but more like a common event in the Galaxy

The nebular theory

• Stars are born from the collapse of an interstellar cloud of dust and gas.

• Planets form as part of the process of the formation of a star

• As part of the formation of a star, a proto planetary disk forms around the star

• Planets are formed from the collapse of the material of a proto planetary disk

• The basic and simple idea suggested by Kant and Laplace needed a lot of modifications before it became the actual nebular theory for the formation of the solar system and other planetary system

Where did all the dust and gas that formed the nebula originated?

• All the dust and gas comes from “recycled” material in the Galaxy.

• The first original stars in the Galaxy were formed from H and He.

• The rest of the heavier elements were synthesized in the interior of the stars as part of the process of energy generation inside the star (fusion, conversion of lighter elements into heavier elements)

• The most massive stars are able to generate the heaviest elements. They end their life in a spectacular explosion called supernova.

• All that material is thrown into the interstellar space and contaminate the original clouds of H and He gas.

• Heavier elements and lighter elements combine and form molecules. These molecules form aggregates which become dust particles.

• Gas and dust form clouds of “contaminated” material.

• From that material new stars are formed, already containing heavier elements.

• The fusion process inside of these stars and the evolution of these stars continues enriching the material in the clouds of dust and gas.

The Orion nebula An example of an interstellar cloud of gas and dust where new stars are being

born

The cloud of gas that gave birth to our solar system resulted from the recycling of

material through many generations of stars within our galaxy.

A schematic representation of the process of contamination of

interstellar clouds

Nebular Hypothesis • Event such as the impact of a shock wave from the

explosion of a supernova or the passage of a compression wave in the galaxy, (spiral density wave) will triggers the gravitational collapse (collapse due to its own gravity) of the cloud

Radiation pressure

Gravitational force

Radiation pressure

Gravitational force

Centrifugal force

Nebular Hypothesis • The collapse cause an increase in density

• Increased density -> increased gravity -> more material gets sucked in -> center heats up

• Because of conservation of angular momentum, as the nebula collapses, it decreases it radius and it will spin faster

• The density and temperature in the center increases

• Gravitational potential energy is converted into kinetic energy

• As the temperature keeps increase in the center , it will reach the fusion point (It needs to reach about 10 million K in the case of the Sun to convert H into He).

• Once the central body start generating energy, a new star is born

• Outer, cooler particles suffer repeated collisions, building planet-sized bodies from dust grains ( Process called accretion)

• The presence of dust is a key element in the formation of small particles which will stick together and form planetesimals. Gas molecules by themselves will not stick and form planetesimals

• The cloud collapse into a

disk

• This is called a

protoplanetary disk.

• Planets form from the

material in the disk

Nebular Hypothesis

• Accretion clears a gap near the new star

• Most of the gas is accreted into the central star and into forming planets

•Young stellar activity produces high stellar winds, which blows off any remaining gas and leaves an embryonic solar system

Nebular Hypothesis The sequence of the collapse and formation of a planetary system

A better sequence of a collapsing cloud and the formation of planets

A detailed view of the process of accretion of planetesimals and formation of planets

Planetary Compositions The temperature decreases with distance from the Sun and

regulates:

• Which elements actually condense

• Which compounds are formed from the elements

• At what rates the compounds are formed.

Volatile species will only be

stable beyond a point in

which the temperature in

the disk s low enough.

Heavier elements (Like

silicon and its compounds)

can condense at higher

temperatures in the inner

part of the disk

This is why the inner

planets are rock-rich and

the outer planets gas- and

ice-rich

Formation of terrestrial planets

• Terrestrial planets formed from the accretion of smaller bodies called planetesimals

• The process started with small solid particles that condensed from the dust and gas in the nebula. Even with the high temperatures, the dust was able to condense. These particles were too small to “stick” together by gravitational forces. Electrostatics forces may be responsible for them to stick together .

• Once they grew bigger, gravitation was responsible for them to attract more particles and began forming planetesimal.

• Collision were frequent during this stage. Some of the small planets and planetesimals may have been shattered. Only the largest ones may have survived and became planets and grew large to form the terrestrial planets.

The formation of Jovian planets

• The process of accretion also took place at the distance in which the Jovian planets formed.

• But at that distance, condensation of ice was possible due to the lower temperature.

• The planetesimals were formed from condensation of large amounts of ice, and some metals and rocks.

• Planetesimals grew bigger and faster. They grew large enough to attract and retain H and He

• They attracted so much H and He so that the original “seeds” of rock and metal became small compared with the gas.

Formation of Jovian planets

• Beyond the frost line, planetesimals could accumulate ice

• Hydrogen and other low-mass compounds are more abundant (98%) than rock/metal (2%) so Jovian planets got bigger, faster.

+ dust particles

Outside the “Frost line”, there was more efficiently capture (by their bigger gravitational

pull) of H/He gas before it was dispersed by the Sun’s radiation and solar wind

Nebular gas collapsed onto rock-ice cores of perhaps 10 Earth masses

Rocky, icy core

• Each young Jovian planet formed its own “miniature” solar nebula out of the gas around them.

• The satellites of the Jovian planets formed out of these gas and dust disks.

What ended the process of planet formation?

• A large part of the H and He of the original nebula never became part of the solar system.

• Once the Sun formed, it developed a strong solar wind. The solar wind are charged particles, electrons, and protons ejected by the Sun. Stellar winds is common in young stars.

• The strong solar wind blew away into the interstellar space all the material including H and He that were not captured in planetesimals and planets.

• Clearing of the gas in the disk sealed the fate of the planets: they did not have more material to accrete.

• If gas may have remained longer, the composition of the planets, specially the terrestrial planets may be different.

• This may be the case of other planetary systems!

Lost of angular momentum of the Sun Magnetic field slow down the rotation of the Sun

Three kinds of planets . . .

• Nebular material can be divided into “gas” (mainly H/He), “ice” and “rock” (including metals)

• Planets tend to be dominated by one of these three end-members

Gas-rich

Ice-rich

Rock-rich

Ratio

100:1:0.1

Terrestrial (silicate) planets

• Consist mainly of silicates (compounds of silicon oxide) and iron

• Volatile elements (H, He) uncommon in the inner solar system because of the initially hot conditions. (some were supplied by comets)

• Satellites like Ganymede have similar structures but have an ice layer on top (volatiles are more common in the outer nebula)

Mercury

Venus Earth

Moon

Mars

Ganymede

Io

Gas and Ice Giants

• Jupiter and Saturn consist mainly of He/H with a rock-ice core of ~10 Earth masses

• Uranus and Neptune are primarily ices covered with a thick He/H atmosphere

• Their cores grew more slowly and captured less gas

90% H/He

75% H/He

10% H/He

10% H/He

Evidence of planet formation beyond our Solar System

• Early stages of a planetary system formation can be imaged directly

• Dust disks have large surface area and radiate effectively in the infrared

Hubble image of a young solar system. Young star clearing part of the gas Thick disk

A proto planetary disk (proplyd) in the Orion nebula

Proto-planetary Disks

-IR image of a disk around a star -Images in IR obtained at wavelengths from 8.7 to 24.3 micrometers - Dust disk around a young star -The gas has been blown out of the system - The star Beta Pictoris is an example

An artist’s impression of a young star and its proto planetary disk in the process of forming planets

The young Sun gas/dust

nebula

solid planetesimals

How do we explain the existence of our moon?

• Our moon is large compared with the size of

the Earth

•Its composition is not similar to the

composition of the Earth. Its density is lower

and it has less iron.

•It did not form from the same material of the

Earth

•The most accepted theory about the

existence of our moon is the impact of a large

size body (Mars-size) with the Earth.

•The material ejected from the impact may

have accreted and condensed to form the

moon.

• Where did this object came from? Some

proto planets may have been large (Mars-

size) and one of them may have collided with

the Earth

•The composition of the moon is more similar

to the composition of the Earth crust

•Computer simulations support the impact

theory for the formation of the Moon