Module 6: Modelling the Formation

of the Solar System

Activity 1:

The Solar Nebula

Trapezium cluster in the Orion nebula

Summary

In this Activity, we will investigate:

(a) the present-day Solar System;

(b) regions of star formation;

(c) how stars form; and

(d) the Solar Nebula hypothesis of planet formation.

The Solar System is made up of the Sun, nine planets, dozens of satellites, planetary ring systems, and thousands of asteroids and comets.

Introduction

The planets can be broken up into two distinct groups: the inner rocky terrestrials and the outer gaseous Jovians, plus Pluto, which fits into neither category.

In order to understand the composition of the Solar System, we first need to understand how it formed. In this Module we will look at how stars and planets form, noting some of the differences between the two planetary groups, and try to relate these differences to their formation histories and how they have evolved.

At the centre of the Solar System lies our ruler – the majestic Sun.

(a) The Present-day Solar System

The Sun contains about 99.8% of the total mass of the Solar System and its gravity governs the motion of almost all the other members.

While it is extremely important to the planets within the Solar System, on the grander scheme of things the Sun is quite an ordinary star powered by nuclear fusion.

The planets, of course, make up a very important part of the Solar System.

All nine planets orbit around the Sun in the same direction (anti-clockwise viewed from above the Earth’s north pole),

SUN

Earth

Mars Jupiter

SaturnUranus

Neptune

Pluto

and almost all lie in approximately the same plane. The exception is Pluto, which is inclined by 17° to the plane of the Solar System.

The planets make up only 0.18% of the total Solar System mass – and Jupiter makes up 70% of that! So in terms of mass, the planets are a pretty small part of the Solar System.

• the inner rocky terrestrials: Mercury, Venus, Earth, Mars

We can divide the nine planets into two main groups:

Pluto fits in neither of these classes, and, as we’ll see later in this Unit, is more like a Jovian satellite.

• and the outer gas giant, the Jovians: Jupiter, Saturn, Uranus, Neptune

Just as the Earth has its own Moon, some of the other planets also have natural satellites.

Earth-Moon system

In fact, the Jovians all have a large family of moons.

Saturn’s satellites

And Saturn is not the only member of the Solar System to host planetary rings: again, all the the Jovians have ring systems.

Jupiter’s ring system

Halley’s cometAsteroid Ida (and its satellite Dactyl)

As well as planets and their accompanying rings and satellites, other members of the Solar System include tens of thousands of rocky asteroids and icy comets.

Beyond the planets, however, astronomers are beginning to detect members of a population of ice and rock conglomerates in a region called the Kuiper Belt, which is believed to supply most of the short-period comets like Halley’s comet.

Extending even further out into space, astronomers suspect that a spherical cloud, the Oort cloud, contains billions of “dirty snowballs” which are the source of the long-period comets that take millions of years to travel once round their orbit.

Our traditional view of the Solar System usually stopsa little past the orbit of Neptune and Pluto.

In order to understand all the objects and motions within the Solar System, we should try to understand how the Solar System was formed.

Since planets form around stars, to understand how planets form we first need to briefly look at how stars form. Let’s take a quick tour of some star forming regions.

(b) Regions of Star Formation

NGC253Whirlpool galaxy

Spiral galaxies contain about 100 billion stars, as well as enormous clouds of gas and dust. It is within these clouds - called giant molecular clouds - that stars are born.

Stars form in dense clouds of gas and dust that are found in the arms of our own Milky Way galaxy and other spiral galaxies. M100

Giant Molecular Clouds

* where 1 light year = 9.46 x 1015m. To learn more about astronomical distances, click here.

Because these clouds are so cold, they are good hosts for molecules (as the cloud thermal energy is not high enough to break molecular bonds) and thus can contain more than 60 different kinds of molecules – hence their name.

Giant molecular clouds are dusty gas clouds that are held together by gravity, with masses of 100,000 to 1,000,000 times that of the Sun, diameters of 50 to 300 light years* and temperatures of about 10 K*, which is very cold! Ophiuchus & Orion clouds in infrared

* where 273 K = 0°C, so 10K = -263°C. To learn more about the Kelvin temperature scale, click here.

Stellar NurseriesThe Hubble Space Telescope allows us to probe these star forming regions in great detail.

Three of the richest star forming regions in our Galaxy are the Eagle Nebula, which is 7000 LY* away, the Orion nebula, 1500 LY* away, and the Taurus-Auriga cloud at a distance of just 450 LY*. Each of these nebulae* contain hundreds of protostars.

* 1 LY = 1 light year = 9.46 x 1015m

* Nebulae are just clouds of gas and dust.

Stellar Cocoons

Anglo-Australian Observatory optical image of the Eagle

nebula, or M16, 7000 LY away. HST images of the Eagle nebula pillars, one light year in length.

Stars are thought to be forming

inside these dusty

cocoons.

The Orion Nebula

One of the richest stellar nurseries in our part of the Milky Way, extensively studied by the Hubble Space Telescope, is the Orion Nebula.

This image is a mosaic of 45 images constructed from 15 separate fields in the centre of the Orion nebula. It is located in the middle of the “sword” region of the constellation of Orion, an area seen by southern hemisphere observers as the “handle of a saucepan”.

A detailed study of the Orion Nebula has revealed over 150 glowing proplyds - or protoplanetary disks -around young stars.

The lumps in this part of the Orion nebula are called proplyds: they are cocoons of leftover gas

and dust surrounding baby stars.

Proplyds

Proplyds are thought to be embryonic Solar Systems - the sites of planet formation.

Each of these images is about 30 times the size of the Solar System and the proplyds are 2–8 times the size of the Solar System.

The red glow in the centre of each disk is a young, newlyformed star, roughly one million years old.

Click here to see a brief movie showing how these images fit together.

Solar Systems in the Making

This proplyd, Orion’s largest and about 17 times Solar System’s diameter, is a good example of a protostellar disk seen silhouetted against the back-lit nebula. The second image - taken through a different filter - clearly shows the hidden protostar.

In this false colour mosaic we see the centre of the Trapezium cluster of the Orion nebula, which contains four massive energetic stars as well as a number of evaporating proplyds (seen as small white “blobs”).

Solar Systems not in the Making...

This is a “false colour image”, with the colours chosen to bring out the details - the disk is actually quite dark.

This close-up image shows the destruction of a disk surrounding a young star in the act of forming. If the disk had been left alone it would be a strong candidate for producing planets.

Let’s now return to our giant molecular clouds and learn

how stars form.

(c) How Stars FormGiant molecular clouds are held up by gas pressure, rotation and magnetic fields. If the clouds become massive enough, they can collapse due to gravity. First let’s just consider pressure and gravity:

gravity

pressure

gravity

pressure

Pressure “wins”,cloud expands

Pressure “wins”,cloud expands

Gravity “wins”,cloud contractsGravity “wins”,cloud contracts

High-mass cloudHigh-mass cloudLow-mass cloudLow-mass cloud

There are actually three factors fighting against gravity:

Magnetic forces from moving charges act

against collapse

Magnetic forces from moving charges act

against collapse

A rapidly rotating cloud tends to

spread out

A rapidly rotating cloud tends to

spread out

gravity

motion

The particles are in motion (causing heat

and pressure)

The particles are in motion (causing heat

and pressure)Gravity works to

collapse the cloud, but ...

Gravity works to collapse the cloud,

but ...

magnetism

rotation

• thermal motions and gas pressure mean that the gas is in motion;• magnetic fields act upon the charged particles in the cloud, making them follow magnetic field lines rather than obey gravity; and• rotation gives an outward centrifugal force that keeps the cloud from collapsing.

Eventually gravity wins... Giant molecular clouds are extremely cold – only a few degrees warmer than the near-zero of space – and rotate very slowly. So at some stage gravity actually wins and the giant molecular cloud begins to collapse under its own gravity.

The giant molecular cloud fragments, forming much smaller dense cloud cores within it.

It is within these cloud cores that stars actually form.

Triggered star formation Most star formation is probably triggered by an event that starts the gravitational collapse.

NGC 604, a huge, star-forming nebula in the galaxy M33

Shock waves travelling through a nebula would cause it to bunch up in places, sometimes enough for gravity to be able to do its work and start the collapse process.

Such shock waves can be caused by the outflows of nearby young stars or the supernova death of nearby old stars.

The collapse of cloud cores

Both momentum and energy have to be conserved during the collapse process.

• To conserve angular momentum, the smaller the cloud core gets, the faster it (or parts of it) will spin.

Large cloud core:• high potential energy• low kinetic energy• low spin speed

Large cloud core:• high potential energy• low kinetic energy• low spin speed

Small cloud core:• lower potential energy• higher kinetic energy• higher spin speed

Small cloud core:• lower potential energy• higher kinetic energy• higher spin speed

Once the collapse process has begun, the cloud cores continue to attract surrounding gas and dust from the nebula, making them more massive, and continuing the gravitational collapse.

• To conserve energy, the smaller the cloud core gets, the hotter it gets (potential energy is converted into kinetic energy, which heats the gas).

Click here to find out more about angular momentum

The collapse of cloud cores

So as the cloud cores collapse, they become more centrally condensed and the central region heats up. They also spin faster as they get smaller - particularly in the central regions.

This rapid rotation creates large centrifugal forces, just as youfeel in a car when you go too fast around a corner - large sideways forces push you away from the corner.These centrifugal forces are greatest in the central regions resulting in the cloud core spreading out to form a disk:

*

* Follow this link to find about more about centrifugal forces.

Inside a cocoon of dust

Start with a vast,rotating, contracting

cloud of gas,dust & molecules

Nebula contracts to form a

protostellar disk

Young protostar isforming in centre

Disk gets cooler as you go further out

If we could see inside one of these dense cloud cores, we would see a rotating disk - or proplyd - with a protostar in the centre.

Our Solar System

This is why all the members of our Solar System – the planets, asteroids and comets – are all orbiting in the same direction.

It is the direction in which the original cloud of gas and dust that formed our Solar System was spinning.

A star is bornIn the centre of the collapsing cloud core, a protostar is forming.

61H+ 4He++ + 2e+ + 2 + 2 + 21H+

6 hydrogen atoms fuse to become one helium nucleus,two positrons, two neutrinos, two gamma rays

and two spare hydrogen atoms to keep the fusion going

61H+ 4He++ + 2e+ + 2 + 2 + 21H+

6 hydrogen atoms fuse to become one helium nucleus,two positrons, two neutrinos, two gamma rays

and two spare hydrogen atoms to keep the fusion going

…and once fusion reactions start, you have a baby star!

As gravity pulls more and more gas to the centre, the temperature, density and pressure of the core get so large that hydrogen atoms actually fuse together...

We’ll learn more about star formation in the Unit Exploring Stars and the Milky Way and in more detail in the Unit Stellar Astrophysics.

Astronomers have long theorised that the Sun and planets were formed from the Solar Nebula - a vast rotating disk-shaped cloud of gas and dust.

Hubble Space Telescope images of proplyds provide other examples to study in order to confirm, refine and, where necessary, revise our theories of planet formation.

(d) The Solar Nebula Hypothesis

The fact that all the planets in the Solar System orbit around the Sun in the same direction and in nearly the same plane strongly constrains any theory of the Solar System’s formation.

The Solar Nebula hypothesis was originally put forward by German philosopher Immanual Kant and French scientist Pierre-Simon de Laplace in the late-1700s. They suggested that the entire Solar System was formed from a huge rotating cloud of gas and dust called the Solar Nebula.

As the rotating cloud collapsed, the proto-Sun formed in the

centre, surrounded by a disk of material out of which the

planets formed about 4.5 billion years ago.

The formation of the solar nebula disk from the collapsing cloud core took about 100,000 years. During the collapse, the density and temperature of the nebula increased enormously. The temperatures were high enough to vaporise the primordial dust of the cloud core, thereby erasing any prior history of the nebula dust.

The resulting protostellar disk was made up of predominantly hydrogen gas, with traces of other elements. Over time the disk radiated its energy and began to cool.

In the conditions believed to exist in the solar nebula, whether a substance existed as a solid or as a gas depended on the local temperature.

As the solar nebula radiated its energy and gradually cooled, different elements and molecules started to condense out of the nebula, forming solid dust grains.

Different substances have different condensation temperatures.

Temperaturedecreases from

the centre

Condensation

The interior of the nebula is hotter than the outer regions, so materials stable at relatively high temperatures tended to dominate in the hot inner Solar System,

whereas more volatile (easily evaporated) compounds tended to dominate in the cooler outer Solar System.

Temperaturedecreases from

the centre

So metals and rock minerals could exist as solids near the Sun, whereas volatile compounds (like methane, ammonia and water ices) were stable further out.

Ura

nu

s

Ne

ptu

ne

Plu

to

Approximate temperatures in the early Solar System

Temperature in Kelvin (K), where 0°C = 273K

Distance in AU, where 1AU = Sun-Earth distance = 1.5x1011m

Tem

per

atu

re (

K)

10

500

1000

Distance from Sun (AU)0.1 1.0 10.0

200

100

Me

rcu

ry

Ve

nu

s

Ea

rth

Ma

rs

Ju

pit

er

Sa

turn

Examples of compounds and distances beyond which it wascool enough in the early Solar

System for them to condense out:

Iron oxide

water

ammonia &methane

Tem

per

atu

re (

K)

10

500

1000

Distance from Sun (AU)0.1 1.0 10.0

200

100metal oxides,

silicates

Once solids began to condense out of the solar nebula, this effectively reset their “internal clocks”. Isotope dating of meteorites tells us that the metals began to condense as soon as the disk formed, about 4.55 – 4.56 billion years ago.

The rocks (mostly silicates) condensed out a little later, about 4.4 – 4.5 billion years ago, once the disk began to cool.

So now the solar nebula is full of tiny dust grains, with metal oxides, iron & nickel close to the Sun, silicate compounds out a little further, and ices (water, ammonia and methane) dominating the outer Solar System.

Planetesimal formation

Slowly they grew in size until they formed large rocky and icy bodies called planetesimals, which were the size of boulders and small asteroids.

The sticking mechanism that turned micron sized grains into metre sized rocks is not well understood…

These tiny newly formed grains began to stick together electrostatically through low speed collisions.

10 micron interplanetary

dust grain

As the planetesimals formed, regions of slightly higher density would have accumulated more of the surrounding material by gravitational attraction.

Planetesimals began to condense out:

- roughly 4.5 billion years ago

Small clumps of planetesimals would have formed gradually.

Once the planetesimals are metre sized, runaway growth occurred. The planetesimals with sizeable masses and therefore appreciable gravity quickly became larger, accumulating all solids in their orbital path, becoming protoplanets of several hundred kilometres.

Protoplanet formation

The resulting size of a protoplanet depended on its position in the solar nebula, since location determined the local density and composition.

In the inner Solar System, protoplanets were the size of asteroids and small moons, made up of metals and rocky materials. In the outer Solar System, protoplanets grew much larger, between one and 15 Earth masses.

Protoplanet size

The large size jump of protoplanets at the Mars-Jupiter boundary was due to the availability of materials. Since the solar nebula contained a much higher proportion of volatiles than metals and silicates, this meant that there was much more material available in the outer Solar System to go into forming planets, resulting in much larger protoplanets.

The formation timescale of protoplanets was a few 100,000 to several million years.

After about a million years, the solar wind would have begun to blow, clearing the nebula of any remaining gas.

The Solar Wind

This puts strong time constraints on the formation of the giant planets, since their huge atmospheres are made up of gas taken directly from the solar nebula. Therefore the cores of the giants must have formed within a million years.

The protoplanets continued to grow slowly via collisions, clearing up the remaining solids in the disk. After about 10 to 100 million years, the Solar System was made, with 9 newly formed planets in stable orbits, as well as some remaining debris - the asteroids and comets.

Once the solar wind had blown away the remaining gas of the solar nebula, all that remained were the protoplanets and planetesimals. Such a disk of solid material is called a protoplanetary disk.

Protoplanetary Disk

In this Activity, we have looked at star and planet formation in a contracting rotating cloud of gas and dust. We have discussed how due to the rotation, the cloud collapses into a disk out of which planets form. This theory of planet formation explains the overall shape and motions within the Solar System.As the disk cooled, rocky materials condensed out in the inner Solar System and volatiles in the outer Solar System, producing inner rocky protoplanets and giant outer icy protoplanets.In the next Activity we will look at how these newly formed protoplanets evolved into fully fledged planets.

Summary

Image CreditsTrapezium cluster in Orion nebula Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASAhttp://antwrp.gsfc.nasa.gov/apod/ap971118.html

Montage of the nine planets - JPLhttp://photojournal.jpl.nasa.gov/catalog/PIA01341.jpg

Image of the Earth and Moon from Galileo - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_earth_moon.jpg

Montage of Saturn and some of its satellites - JPLhttp://photojournal.jpl.nasa.gov/catalog/PIA01482.jpg

Jupiter and its rings in infrared - NASAhttp://antwrp.gsfc.nasa.gov/apod/ap970205.html

Ida and Dactyl - NASAhttp://nssdc.gsfc.nasa.gov/image/planetary/asteroid/idasmoon.jpg

Comet Halley © David Malin, AAO, used with permissionhttp://www.aao.gov.au/local/www/dfm/image/uks019.gif

Image CreditsSpiral galaxy M100 © David Malin, AAO, used with kind permissionhttp://www.aao.gov.au/images.html/captions/aat058.html

Central region of spiral galaxy NGC253Credit: Hubble Heritage Team (AURA/STScI/NASA)http://oposite.stsci.edu/pubinfo/pr/1998/42/

Central region of Whirlpool Galaxy (M51)Credit: Nino Panagia (STScI and ESA) and NASAhttp://oposite.stsci.edu/pubinfo/pr/96/17.html

Orion nebula mosaic, O’Dell & Wong (Rise U.) and NASAhttp://antwrp.gsfc.nasa.gov/apod/ap951121.html

Eagle nebula in M16 © David Malin, AAO, used with kind permissionhttp://www.aao.gov.au/images.html/captions/aat047.html

Gas pillars in M16, Eagle nebulaCredit: Jeff Hester & Paul Scowen (Arizona State University), and NASAhttp://www.stsci.edu/pubinfo/pr/95/44.html

Image CreditsCloseup of “Proplyds” in Orion Nebula - Robert O’Dell (Rice U.) & NASA http://oposite.stsci.edu/pubinfo/gif/OrionProplyds.gif

Four proplyds in OrionMark McCaughrean (MPIA), Robert O’Dell (Rice U.) & NASAhttp://oposite.stsci.edu/pubinfo/gif/OriProp4.gif

Edge on protoplanetary disks in Orion nebula Credit: Mark McCaughrean (MPIA), Robert O'Dell (Rice U.) & NASAhttp://oposite.stsci.edu/pubinfo/jpeg/OriEODsk.jpg

Closeup of “Proplyds” in Orion Nebula Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASAhttp://www.cita.utoronto.ca/~johnston/orion.html#figures

Trapezium cluster of Orion Nebula Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASAhttp://www.cita.utoronto.ca/~johnston/orion.html#figures

NGC 604, courtesy of Hui Yang (University of Illinois) and NASAhttp://oposite.stsci.edu/pubinfo/gif/NGC604.gif

Image CreditsInterplanetary dust grainhttp://stardust.jpl.nasa.gov/science/sd-particle.html#idp-s

Protoplanet disk (artists conception) - Credit: Pat Rawlings (JPL)http://eis.jpl.nasa.gov/origins/poster/protodisk.html

Solar flare - Credit: SOHO - EIT Consortium, ESA, NASA http://antwrp.gsfc.nasa.gov/apod/ap970918.html

Dusty disk of Beta Pictoris - Credit: C. Burrows & J. Krist (STScI) & NASAhttp://oposite.stsci.edu/pubinfo/jpeg/BetaPicB.jpg

Now return to the Module home page, and read more about modelling the formation of the Solar System in the Textbook Readings.

Hit the Esc key (escape) to return to the Module 6 Home Page

Astronomers define a convenient unit of length:The AU (astronomical unit) = average distance between Sun and Earth = 1.496 x 1011 m

Distances in the Solar System are very large! To compare the average distances between the Sun and each of the planets, it is convenient to do it in terms ofthe average Earth - Sun separation.

Astronomical Distances

1 AU

The lightyear (LY) is the distance that light will travel in a year, where: 1 LY = 9.461 x 1015 m = 63,240 AU

Another astronomical unit of measure is the lightyear, which comes from the knowledge that light takes a finite length of time to travel through space.

An event happens here ...

An event happens here ...

1 ly (distance)1 ly (distance)

… and is seen here a year later

… and is seen here a year later

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The Kelvin temperature scale

The Kelvin temperature scale is the same as the Celsius scale, except that the definition of zero is different.

The Celsius scale specifies 0 degrees as the temperature at which water freezes (0°C).

On the other hand, the Kelvin scale specifies 0 degrees as the temperature of an object in which the kinetic energy of the particles making up the object is at a minimum. This is called absolute zero (0°K).

Therefore,

273.15 degrees Kelvin is the freezing point of water and 373.15 degrees Kelvin is the boiling point of water.

The Celsius scale is 273.15 degrees “out of sync”:

Kelvin 0 100 200 300 400 500 600

Celsius -273 -173 -73 27 127 227 327

Melting point of

ice

Boiling point of water

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Angular momentum

If something is spinning, it has angular momentum - which is a conserved quantity.

For each part of an object, the angular momentum

AM = mvr

where m = mass v = speed r = distance from centre

For each part of an object, the angular momentum

AM = mvr

where m = mass v = speed r = distance from centre

radius

r Speed v

m

The angular momentum of an object depends on how the mass is arranged around the axis it is spinning, and the speed at which it spins.

The classic ice skater concept

If an iceskater is spinning with his arms and a leg out, he will spin slowly.

But if he pulls these limbs closer to his body, he will spin faster.

Angular momentum is conserved

This is because the total sum, for each part of his body, of the angular momentum

mass x speed x distance from axis

must stay the same.

If the distances of his hands, arms and legs from the axis get smaller, the speeds must get bigger to compensate!

Distance large,speed small

Distance large,speed small

Distance small,speed large

Distance small,speed large

axis

Angular momentum in space

If a molecular cloud, for instance, contracts under gravity, it will spin faster.

Distance large,speed small

Distance large,speed small

Distance small,speed large

Distance small,speed large

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As seen from above

As seen from the merry-go-round

Centrifugal ForcesCentrifugal force is not a “real” force in the same way that body forces such as gravity are - it is “fictitious forces” that is only felt in an accelerating reference frame.

For an observer in a rotating (and therefore accelerating) reference frame, such as a child sitting on a merry-go-round or a passenger sitting in a cornering car, the observer feels an outward fictitious force. In fact, this sensation just comes from their body wanting to continue in a straight line.

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