Download The Solar Nebula

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.
Introduction
The Solar System is made up of the Sun, nine planets,
dozens of satellites, planetary ring systems, and thousands
of asteroids and comets.
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 need to understand how it formed. In this
Module we will look at how stars and planets form, and in
particular note some of the differences between these two
planetary groups and try to relate these differences to their
formation histories and how they have evolved.
(a) The Present-day Solar System
At the centre of the Solar
System lies our ruler
– the majestic Sun.
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),
Pluto
SUN
Earth
Neptune
Mars
Jupiter
Saturn
Uranus
and all lie in approximately the
same plane, with the exception of
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.
We can divide the nine planets into two main groups:
• the inner rocky terrestrials:
Mercury, Venus, Earth, Mars
• and the outer gas giant, the Jovians:
Jupiter, Saturn, Uranus, Neptune
Pluto fits in neither of these classes,
and, as we’ll see later in this Unit, is
more like a Jovian satellite.
Just as we are familiar with our own Moon, some
of the other planets also have natural satellites.
In fact, the Jovians all have
Earth-Moon system
a large family of moons.
And Saturn is not the only member of the
Solar System to host planetary rings:
again, all the the Jovians
Saturn’s satellites
have ring systems.
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.
Asteroid Ida (and its satellite Dactyl)
Jupiter’s ring system
Halley’s comet
Our traditional view of the Solar System usually stops
a little past the orbit of Neptune and Pluto.
Past 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 called the Oort cloud contains billions
of “dirty snowballs” which are the source of long-period comets
that take millions of years to travel once round their orbit.
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
forms. Let’s take a quick tour of some star forming
regions.
(b) Regions of Star Formation
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.
NGC253
M100
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.
Whirlpool galaxy
Giant Molecular Clouds
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
Because these clouds are so cold, they are good hosts for
molecules and can contain more than 60 different kinds of
molecules – and hence their name.
* where 1 light years = 9.46 x 1015m. To learn
more about astronomical distances, click here.
* where 273 K = 0°C, so 10K = -263°C. To learn
more about the Kelvin temperature scale, click here.
Stellar Nurseries
The Hubble Space Telescope
now 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 TaurusAuriga cloud at a distance of just
450 LY*.
Each of these nebulae* contain
hundreds of protostars.
* Nebulae are just clouds of gas and dust.
* 1 LY = 1 light year = 9.46 x 1015m
Stellar Cocoons
Stars are thought
to be forming
inside these
dusty
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.
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 taken of15
separate fields in the centre
of the Orion nebula, located
in the middle of the “sword”
region of the constellation of
Orion - seen by southern
hemisphere observers as
the “handle of a saucepan”.
Proplyds
A detailed study of the Orion Nebula has revealed over
150 glowing proplyds - or protoplanetary disks around young stars.
Proplyds are thought
to be embryonic
Solar Systems the sites of planet
formation.
The lumps in this part of the Orion nebula are
called proplyds: they are cocoons of leftover gas
and dust surrounding baby stars.
Each of these images is about 30 times the size of the
Solar System and the proplyd are 2–8 times the size of
the Solar System.
The red glow in the
centre of each disk is
a young, newly
formed 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 the best 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.
Solar Systems not in the Making...
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”).
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.
This is a “false colour
image”, with the
colours chosen to
bring out the details the disk is actually
quite dark.
Let’s now return to our giant
molecular clouds and learn
how stars form.
(c) How Stars Form
Giant 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:
Low-mass cloud
High-mass cloud
gravity
gravity
pressure
pressure
Pressure “wins”,
cloud expands
Gravity “wins”,
cloud contracts
There are actually three factors fighting against gravity:
• 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 field lines rather than obey gravity; and
• rotation gives an outward centrifugal force that keeps the cloud
from collapsing.
The particles are in
motion (causing heat
and pressure)
Gravity works to
collapse the cloud,
but ...
magnetism
gravity
Magnetic forces from
moving charges act
against collapse
A rapidly rotating
cloud tends to
spread out
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.
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
NGC 604, a huge, star-forming
stars.
nebula in the galaxy M33
The collapse of cloud cores
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. Large cloud core:
• high potential energy
Both momentum and energy have
• low kinetic energy
to be conserved during the collapse • low spin speed
process.
• To conserve energy, the smaller
the cloud core gets, the hotter it gets
(potential energy is converted into
kinetic energy, which heats the gas).
• To conserve angular momentum,
the smaller the cloud core gets, the
faster it (or parts of it) will spin.
Click here to find out more about
angular momentum
Small cloud core:
• lower potential energy
• higher kinetic energy
• higher spin speed
The collapse of cloud cores
So as the cloud cores collapse, they become more centrally
condensed and the central region heats up. It also spins faster
as it gets smaller - particularly in the central regions.
This rapid rotation creates large centrifugal forces,* just as you
feel 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 at the equator and the core
starts to spread out and form a disk:
* Follow this link to find about more about centrifugal forces.
Inside a cocoon of dust
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.
Young protostar is
forming in centre
Start with a vast,
rotating, contracting
cloud of gas,
dust & molecules
Nebula contracts
to form a
protostellar disk
Disk gets cooler as
you go further out
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’s the direction that the original cloud of gas and dust
that formed our Solar System was spinning.
A star is born
In the centre of the collapsing cloud
core, a protostar is forming.
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...
…and once fusion reactions
start, you have a baby star!
We’ll learn more about star
formation in the Unit Exploring
Stars and
61H+  4He++ + 2e+ + 2 + 2 + 21H+
the Milky Way.
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
(d) The Solar Nebula Hypothesis
Astronomers have long theorised that the Sun and
planets were formed from the Solar Nebula - a vast
swirling disk-shaped cloud of gas and dust.
Hubble Space Telescope
images of proplyds provide
concrete examples to study
in order to confirm, refine
and, where necessary,
revise our theories of planet
formation.
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.
Condensation
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.
Temperature
decreases from
the centre
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.
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,
Temperature
decreases from
the centre
whereas more volatile (easily evaporated) compounds
tended to dominate in the cooler outer Solar System.
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.
Approximate temperatures
in the early Solar System
500
200
Pluto
Neptune
Uranus
Saturn
Mercury
Temperature
in Kelvin (K),
where
0°C = 273K
Jupiter
100
Venus
Earth
Mars
Temperature (K)
1000
10
0.1
1.0
10.0
Distance from Sun (AU)
Distance in AU, where
1AU = Sun-Earth distance
= 1.5x1011m
Examples of compounds and
distances beyond which it was
cool enough in the early Solar
System for them to
condense out:
Temperature (K)
1000
500
200
metal oxides,
silicates
Iron oxide
100
water
ammonia &
methane
10
0.1
1.0
10.0
Distance from Sun (AU)
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.
Planetesimal formation
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.
These tiny newly formed
grains began to stick
together electrostatically
through low speed collision.
10 micron
interplanetary
dust grain
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…
Small clumps of planetesimals would have formed
gradually.
Planetesimals began to condense out:
- roughly 4.5 billion years ago
As the planetesimals formed, regions of slightly higher
density would have accumulated more of the
surrounding material by gravitational attraction.
Protoplanet formation
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.
The resulting size of a
protoplanet depended
on its position in the
solar nebula, since
location determined
the local density and
composition.
Protoplanet size
 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.
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.
The Solar Wind
After about a million years,
the solar wind would have
begun to blow, clearing the
nebula of any remaining
gas.
This puts some strong time constraints on the formation of
the giant planets, since their huge atmospheres are made
up of gas from the solar nebula. Therefore the cores of the
giants must have formed within a million years.
Protoplanetary Disk
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.
The protoplanets continued to grow slowly via collisions,
clearing up the remaining solids in the disk. After about 10
to100 million years, the Solar System was made, with 9 newly
formed planets in stable orbits, as well some remaining debris
- the asteroids and comets.
Summary
In this Activity, we have looked at star and planet formation
in a contracting rotating cloud of gas and dust. Due to the
rotation, the cloud collapsed in a disk out of which the
planets formed. 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’ll look at how these newly formed
protoplanets evolved into fully fledged planets.
Image Credits
Trapezium cluster in Orion nebula
Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASA
http://antwrp.gsfc.nasa.gov/apod/ap971118.html
Montage of the nine planets - JPL
http://photojournal.jpl.nasa.gov/catalog/PIA01341.jpg
Image of the Earth and Moon from Galileo - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_earth_moon.jpg
Montage of Saturn and some of its satellites - JPL
http://photojournal.jpl.nasa.gov/catalog/PIA01482.jpg
Jupiter and its rings in infrared - NASA
http://antwrp.gsfc.nasa.gov/apod/ap970205.html
Ida and Dactyl - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/asteroid/idasmoon.jpg
Comet Halley © David Malin, AAO, used with permission
http://www.aao.gov.au/local/www/dfm/image/uks019.gif
Image Credits
Spiral galaxy M100 © David Malin, AAO, used with kind permission
http://www.aao.gov.au/images.html/captions/aat058.html
Central region of spiral galaxy NGC253
Credit: 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 NASA
http://oposite.stsci.edu/pubinfo/pr/96/17.html
Orion nebula mosaic, O’Dell & Wong (Rise U.) and NASA
http://antwrp.gsfc.nasa.gov/apod/ap951121.html
Eagle nebula in M16 © David Malin, AAO, used with kind permission
http://www.aao.gov.au/images.html/captions/aat047.html
Gas pillars in M16, Eagle nebula
Credit: Jeff Hester & Paul Scowen (Arizona State University), and NASA
http://www.stsci.edu/pubinfo/pr/95/44.html
Image Credits
Closeup of “Proplyds” in Orion Nebula - Robert O’Dell (Rice U.) & NASA
http://oposite.stsci.edu/pubinfo/gif/OrionProplyds.gif
Four proplyds in Orion
Mark McCaughrean (MPIA), Robert O’Dell (Rice U.) & NASA
http://oposite.stsci.edu/pubinfo/gif/OriProp4.gif
Edge on protoplanetary disks in Orion nebula
Credit: Mark McCaughrean (MPIA), Robert O'Dell (Rice U.) & NASA
http://oposite.stsci.edu/pubinfo/jpeg/OriEODsk.jpg
Closeup of “Proplyds” in Orion Nebula
Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASA
http://www.cita.utoronto.ca/~johnston/orion.html#figures
Trapezium cluster of Orion Nebula
Credit: John Bally, Dave Devine & Ralph Sutherland, HST and NASA
http://www.cita.utoronto.ca/~johnston/orion.html#figures
NGC 604, courtesy of Hui Yang (University of Illinois) and NASA
http://oposite.stsci.edu/pubinfo/gif/NGC604.gif
Image Credits
Interplanetary dust grain
http://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) & NASA
http://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
Astronomical Distances
Distances in the Solar System are very large!
To compare the average distances between the Sun and
each of the planets, it’s convenient to do it in terms of
the average Earth - Sun separation.
Astronomers define a convenient unit of length:
The AU (astronomical unit)
= average distance between Sun and Earth
= 1.496 x 1011 m
1 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.
The lightyear (LY) is the distance that light will travel
in a year, where:
1 LY = 9.461 x 1015 m = 63,240 AU
1 ly (distance)
… and is
seen here a
year later
An event
happens
here ...
Click here to return to Activity
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
Celsius -273
100
200
300
400
500
600
-173
-73
27
127
227
327
Melting
point of
ice
Boiling
point of
water
Click here to return to Activity
Angular momentum
If something is spinning, it has angular momentum which is a conserved quantity.
The angular momentum of an object depends on how
the mass is arranged about the axis it is spinning
around, and the speed at which it spins.
For each part of an object,
the angular momentum
radius
r
m
Speed
v
AM = mvr
where
m = mass
v = speed
r = distance from centre
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
Distance large,
speed small
axis
Distance small,
speed large
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!
Angular momentum in space
Distance large,
speed small
Distance small,
speed large
If a molecular cloud, for instance, contracts under
gravity, it will spin faster.
Click here to return to Activity
Centrifugal Forces
Centifugal 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.
As seen from the
merry-go-round
As seen from
above
Click here to return to Activity