ASTR 330: The Solar System
Lecture 5 Review Quiz
1. Distinguish between remote sensing and in situ sensing, and
give examples.
2. What is meant by an atmospheric spectral window?
3. What information can we tell about a planet from infrared
spectral lines?
4. What are the two main types of telescopes, and name some
recent advances in telescope technology.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Announcements
• Yellow forms still required for students: Gilkey, Snyder &
Talebi!
• E-mail: anyone not getting e-mail on the class list?
• Homework #1 returned.
 Standard was high: mean=42.5.
 Question 4 problems.
• Materials on-line: HW #1 solutions. Lectures 6 & 7.
• Calculators.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 5:
Formation of the
Planetary System
Picture from emuseum@Minnisota State Univ, Mankato
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Questions For This Class
1. From what did the solar system form?
2. How did it form?
3. Why are the objects in the solar system all so different?
4. Could it have formed differently, e.g. with a binary star?
5. How long did it take for the planets to accrete most of their mass?
6. How have the planets evolved since the end of their major accretion?
7. Is this evolution continuing today?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Data and Evidence
• In trying to answer these questions we have limited evidence at our
disposal today.
• In our own solar system, we have only the end-point of a complex
evolutionary process to go on: the observed sizes, orbits,
compositions etc of the planets and other bodies.
• It is somewhat like trying to deduce the childhood and experiences of
a human, having only a picture of the adult.
• As astronomical technology has progressed, in the last decade we
have been able to now view the beginnings of solar systems around
other stars: a valuable insight into our own history.
• However, many aspects of solar system formation at this stage are
still not certain.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Facts We Can Use:
Chemical Composition
1. More than 99% of the material in the solar system is in the Sun.
2. The Sun is composed almost entirely of Hydrogen and Helium.
 Hence the initial raw material must have had close to this
composition.
3. Jupiter and Saturn have almost the same composition as the Sun.
4. The smaller bodies are depleted in H, He and other light gases.
 Hence the inner planets were probably formed without ices or
other volatiles.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Facts We Can Use:
Orbits and Rotations
1. All the planets have orbits which are approximately circular.
2. These orbits all lie roughly in the same plane.
3. The planets all rotate in the same direction around the Sun.
4. The Sun rotates in the same direction as the planets orbit.
5. The Sun’s equator lies essentially in the same plane of the planet’s
orbits.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Early Theories
• The first serious speculations about the formation of the solar and
planetary system using the laws of gravity and physics were due to
Pierre-Simon, Marquis de Laplace (1796).
• Laplace envisioned a vast
rotating interstellar gas ‘cloud’
which collapsed under its own
gravity, to form a disk.
• These ideas were improved on
by Roche (1854) and are still
valid today, though with many
changes in the details!
Picture credit:Univ. St. Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Overview Of
Formation
Now let’s look at the
individual stages in
more detail…
Figure: Universities Corp. For Atmospheric Research (UCAR)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
In the beginning…
• … was a huge cloud of
molecular material, known as the
proto-solar or primordial nebula,
similar to the Orion Nebula (right).
•
This nebula may have only
contained only 10-20% more
mass than the present solar
system.
•
Due to some disturbance,
perhaps a nearby supernova, the
gas was perturbed, causing
ripples of increased density.
•
The denser material began to
collapse under its own gravity…
Picture from stardate.org
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Initial Collapse
• The nebula must have possessed some rotation. Due to the spin, the
cloud collapsed faster along the ‘poles’ than the equator.
• The result is that the cloud collapsed into a spinning disk.
• The disk material cannot easily
fall all the remaining way into
the center because of its
rotational motion, unless it can
somehow lose some energy,
e.g. by friction in the disk
(collisions).
• The initial collapse takes just a
few 100,000s of years.
Picture credit: AnimAlu Productions
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rotation and Angular Momentum
•
Angular momentum is a conserved quantity: in the absence of
dissipation the total angular momentum of the cloud stays the same.
• Angular momentum is the product of three quantities: mass, size
(radius) and rotation speed (or velocity):
L = mrv
•
If L is constant, then clearly if any one of the other quantities
decreases, another quantity must increase proportionately.
•
I.e., if the cloud collapses and becomes smaller (r decreases) and
the mass stays the same, then the rotational speed (v) increases: the
cloud ‘spins up’.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rotational Spin-Up
• The spin-up of a shrinking
object can be demonstrated
by a familiar example:
• An ice skater performing a
spin (Michelle Kwan, right)
draws in her arms to spin
faster without expending
any extra effort.
• Now let’s look at some
actual protoplanetary
disks…
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Actual Protoplanetary Disks
• The images (left) show four
protoplanetary disks in the
Orion Nebula, 1500 light
years away, imaged by the
Hubble Space Telescope
(HST).
• The disks are 99% gas and
1% dust. The dust shows as
a dark silhouette against the
glowing gas of the nebula.
• Each frame is 270 billion km
across: about 1800 AU. The
central stars are about 1
million years old – infants!
These image are visible composites from red, green and blue light.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Disk Composition
• The central parts of the nebula were very hot: over 10,000 K.
• Going outwards in the nebula, the temperature drops, and different
compounds condense out at different distances from the protostar:
•
•
•
•
•
•
•
•
Calcium, Aluminum oxides first,
then Iron-Nickel alloys (by 0.2 AU, Mercury),
Magnesium silicates and oxides next (by 1.0 AU),
Olivine and Pyroxene (Fe-Si-O compounds),
Feldspars (K-Fe-Si-O compounds),
Hydrous silicates,
Sulfates,
And finally ices (water ice by 5.0 AU).
• This radial variation in composition in the nebula is one cause of the
variation in composition of the planets with solar orbit distance.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planetesimals
• Dust grains and ices were ‘sticky’ (not just chemically, but electrically
and magnetically cohesive) and began to clump together (‘accretion’),
forming small bodies 0.01 to 10 m across, all orbiting the proto-star in
the same direction like Saturn’s rings.
• As their size grew, gravity
began to have an effect,
and larger bodies around 1
km in size called
planetesimals formed.
• The details of planetesimal
formation are still
uncertain, but km-sized
bodies would have
appeared by 10,000 years
after the disk formed.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Proto-Sun
• Gravity caused the center of the cloud to collapse into a ball: the
proto-sun. The gravitational energy released begins to heat things up.
• When the protosun became hot and dense enough, nuclear fusion
was ignited.
Picture credit: AnimAlu Productions
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
T Tauri Phase
• Young solar-type stars are said to be in the T Tauri phase (named
after the first example), and can have wind velocities of 200-300
km/s. This phase lasts about 10 million years.
• Once the star begins to shine,
the stellar wind ‘turns on’, and
the star begins to blow material
which has not yet accreted
outwards.
• T Tauri stars are characterized
by vigorous outflows
perpendicular to the relatively
dense disk.
• After 105 or 106 years, the
original gas nebula has been
dissipated.
Picture credit: James Schimbert, U. Oregon, Eugene
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planetesimal Growth
• Gravitational interactions between planetesimals perturbed their
orbits into non-circular, collisional trajectories.
• Time passed, and the planetesimals impacted one another. In lower
energy collisions or where the sizes are unequal, the planetesimals
merged into a new larger object.
• But in higher-energy collisions, two similarly-sized original bodies
were disrupted back into fragments.
• Over time, the larger planetesimals gathered up more and more
mass from collisions with smaller impacting bodies.
• In this way, the cores of the inner and outer planets were formed.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Inner Planets
• After about 108 years, the solar wind and accretion of planetesimals
had cleared the inner solar system of debris.
• The inner planets had by
then accreted almost all
their eventual mass.
• A period called the Late
Heavy Bombardment,
around 3.9 billion years ago
is associated with clearing
up the last planetesimals on
inclined orbits, as inferred
from lunar cratering.
• However, the process of collision and accumulations continues to the
present day: e.g. meteors, SL-9.
Picture credit: AnimAlu Productions
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Outer Planets
• The outer planets continued to
accrete for longer than the
inner planets, and gathered
much more ices and
volatiles.
• The outer planets are also
responsible for the asteroid
belt and comets.
Picture: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Differentiation
• As the planets accreted,
temperature and
pressure rose in the inner
regions.
Below: proposed Ganymede interior:
rock core and ice mantle.
• Heavier substances fell
to the core (e.g. metal for
the inner planets) and
lighter substances
floated on top.
• This process, called
differentiation, occurred
in all the planets but the
end result depended on
the initial ingredients!
Picture: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Asteroids
• The major asteroid belt lies between the orbits of Mars and Jupiter, at
a distance of around 2.7 AU.
• The asteroids were once thought to be the remains of a planet
destroyed by a massive impact.
Picture credit: NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Asteroids
• Current theories hold that the fragmented belt of material is the
natural consequence of the presence of the giant planet Jupiter
nearby during the planetary accretion phase.
• The massive Jupiter core formed first, and then either gobbled up
nearby planetesimals, or, in the case of the asteroids slightly further
away; Jupiter was able to disrupt any attempts they made to cling
together into a planet! The Asteroids are all less than 1000 km in size.
• Asteroids also exist in groups either preceding or trailing Jupiter in its
orbit (Jupiter Trojans) or Mars (Martian Trojans). There are also
asteroids which cross the Earth’s orbit, and others.
• Asteroids are important because they are examples of the original
planetesimals from 4.6 billion years ago. We will talk more about
asteroids in a later lecture.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Edgeworth-Kuiper Belt
• The Edgeworth-Kuiper belt is a band of icy planetesimals outside the
orbit of Neptune (40-120 AU), hypothesized in the 1940s.
• These objects are relics from
the early formation phase of
the solar system, which did
not manage to form into
planets.
• The first EKO detected was
found in 1992 (not counting
Pluto and Charon!) – now
over 800 are known.
Picture: Johns Hopkins University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
EB 313 and Pluto
• The object EB 313, first seen in 2003, caused a major upset to
astronomy when its size was announced in mid-2005 to be larger than
Pluto! (2400 or 3000 km, according to 2 studies: Pluto is 2300 km)
• This animation shows EB 313
moving against the star
background in the upper left.
• Astronomers have been grappling
ever since with the question of
how to define what is a planet!
• A decision in August 2006 has
resulted in Pluto being
downgraded to a new ‘dwarf
planet’ category.
Graphic: wikipedia
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Eris and Dysnomia
• Follow-up observations with
the Keck adaptive optics
system showed that EB 313
was accompanied by a small
moon.
• Originally dubbed ‘Xena’ and
‘Gabrielle’ by the discoverers,
they gained official names on
Sept 13: Eris and Dysnomia.
• The names mean ‘strife’ or
‘discord’, and ‘lawlessness’ appropriate to the trouble they
are causing!
Graphic: wikipedia
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Graphic: wikipedia
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
KBOs and SDOs
• Kuiper belt objects are actually clustered quite closely between 39
and 48 AU - stable orbital zones with respect to Neptune.
• Eris lies at a=68 AU, but its 557-year orbit is highly elliptical, ranging
from 38 to 100 AU, and inclined at 45 degrees.
• For this
reason, Eris
is classified
as a SDO:
or scattered
disk object.
Graphic: wikipedia
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Other Kuiper Belts
• We cannot gain a good view of the Kuiper belt as a whole due to our
position in the inner solar system … but, we can look elsewhere.
• These HST images show 2 Kuiper Belts around other stars, face on
(left) and edge-on (right).
Graphic: wikipedia
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Oort Cloud
• A vast reservoir of icy
planetesimals at 100s out to
100,000s of AU.
• Named the Oort Cloud, after
Jan Oort who guessed its
existence in 1950, by noting
that long-period comets
came from all directions of
the sky.
• Ironically, Oort cloud objects
formed closer to the Sun the
EKOs, but are on extremely
eccentric orbits.
Graphic: SWRI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Oort Cloud Formation
• Any planetesimals coming close to mighty Jupiter and Saturn were
ejected from the solar system entirely.
• However, icy bodies coming close to Neptune and Uranus were
merely flung into very distant and eccentric orbits around the Sun.
• These orbits were no longer confined to the plane of the solar system:
and so these icy bodies formed a huge spherical cloud around the
Sun, reaching out to 100,000 AU.
• These objects periodically visit the inner reaches of the solar system,
and we see their long tails of gas and dust as comets.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pictorial
Summary
Picture credit: James Schimbert, U. Oregon, eugene
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz - Summary
1. Describe the conditions which existed in our part of the Milky Way
prior to the birth of the solar system.
2. Why did the gas cloud collapse to a disk and not a point; why did
everything not fall into the Sun?
3. Describe how planets formed from the disk.
4. Describe the early history (pre-main sequence) of the Sun.
5. Why are the inner planets volatile-poor while the outer planets are
volatile-rich?
Dr Conor Nixon Fall 2006