ASTR 330: The Solar System
Announcements
• Homework assignment #6 out today.
• Extra credit term paper: due December 5th (next
Tuesday).
• This lesson will have a slightly different format from
usual.
• We will begin by splitting into groups to discuss one fact each
about the solar system. Try to formulate an explanation in your
group.
• We will then discuss the explanations to each fact in turn.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 25:
Planetary System Formation
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Why study formation again?
• The idea of this lecture is to revisit the subject we broached back in
Lecture 6: the theory of solar system formation.
• At this stage in the course, we have finished our ‘tour’ of the properties
of the solar system in detail, so it is an appropriate time to see if we can
explain how all these objects arose.
• If our theories still have problems, we should give them prominence for
further research.
• I will not re-capitulate the basic formation scenario from Lecture 6: I will
assume everyone is familiar with this!
• Our approach for this class will be to examine thirteen principal ‘facts’
about the solar system requiring explanation, as listed in M&O page 451.
Will attempt to explain each.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
1: Age
Fact 1: The oldest recorded age in the solar system is less
than 4.6 billion years, even though the Galaxy that contains
the solar system is much older than this. Furthermore, many
meteorites share this common age, which we have called
“the age of the solar system”.
• Around 100 years ago, a very different theory for the formation of the
planets was in favor.
• In this theory, the Sun had existed for much longer than the planets. A
chance encounter with a nearby star had gravitationally pulled material
from the Sun, which then coalesced to form the planets.
• At the present time, our theories of stellar evolution predict that the Sun
is about the same age as the date we derive for the planets.
• Therefore, the Sun formed together with the solar system, 4.6 Gyr ago.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Star Forming Regions
• McNeil’s Nebula, a newly discovered reflection nebula surrounding a
newborn star in the region of the Orion Nebula (M78). The star is at the
bottom tip of the bright nebula. The entire frame is less than 10 ly across,
• This nebula was
discovered by
amateur
astronomer Jay
McNeil on January
23rd 2004.
• Subsequent
searches through
older images
showed that it
became visible in
September 2003.
Picture credit: Adam Block/KPNO/NOAA/AURA/NSF
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
‘Proplyds’
• This image snapped by the HST in 1993 just after the corrective optics
were installed shows a portion of M42, the Orion Nebula.
• Five stars surrounded
by gas and dust
clouds, which we call
proto-planetary disks
(or ‘proplyds’ for short)
are seen here.
• Four of the disks
appear bright, and one
appears dark,
depending on how
close they are to the
brightest stars in the
nebula.
Picture credit: C.R. O’Dell/Rice Univ/NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
2: Rotation
Fact 2: The planets all move around the Sun in the same
direction that the Sun rotates, and nearly in the plane that
passes through the Sun’s equator.
• It is clear from the nebula and spinning disk model, that the central star
forms out of the same material as the disk: the material from the original
collapsing, rotating cloud.
• Hence, the central star preserves the same spin axis as the original
cloud. This is also true of the disk.
• The disk must form in the equatorial plane of the star.
• Then, when the planets form from the disk, they also orbit the star in the
same sense as the disk particles: in the equatorial plane of the star.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
From Cloud To Disk
• The left-side
image shows a
disk (500 AU
diameter) with
a gap, due to
the presence of
an unseen
planet.
• The right-side
image shows a
ring-shaped
disk. The ring is
17 AU wide,
with a 80 AU
hole in the
middle.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
3: Angular Momentum
Fact 3: Although the Sun has 99.9% of the mass in the solar
system, the planets have 98% of the system’s angular
momentum.
•
This is a serious problem! From our skater analogy, material accreting
onto the Sun cannot have retained all its original angular momentum.
•
There are two parts to the problem:
1. How does material lose angular momentum and fall into the star in
the first place?
2. How does the star lose angular momentum and slow down? An
interesting finding is that stars 15% more massive than the Sun
rotate much more rapidly, whereas solar-type stars all rotate at
about the same speed at the Sun.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
3: Angular Momentum Transport
Three processes have been proposed:
1. Magnetic braking.
2. Solar Wind.
1. Turbulence.
We’ll examine the pros and cons of each in turn.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
3a: Magnetic Braking
• Material in the inner part of the disk must be ionized.
• Outer layers of the Sun must spin more quickly than inner disk
material, as we know from our skating analogy.
• Therefore, the magnetic field of the Sun will feel resistance as it drags
through the plasma, which will slow down the rotation of the Sun.
• Main problem: predicts that more massive stars, having greater
magnetic fields and ionizing radiation, should brake more strongly than
solar-type stars, and so rotate more slowly. The opposite is observed.
• Also, any angular momentum lost by the Sun must be gained by the
disk material, so we are left with the problem of how the inner disk
material loses angular momentum and accretes onto the Sun.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
3b: Solar Wind
• In this scenario, we literally blow the angular momentum
problem away.
• We know that stars like the Sun have strong T-Tauri winds
in their youth, which ultimately clear away residual gas
and dust in the disk.
• The particles lost from the star also carry away angular
momentum.
• This theory also explains why more massive stars rotate
faster: they do not have proper winds like the Sun, and
therefore are not as good at losing angular momentum.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
3c: Turbulence
• As well as the transfer of angular momentum from the star to the disk,
we must also transport angular momentum within the disk.
• One way we can do this is by convection: computer modeling shows
that turbulent eddies can develop, which can transport angular
momentum.
• This scenario depends on whether the disk material cools rapidly or
not: the more rapid the cooling, the less time in which convection can
work.
• Finally, note that gravitational interactions between clumps of material
in the disk (not to mention planets and planetesimals) can cause
angular momentum to be transported outwards, while gas and dust
move inwards.
• In summary, our understanding of angular momentum transport is far
from complete.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
4: Composition
Fact 4: The inner planets, which are composed primarily of
the cosmically rare silicates and metals, are smaller and
denser than the outer planets. The giant planets, in contrast,
have a more nearly cosmic (or solar) composition, and their
satellites, with the exception of Io and Europa, have lower
densities, indicative of water ice and other volatiles.
• Our explanation for this fact is basically the temperature gradient in the
disk, hotter at the center, and gradually cooling with radial distance from
the center. Why?
• At the very center, gravitational potential energy of infalling material
heats up the protostar and allows ignition to occur.
• Further out, in the disk, material falling from the extended cloud onto the
disk (in the vertical direction) causes heating by the same way.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Evaporation and Condensation
• Towards the inner parts of the disk, temperatures would reach 2000 K,
enough to vaporize ‘dust’: i.e. rock, along with ices. Hence, all the
elements would be mixed together in the their original abundances.
• Subsequent cooling would sort the condensates in radial order: the
ones with the highest melting temperatures condensing closest to the
center.
• Inner planets: mostly rock and metal condensed in the inner solar
system, where the young Sun provided enough energy to keep the lighter
volatiles and ices from condensing. The gas was then removed by the
solar wind before condensation could occur.
• Giant planets: in the outer solar system, temperatures were cooler, and
the giant planets were able to accumulate large amounts of ices, and also
H and He.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mini-disks and Sub-nebulae
• The giant planets each formed out of a ‘mini-disk’, or sub-nebula, in
which there must have been a radial decrease in temperature: a
miniature solar system.
• Therefore, we would expect that in the cool outer parts of the subnebula, all species would condense, and we would see cosmically correct
proportions of the elements, about half rock and half ice.
• This is indeed the what we find for Callisto and Ganymede, the
outermost (regular) Jovian moons, with densities around 1.9 g/cm3.
• The inner (regular) moons of Jupiter, Io and Europa, show higher
densities: more rock compared to ices. This again is what our theory
would predict for the warmer inner parts of the nebula.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
5a: Asteroids
Fact 5a: The asteroids, which represent a composition
intermediate between the metal-rich inner planets and the
volatile-rich outer solar system, are located primarily
between the orbits of Mars and Jupiter.
• The asteroids are basically material left over from the formation of the
planetary system.
• Some represent original denizens of the Mars-Jupiter gap, where the
formation of an actual planet was probably inhibited by the nearby
presence of massive Jupiter.
• Hence, growth of planetesimals stalled at a size of about 100-1000 km.
• Jupiter’s gravity removed most of the protoplanetary material in this
region of space, perhaps also causing Mars to be smaller than would
otherwise have been the case.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
5b: Asteroid Variations
Fact 5b: Within the main asteroid belt, there is a distinct
gradient in composition, with the outermost asteroids being
richest in volatile elements and compounds.
• This is what we would expect: the same gradient as we see in the
planets, with the inner planets mostly metal and rock, and the outer
planets volatile rich.
• An interesting theory holds that some of the dark C-type asteroids
(carbonaceous) at the outer edge of the belt might actually be captured
comets, originally from the outer solar system.
• Once captured into their present orbits, much closer to the Sun, these
comets would gradually lose their volatiles, until only ‘soot’ remained.
• In some respects, the asteroid belt may be a kind of ‘dumping ground’
like the central gravel on a freeway.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
6: Meteorites
Fact 6: The primitive meteorites are composed of
compounds representative of solid grains that probably
formed in a cooling gas cloud of cosmic (solar) abundance at
temperatures of a few hundred Kelvins. Some of these grains
even predate the formation of the solar system.
• (In other words) We have evidence that certain microscopic grains in
some meteorites and IDPs are interstellar material: from the original
nebula. However, we do not have enough material to use radio-dating.
• How were these grains preserved?
• Quite simply, these grains must have been incorporated into KBOs and
comets, far enough from the inner disk to avoid heating and vaporization.
Hence, they are preserved intact from the time before the planets formed.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ancient IDPs?
• IDPs (cometary dust)
collected by NASA’s
ER-2 high-flying aircraft
appear to show a 1%
composition of
interstellar material, as
judged by analysis of
oxygen isotopic ratios.
• The Stardust mission
collected dust from
Comet Wild-2 in Jan
2004, and dropped a
sealed capsule of
comet dust to Earth in
Jan 2006 now
undergoing analysis.
Picture credit: (I) Washington Univ. (ii) NASA (iii) JPL/NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
7: Comets
Fact 7: Comets, like some outer-planet satellites, appear to
be composed largely of water ice, with significant quantities
of trapped or frozen gases like carbon dioxide and methane,
plus silicate dust and dark carbonaceous material.
• It is not a surprise that comets resemble the satellites of giant planets in
composition.
• Our theory for the formation of comets is that they were largely formed
in the giant planet region of the solar system, at temperatures of 30-100
K. They could not have formed out in the Oort Cloud: pressures were too
low to allow solids to form.
• After formation, they must have been gravitationally scattered to the
Oort Cloud by close encounters with the giant planets.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Oort Cloud
• Note that many icy planetesimals must have been ejected from the solar
system entirely, or ventured too close to the Sun, and burned up.
• Some will have traversed the inner solar system before being ejected:
and hence their material may have been ‘reprocessed’.
• Once in the Oort region, comets may
have accumulated primitive grains, and
hence be mixtures of primitive and nonprimitive material.
• This image shows an artist’s
impression of the Oort Cloud objects
(blue points) surrounding the central
Sun.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
8: Isotopes
Fact 8: Specific isotope ratios established prior to solar
system formation are preserved in objects formed at different
distances from the Sun.
• This is a fascinating but rather complex issue.
• In short: there appear to be variations in isotopic abundances
throughout the solar system which are either (i) preserved ‘fossilized’
anomalies inherent in the original nebula, or (ii) fossilized anomalies from
early processes in the solar nebula.
• It is reasonable to expect that varying isotopic abundances can arise in
stars, but there are many processes at work to erase such anomalies.
• So now we can explain Fact 8: although there was mixing (see Fact 9),
the mixing was small enough that the variation in isotopic ratios through
out the solar system has been preserved.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Oxygen Isotope Variation
• This figure
shows that the
isotope
abundances in
oxygen vary
between the
Earth, Mars, and
meteorites and
asteroids.
• The material
returned by the
Genesis
spacecraft may
answer these
questions.
Figure credit: Caltech Genesis GPS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
9: Inner Planet Volatiles
Fact 9: Volatile compounds (such as water) have reached
the inner planets even though the bulk composition of these
objects suggests formation at temperatures too high for
these volatiles to form solid grains.
• This point hints at the problem of mixing, of material between the outer
and inner solar system.
• Consider: how did the inner planets wind up with any volatile ices at all
(e.g. the Earth’s oceans), if the disk temperatures were too high for ices
to condense?
• Answer: the ices came later on from the outer solar system. The key to
this was objects on eccentric orbits, which we call comets.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Late Heavy Bombardment
• One explanation for the presence of volatiles in the inner solar system,
is that they arrived during the Late Heavy Bombardment.
• In the outer solar system, the distances between the planets are much
greater, and hence, it would take longer to ‘clear up’ any remaining
planetesimals.
• The LHB may be the time when Uranus and Neptune were scattering
the last planetesimals of the outer solar system, some of which reached
the inner solar system and added volatiles.
• As well as volatiles, such as water for the oceans, comets could also
have brought amino acids and other raw materials which may have
contributed to the formation of life.
• Many comets of course were scattered outward to form the Oort Cloud
(Fact 7).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
10: Retrograde Rotation
Fact 10: Despite the general regularity of planetary
revolution and rotation, Venus, Uranus and Pluto all rotate in
a retrograde direction (although their revolution is normal).
• Remember that, if all the planets formed out of a planar disk having a
certain rotation axis, the planets should all be rotating with their axes
nearly aligned with the axis of the Sun.
• The most likely explanation for these retrograde rotators is… what?
• Impacts of course!
• Besides the obvious features like impact basins and craters, impacts
are probably responsible for: the Moon; the anomalously high metal
content of Mars; and some planetary ring systems.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Uniformitarianism vs Catastrophism
• One of the great problems of 18th and 19th century geology was explain
the shape of the Earth’s surface terrain. The prevailing view was that
topology had all originated catastrophically, and that the Earth was
relatively young (thousands of years).
• A different view was taken by James Hutton (1726-1797), and
championed by Charles Lyell (1797-1875): that the Earth’s landforms had
arisen by the action of comparatively small forces, such as wind and rain,
acting over very long times (billions of years).
• So successful was uniformitarianism, that it prevented scientists from
recognizing the importance of catastrophic processes, e.g. the crater
density of the Moon, which is underestimated using present-day rates.
• Catastrophes (unusually rare and energetic events) are also implicated
in the formation of the Moon, and the tipping of Uranus.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
11: Satellite Systems
Fact 11: All the giant planets have systems of regular
satellites orbiting in their equatorial planes. They resemble
miniature versions of the solar system itself, …
• We envisage that, when the giant planets formed, the gravitation effects
produced both a central condensation (which would become the planet)
and a mini-disk (or sub-nebula).
• The mini-disk would then go on to form satellites in orbit about the
parent planet, in an analogous way to the planets themselves forming
around the parent star.
• The moon systems would clearly form in the same plane as the planet’s
equator, and also orbit in the same direction as the planet turned.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rings
Fact 11 (continued) …with the addition of rings, which the
Sun does not possess (unless you consider the asteroid
belt and the Kuiper Belt as ring systems circling the Sun).
•
There are two possible explanations for ring systems:
1. Debris from original satellite formation. Material which was inside the
Roche limit for the planet would not have coalesced into a satellite.
This is more likely for the rings of Saturn than for the other ring
systems, why?
2. A shattered moon (or moons). This seems more probable for the dark
rings of the other three giant planets. Due to the expected high impact
rates of inner moons, it seems likely that the inner moons could be
completely destroyed (look at Mimas and Miranda!) If the resulting
material fell inside the Roche limit, it could not reform into a satellite.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
12: Irregular satellites
Fact 12: All four of the giant planets: Jupiter, Saturn, Uranus
and Neptune; have one or more highly irregular satellites either in retrograde orbits or with with high eccentricities.
• These moons were surely captured. Satellites forming with the planets
could not assume such orbits, in such different rotational motion from the
sub-nebulas which created the outer planets.
• The capture scenario may have involved a planetesimal venturing close
to a giant planet, and experiencing frictional drag, thereby losing energy
and falling into a capture orbit.
• Friction could come from either (i) a very extended atmosphere of the
early planet, or (ii) the sub-nebular disk.
• Or, a third body must have been present.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
13: Giant Planets
Fact 13a: All of the giant planets are enriched in heavy
elements equivalent to 10-15 Earth masses; all have
atmospheres rich in hydrogen and helium …
• The 10-15 ME enrichment in heavy elements refers to the core of the
giant planets. These cores formed before the outer layers of the planet.
• After a certain maximum size (10-15 ME, apparently) the surrounding
gas from the nebula could then be accreted directly onto the planet core.
This nebular material formed the H- and He-dominated atmosphere.
• The current atmospheres are a mixture of the captured H and He, and
also out-gassed volatiles (methane for example) from the core.
• Why did Uranus and Neptune capture less gas? Did they form too late?
We do not yet know the answer.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Heat Excess
Fact 13b: .. and all (the giant planets) except Uranus are
radiating substantial quantities of heat from their interiors.
• This fact takes in several processes.
• For Jupiter and Saturn, the heat excess seems to be generated by the
ongoing differentiation of helium, condensing or raining out within the
metallic hydrogen core.
• For Neptune, there is no metallic hydrogen core. But, Neptune is slightly
heavier and denser than Uranus, and the heat seems to be heat from the
original formation which is still escaping.
• Uranus, being less dense than Neptune must have been able to cool
more thoroughly.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. Why do we see such a uniformity in ‘oldest ages’ for rocks in the solar
system of 4.6 Gyr?
2. Why do we not see any rocks 10 billion years old?
3. Why do all the planets move around the Sun in the same plane and
direction?
4. If angular momentum is conserved, why does the Sun not have most of
the angular momentum in the solar system, when it has most of the
mass?
5. Why are the inner planets smaller, denser, and depleted in volatiles
compared to the outer planets?
6. Why are Io and Europa denser than Ganymede and Callisto?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
7. What caused the main asteroid belt?
8. What is the gradient in composition which exists in the asteroid belt, and
what might have caused it?
9. What is an IDP? Why have we never dated a substance older than 4.6
Gyr? Where might we reasonably expect to look for such remnants?
10. Why do comets have similar compositions to outer planet satellites?
11. How did comets get where they are now?
12. What is curious about oxygen isotope ratios in the solar system?
13. How did the inner planets acquire volatiles?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
14. Which planets rotate backwards? What could have caused this?
15. How did the regular satellite systems of the outer planets form?
16. What about the irregular satellites systems?
17. How did the (i) cores (ii) atmospheres of the outer planets form?
18. Why does Uranus alone not generate more heat energy than it recieves
from the Sun?
Dr Conor Nixon Fall 2006