ASTA01 @ UTSC – Lecture 15
Chapter 12
The Origin of the Solar System
The story of planet formation:
- From the beginning to planetesimal formation
-Accumulation into protoplanets
- Solids: Condensation
-Debris Disks
1
The Age of the Solar System
• That is in agreement with the age of the Sun,
which is estimated to be (5 +-1.5) Gyr
• This has been calculated using mathematical
models of the sun’s interior that are completely
independent of meteorite radioactive ages.
• Apparently, all the bodies of the solar system formed at
about the same time, some 4.56 billion years ago.
2
The Origins of the Solar System
• According to the solar nebula theory, the planets
should be about the same age as the Sun. Here
is the brief timeline of events:
• The primordial gas cloud collapses in about 105 yrs,
forming a rotating disk/nebula
• The nebula cools down sufficiently for silicate rocks to
condense in the form of dust in ~105 yrs
• Turbulence in the disk dies down sufficiently to allow
settling of dust into a thin layer in the midplane of the disk
in several times 105 yrs (Remember Leukippus and
Democritus? They’ve predicted both the rotating nebula
and that process! Of course not the time scale.)
3
The Origins of the Solar System
• During the settling, the dust agglomerates into sand and
pebbles by collisions and electrostatic forces.
• Next, the sub-layer undergoes instability: gravity of the
thin layer of dust and stones fragments it into dense
chunks which soon shrink to become km-size
planetesimals. Less than 1 Myr (1 million years) passed
at this time from the beginning of the stellar formation
process.
• The nebula enters a period of slow evolution lasting 1-3
Myr, in most cases (although we have observations of
disks which still have a substantial amount of primordial
hydrogen+helium gas while 10 Myr old; one of them is
called TW Hydrae).
4
The Origins of the Solar System
• During these several millions of years, terrestrial planets
and solid cores of giant planets assemble in mutual
collisions of smaller solid bodies, planetesimals. Since a
large portion of the nebula is at low temperatures, ices as
well as silicates dominate the chemical composition of
planetesimals.
• After the largest bodies reach the size > 10 km, their
gravity is substantial enough to speed up their buildup in
a “runaway” fashion. Isolated protoplanets grow in such a
way, out of reach of each other’s perturbing gravity force.
• The growth of cores is curbed by the lack of material
located on close enough orbits, which can be destabilized
and accreted (meaning: absorbed) by a protoplanet.
5
The Age of the Solar System
• After several Myr, planets in the inner solar system
exhaust the supply of material and stop growing.
• At the same time, giant planet cores grow to the mass
~10 Earth masses. At that point, their massive hydrogen
& helium atmospheres become unstable and in ~0.1 Myr
they acquire a very massive gaseous envelope.
• After the lifetime of the disks expires (3-10 Myr), they are
dispersed, but giant planets are already gas-rich.
• Active planet formation is over after 10-30 Myr
• The next long stage is the removal of Kuiper belt bodies,
mainly comets in the region beyond Jupiter; it can last up
to 500 Myr.
6
Chemical Composition of the Solar Nebula
• Everything astronomers know about the
solar system and star formation suggests
that the solar nebula was a fragment of an
interstellar gas cloud.
• Such a cloud would have been mostly
hydrogen (75% mass) with some helium
(23%) and minor traces of the heavier
elements (1.22-1.94)%.
• The lower value is the recently revised
average solar composition.
7
Chemical Composition of the Solar Nebula
• Of course, in the sun nuclear reactions
have fused some hydrogen into helium.
• This, however, happens in the core and has
not affected its surface composition.
• Thus, the composition revealed in its spectrum is
essentially the same composition of the solar
nebula gases from which it formed.
8
Chemical Composition of the Solar Nebula
• You can see that same solar nebula
composition is reflected in the chemical
compositions of the planets.
9
Chemical Composition of the Solar Nebula
• The composition of the Jovian planets
resembles the composition of the Sun.
• Jupiter is only 3 times more enriched in heavy
elements than the sun, but they still make a
small contribution to the overall mass
• Furthermore, if you allowed low-density gases
to escape from a blob of sun-stuff, the
remaining heavier elements would resemble
the composition of the other terrestrial planets
– as well as meteorites.
10
Chemical Composition of the Solar Nebula
• The key to understanding the process that
converted the nebular gas into solid matter
is the observed variation in density among
solar system objects.
• The four inner planets are high-density,
terrestrial bodies.
• The outer, Jupiter-like planets are low-density,
giant planets.
• This division is due to the different ways gases are
condensed into solids in the inner and outer
regions of the solar nebula.
11
Condensation of Solids
• Even among the terrestrial planets, you
find a pattern of slight differences in
density.
• The uncompressed densities – the densities
the planets would have if their gravity did not
compress them – can be calculated from the
actual densities and masses of each planet.
12
Condensation of Solids
• In general, the closer a planet is to the
Sun, the higher is its uncompressed
density.
• This density variation is understood to have
originated when the solar system first formed
solid grains.
• The kind of matter that is condensed in a particular
region would depend on the temperature of the
gas there.
13
Condensation of Solids
• In the inner regions, the temperature
seems to have been 1500 K or so.
• The only materials that can form grains at this
temperature are compounds with high melting
points, such as metal oxides and pure metals.
• These are very dense, corresponding to the
composition of Mercury.
14
Condensation of Solids
• Farther out in the nebula, it was cooler.
• Silicates (rocky material) could condense.
• These are less dense than metal oxides and
metals, corresponding more to the compositions of
Venus, Earth, and Mars.
15
Condensation of Solids
• Somewhere further from the Sun, there
was a boundary called the ice line –
beyond which the water vapour could
freeze to form ice.
16
Condensation of Solids
• Further out, compounds such as methane
and ammonia could condense to form
other ices.
• Water vapour, methane, and ammonia were
abundant in the solar nebula.
• So, beyond the ice line, the nebula was filled
with a blizzard of ice particles.
• Those ices have low densities like the Jovian
planets. satellites
• [The planets are low density due gases they
contain.]
17
Condensation of Solids
• The sequence in which the different materials
condense from the gas as you move away from the
Sun is called the condensation sequence.
• It suggests that the planets,
forming at different
distances from the Sun,
accumulated from different
kinds of materials.
• This is not at all certain:
• Solid material migrated
18
Condensation of Solids
• The important factor was temperature.
• The inner nebula was hot, and only metals and rock could
condense there.
• The cold outer nebula could form lots of ices in addition to
metals and rocks.
• The ice line seems to have been between Mars and
Jupiter – it separates the formation of the dense
terrestrial planets from that of the low-density Jovian
planets.
• Astronomers have recently found that Jupiter is rather
poor in Oxygen but overabundant in Carbon.
19
Condensation of Solids
• Astronomers have recently found that Jupiter is rather poor
in Oxygen but overabundant in Carbon. Isotopic and
elemental abundances (mass ratios of elements and
different sub-species of elements) in Jupiter are like those
in asteroids, not in comets, which contradicts the
importance of the ice line for inner/outer planet division
• Other reasons for relative unimportance of ice line are:
• Migration and thus mixing of solids in the solar nebula
• Roughly 1:1 mass ratio of ice and rock in comets – not
important enough for locating Jupiter at 5.2 AU from sun.
20
• Condensation
sequence
Solar composition
Gas that cools
Would produce
These compounds
at appropriate T
21
Pyroxenes (Mg,Fe)SiO3
• The original chemical composition of the
solar nebula should have been roughly the
same throughout the nebula.
• Pyroxenes (usually Mg & Fe-rich silicates)
22
Olivines
• (Mg,Fe)2SiO4
• Forsterite to fyalite (solid solution)
23
Microstructure of circumstellar
disks: identical with IDPs
(interplanetary dust particles)
mostly Fe+Mg silicates
Pyroxene (Mg,Fe)SiO3
Olivine (Mg,Fe)2SiO4
24
The unity of chemistry in the Universe
It’s hard to tell if these
rocks are different; did
they come from Solar
System or maybe from
Beta Pictoris system?
The whole universe has
remarkably uniform
chemistry (chemical
abundances)
25