1 Chapter 12 The Origin of the Solar System The story of planet formation: - From the beginning to...

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Chapter 12The Origin of the Solar System

The story of planet formation: - From the beginning to planetesimal formation

-Accumulation into protoplanets

- Solids: Condensation-Debris Disks

ASTA01 @ UTSC – Lecture 15

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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.

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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.)

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• 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).

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• 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.

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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.

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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.

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• 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.

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• You can see that same solar nebula composition is reflected in the chemical compositions of the planets.

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• 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.

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• 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.

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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.

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• 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.

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• 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.

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• 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.

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• Somewhere further from the Sun, there was a boundary called the ice line – beyond which the water vapour could freeze to form ice.

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• 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

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• 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.

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• Condensation

sequence

Solar composition

Gas that cools

Would produce

These compounds

at appropriate T

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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)

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• (Mg,Fe)2SiO4

• Forsterite to fyalite (solid solution)

Olivines

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Microstructure of circumstellardisks: identical with IDPs

(interplanetary dust particles)

mostly Fe+Mg silicatesPyroxene (Mg,Fe)SiO3

Olivine (Mg,Fe)2SiO4

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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)

The unity of chemistry in the Universe

1 Chapter 12 The Origin of the Solar System The story of planet formation: - From the beginning to...

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