2.2 Timescales and stages of solar system evolution

The solar system's formation and evolution is a cosmic dance spanning billions of years. It all started with a collapsing cloud of gas and dust, leading to the birth of our sun and the swirling disk that would become the planets.

From tiny dust grains to massive gas giants, the solar system's components took shape over millions of years. The inner rocky planets formed slowly, while the outer gas giants raced to gather gas before it disappeared. This complex process shaped our cosmic neighborhood.

Stages of Solar System Evolution

Formation of the Protostar and Protoplanetary Disk

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• The solar system formed from the gravitational collapse of a molecular cloud composed primarily of hydrogen and helium, with trace amounts of heavier elements (carbon, oxygen, nitrogen, and heavier elements)
• The initial collapse led to the formation of a protostar at the center, surrounded by a protoplanetary disk of gas and dust
  • The protostar formed as the collapsing material heated up due to gravitational compression and began to undergo nuclear fusion
  • The protoplanetary disk formed from the remaining material that had too much angular momentum to fall directly into the protostar

Accretion of Dust Grains and Formation of Planetesimals

• Dust grains in the protoplanetary disk began to collide and stick together through a process called accretion, forming larger particles and eventually planetesimals
  • Electrostatic forces and van der Waals forces helped the dust grains stick together initially
  • As the particles grew larger, gravitational forces became more important in holding them together
• Planetesimals continued to grow through collisions, with some becoming large enough to differentiate into rocky cores and metallic cores due to heating from radioactive decay and gravitational compression
  • Radioactive isotopes (aluminum-26 and iron-60) provided heat that melted the interiors of larger planetesimals
  • Denser materials (metals) sank to the center, forming metallic cores, while lighter materials (silicates) remained in the outer layers, forming rocky mantles and crusts

Formation of Planets in the Inner and Outer Solar System

• In the inner solar system, rocky planets formed from the accretion of planetesimals composed primarily of refractory materials (metals and silicates), while gas giants formed in the outer solar system where volatile materials (ices) were more abundant
  • The higher temperatures in the inner solar system prevented the condensation of volatile materials, leading to the formation of smaller, rocky planets (Mercury, Venus, Earth, and Mars)
  • The lower temperatures in the outer solar system allowed for the condensation of volatile materials, leading to the formation of larger, gas-rich planets (Jupiter, Saturn, Uranus, and Neptune)
• The gas giants grew rapidly, accreting large amounts of gas from the protoplanetary disk before it dissipated
  • The gas giants had to form quickly, within a few million years, before the gas in the protoplanetary disk was blown away by the intense solar wind from the young Sun

Clearing of Debris and Establishment of the Modern Solar System

• The final stage of solar system evolution involved the clearing of remaining debris through collisions and gravitational interactions, leading to the relatively stable configuration we observe today
  • The Late Heavy Bombardment, a period of intense asteroid and comet impacts, occurred approximately 4.1 to 3.8 billion years ago and may have been caused by the migration of the gas giants
  • Gravitational interactions between planets and smaller bodies (asteroids and comets) scattered many of these objects into the Oort Cloud and Kuiper Belt, or ejected them from the solar system entirely
  • The remaining debris settled into relatively stable orbits, forming the asteroid belt between Mars and Jupiter and the Kuiper Belt beyond Neptune

Timescales for Solar System Evolution

Collapse of the Molecular Cloud and Formation of the Protostar

• The initial collapse of the molecular cloud and formation of the protostar and protoplanetary disk occurred within a few hundred thousand years
  • The collapse of the molecular cloud was triggered by a gravitational instability, possibly caused by a nearby supernova or the passage of the cloud through a spiral arm of the galaxy
  • The collapse proceeded rapidly once initiated, with the central protostar forming within a few hundred thousand years

Accretion of Dust Grains and Formation of Planetesimals

• Dust grains in the protoplanetary disk began to accrete and form planetesimals within about 1-10 million years after the formation of the protostar
  • The growth of dust grains into larger particles and eventually planetesimals was a gradual process, taking several million years
  • The formation of the first planetesimals marked the beginning of the planet formation process

Formation of Rocky Planets and Gas Giants

• Rocky planets in the inner solar system formed within about 10-100 million years after the formation of the first planetesimals
  • The accretion of planetesimals into larger bodies and the differentiation of these bodies into rocky planets took several tens of millions of years
  • The formation of the Earth-Moon system, likely through a giant impact event, occurred around 4.5 billion years ago, approximately 50-100 million years after the formation of the first solids in the solar system
• Gas giants in the outer solar system formed within a few million to a few tens of millions of years after the formation of the first planetesimals, before the gas in the protoplanetary disk dissipated
  • The gas giants had to accrete their massive gas envelopes before the gas in the protoplanetary disk was removed by the solar wind, setting an upper limit on their formation timescale
  • Jupiter and Saturn likely formed within a few million years, while Uranus and Neptune may have taken slightly longer, up to a few tens of millions of years

Dissipation of the Protoplanetary Disk and Clearing of Debris

• The protoplanetary disk dissipated within about 1-10 million years after the formation of the protostar, setting an upper limit on the timescale of gas giant formation
  • The intense solar wind from the young Sun gradually removed the gas from the protoplanetary disk, ending the period of gas giant formation
  • The dissipation of the protoplanetary disk also marked the end of the main phase of planet formation
• The clearing of remaining debris and the establishment of the relatively stable configuration of the solar system took several hundred million years, with the Late Heavy Bombardment occurring around 4.1-3.8 billion years ago
  • The gravitational interactions between planets and smaller bodies gradually cleared the remaining debris from the solar system, a process that took several hundred million years
  • The Late Heavy Bombardment, a period of intense asteroid and comet impacts, may have been caused by the migration of the gas giants and represents the final major event in the clearing of debris from the early solar system

Inner vs Outer Solar System Evolution

Composition and Formation of Planetesimals

• Inner solar system objects (terrestrial planets) formed from the accretion of rocky planetesimals, while outer solar system objects (gas giants) formed from the accretion of both rocky planetesimals and large amounts of gas
  • The composition of planetesimals in the inner solar system was dominated by refractory materials (metals and silicates), as the higher temperatures prevented the condensation of volatile materials (ices)
  • Planetesimals in the outer solar system had a higher proportion of volatile materials (ices) due to lower temperatures, allowing for the formation of ice-rich cores that could accrete large amounts of gas

Physical Properties and Formation Timescales

• Rocky planets in the inner solar system are smaller and have higher densities compared to the gas giants in the outer solar system, which have lower densities and are primarily composed of hydrogen and helium
  • The terrestrial planets (Mercury, Venus, Earth, and Mars) have radii ranging from about 2,400 to 6,400 km and densities ranging from 3.9 to 5.5 g/cm³
  • The gas giants (Jupiter and Saturn) have radii of about 60,000 to 70,000 km and densities of 0.7 to 1.3 g/cm³, while the ice giants (Uranus and Neptune) have radii of about 25,000 km and densities of 1.3 to 1.6 g/cm³
• The formation of gas giants in the outer solar system was more rapid compared to the formation of rocky planets in the inner solar system, as gas giants had to accrete their gas envelopes before the protoplanetary disk dissipated
  • Gas giants formed within a few million to a few tens of millions of years, while rocky planets took several tens of millions of years to form
  • The rapid formation of gas giants was necessary to accrete large amounts of gas from the protoplanetary disk before it was removed by the solar wind

Bombardment History and Planetary Migration

• Inner solar system objects experienced more intense bombardment and collision events during the early stages of solar system evolution, as evidenced by the heavily cratered surfaces of Mercury, Venus, Earth, and Mars
  • The Late Heavy Bombardment, a period of intense asteroid and comet impacts around 4.1 to 3.8 billion years ago, left a significant imprint on the surfaces of the terrestrial planets
  • The Earth-Moon system is thought to have formed as a result of a giant impact event between the proto-Earth and a Mars-sized object, which likely occurred during the early stages of solar system evolution
• Outer solar system objects, particularly the ice giants (Uranus and Neptune), may have experienced significant migration during the early stages of solar system evolution, as suggested by the Nice model
  • The Nice model proposes that the gas giants formed in a more compact configuration and later migrated to their current positions through gravitational interactions with a disk of planetesimals
  • The migration of the gas giants may have triggered the Late Heavy Bombardment by disrupting the orbits of asteroids and comets in the outer solar system, sending them into the inner solar system

Evidence for Solar System Evolution Timescales

Radiometric Dating of Meteorites and Lunar Samples

• Radiometric dating of meteorites and lunar samples provides evidence for the age of the solar system and the timescales of planetary formation
  • The oldest dated meteorites, known as carbonaceous chondrites, have ages of about 4.568 billion years, setting a lower limit on the age of the solar system
  • Radiometric dating of lunar samples returned by the Apollo missions has shown that the Moon formed around 4.51 to 4.44 billion years ago, providing constraints on the timing of the Earth-Moon system formation
• The hafnium-tungsten (Hf-W) isotopic system provides evidence for the rapid formation of planetary cores, as the decay of radioactive 182Hf to 182W occurs on a timescale of about 9 million years
  • The Hf-W isotopic compositions of meteorites and terrestrial rocks suggest that planetary cores formed within the first 10-30 million years of solar system history
  • The rapid formation of planetary cores is consistent with the idea that planetesimals differentiated early in solar system history due to heating from radioactive decay and gravitational compression

Analysis of Primitive Solar System Materials

• The presence of calcium-aluminum-rich inclusions (CAIs) in meteorites, which are among the oldest known solid objects in the solar system, provides evidence for the early formation of refractory materials in the protoplanetary disk
  • CAIs have ages of about 4.567 billion years, indicating that they formed within the first few million years of solar system history
  • The presence of CAIs in meteorites suggests that refractory materials condensed early in the protoplanetary disk and were later incorporated into planetesimals and planets
• The presence of isotopic anomalies in meteorites, such as oxygen isotope variations and extinct radionuclides, provides evidence for the heterogeneity of the protoplanetary disk and the timescales of mixing and accretion processes
  • Oxygen isotope variations in meteorites indicate that the protoplanetary disk was not completely homogenized and that mixing of material from different regions occurred over timescales of several million years
  • The presence of extinct radionuclides, such as aluminum-26 and iron-60, in meteorites provides evidence for the rapid formation of the solar system and the incorporation of material from nearby supernovae

Crater Counting and Planetary Surface Ages

• The cratering records on the surfaces of terrestrial planets and moons provide evidence for the intensity and duration of bombardment events during the early stages of solar system evolution, such as the Late Heavy Bombardment
  • The heavily cratered surfaces of the Moon, Mercury, and Mars indicate that these bodies experienced intense bombardment early in their histories
  • The relative ages of different regions on these bodies can be estimated by counting the number and size distribution of craters, with more heavily cratered regions being older than less cratered regions
• The age and composition of the Moon, as determined by radiometric dating and analysis of lunar samples, provide constraints on the timescale of the Earth-Moon system formation and the timing of the proposed Giant Impact event
  • The similar ages of the oldest lunar rocks and the oldest Earth rocks suggest that the Moon formed relatively soon after the formation of the Earth, likely within the first 100 million years of solar system history
  • The unique composition of the Moon, with its depletion in volatile elements and enrichment in refractory elements, is consistent with the Giant Impact hypothesis, in which a Mars-sized object collided with the proto-Earth, leading to the formation of the Moon

Dynamical Models and Planetary Migration

• The size distribution and orbital characteristics of asteroids and Kuiper Belt objects provide evidence for the dynamical evolution of the solar system and the timescales of planetary migration and debris clearing
  • The Kirkwood gaps in the asteroid belt, which are regions of low asteroid density corresponding to orbital resonances with Jupiter, provide evidence for the gravitational influence of Jupiter on the asteroid belt over billions of years
  • The orbital distribution of Kuiper Belt objects, with many objects in resonant orbits with Neptune, suggests that Neptune migrated outward early in solar system history, capturing objects into resonant orbits
• Numerical simulations, such as the Nice model, provide theoretical support for the idea of planetary migration and its role in shaping the current configuration of the solar system
  • The Nice model proposes that the gas giants formed in a more compact configuration and later migrated to their current positions through gravitational interactions with a disk of planetesimals
  • The model has been successful in explaining various features of the solar system, such as the orbital distribution of Kuiper Belt objects and the timing of the Late Heavy Bombardment
  • Dynamical models like the Nice model provide a framework for understanding the timescales and mechanisms of planetary migration and debris clearing in the early solar system