3.2 Planetary formation and solar system architectures

Planets form from swirling disks of gas and dust around young stars. Tiny particles collide and stick, growing into planetesimals, then embryos, and finally full-fledged planets. This process shapes the architecture of solar systems, including our own.

Our solar system has rocky planets close in and gas giants farther out. But exoplanet systems show wild diversity. Some have "hot Jupiters" hugging their stars, while others boast tightly packed "super-Earths." Planetary migration plays a key role in these varied layouts.

Planetary Formation

Process of planetary formation

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• Protoplanetary disk formation occurs when a molecular cloud gravitationally collapses and conservation of angular momentum leads to the formation of a flattened disk of gas and dust around a central protostar
• Dust grain growth happens as small dust particles collide and stick together due to electrostatic forces, gradually settling towards the midplane of the protoplanetary disk
• Planetesimal formation involves the growth of dust grains into kilometer-sized objects called planetesimals through further collisions and gravitational interactions
• Planetary embryos form as planetesimals continue to collide and merge, growing into Moon-to-Mars-sized objects through a process called oligarchic growth where larger embryos grow faster than smaller ones
• Final assembly stage occurs when planetary embryos collide and merge to form the final planets, with giant impacts playing a crucial role in shaping their characteristics (Earth-Moon system)

Terrestrial vs jovian planet formation

• Terrestrial planet formation
  • Occurs in the inner solar system where temperatures are too high for ices to condense
  • Planets are composed primarily of rock and metal materials
  • Limited material availability leads to smaller planet sizes compared to jovian planets (Mercury, Venus)
• Jovian planet formation
  • Takes place in the cooler outer solar system where ices can condense
  • Planet cores form from the accretion of icy planetesimals
  • Rapid gas accretion occurs once the core reaches a critical mass of about 10 Earth masses
  • Massive atmospheres are primarily composed of hydrogen and helium gas (Jupiter, Saturn)

Solar System Architectures

Solar system vs exoplanet architectures

• Solar system architecture
  • Terrestrial planets are located in the inner solar system (Mercury, Venus, Earth, Mars)
  • Gas giant planets are found in the outer solar system (Jupiter, Saturn, Uranus, Neptune)
  • Debris belts are present, such as the asteroid belt between Mars and Jupiter and the Kuiper belt beyond Neptune
  • Oort cloud is a spherical region of comets in the outer reaches of the solar system
• Exoplanet system architectures
  • Hot Jupiters are gas giant planets orbiting extremely close to their host stars (51 Pegasi b)
  • Super-Earths are planets with masses between that of Earth and Neptune (Kepler-10b)
  • Compact systems have multiple planets orbiting close to the host star (TRAPPIST-1)
  • Debris disks around other stars are analogous to the asteroid belt and Kuiper belt in our solar system (Fomalhaut)
• Differences between solar system and exoplanet architectures
  • Exoplanet systems display a higher diversity in their architectures compared to our solar system
  • Some exoplanet systems have planets in resonant orbits with orbital periods in integer ratios (Kepler-223)
  • Misaligned orbits are observed in some exoplanet systems, with planets orbiting in the opposite direction of the star's rotation (HAT-P-7b)

Role of planetary migration

• Types of migration
  1. Type I migration involves small planets embedded in a gaseous disk experiencing torques and migrating inward
  2. Type II migration occurs when massive planets open gaps in the disk and migrate along with the disk's viscous evolution
• Migration in the solar system
  • Jupiter and Saturn may have undergone inward and then outward migration according to the Grand Tack model, explaining the small size of Mars and the presence of the asteroid belt
  • Neptune's outward migration could have scattered Kuiper belt objects as described by the Nice model
• Migration in exoplanet systems
  • Hot Jupiters likely formed farther from their stars and migrated inward to their current close-in orbits
  • Resonant orbits in multi-planet systems can be explained by convergent migration (TRAPPIST-1)
  • Misaligned orbits may result from migration influenced by planet-planet scattering or the Kozai mechanism (HD 80606b)