Our solar system's formation is a cosmic dance of gravity, collisions, and migration. Planets orbit in the same direction, with inner rocky worlds and outer gas giants. This pattern hints at a common birth from a spinning disk of material around our young Sun.

Discoveries of diverse exoplanets have challenged our understanding of planet formation. Hot Jupiters, super-Earths, and eccentric orbits show that planetary systems can take unexpected forms. These findings are reshaping theories about how worlds are born and evolve.

Solar System Formation

Key planetary characteristics for formation models

• Orbital characteristics
  • Planets orbit the Sun in the same direction (prograde) indicating they formed from a common rotating disk
  • Planetary orbits are nearly circular (low eccentricity) suggesting a smooth formation process
  • Planets orbit near the ecliptic plane (low inclination) supporting formation from a flattened disk
• Compositional characteristics
  • Inner planets (terrestrial) are rocky and dense formed from high-temperature materials (iron, silicates)
  • Outer planets (jovian) are gaseous and less dense accreted lighter elements (hydrogen, helium)
  • Asteroid belt separates inner and outer planets marks transition in composition and formation conditions
• Rotational characteristics
  • Most planets rotate in the same direction as their orbit (prograde) inherited from the rotating protoplanetary disk
  • Rotational axes are tilted relative to the orbital plane (obliquity) caused by collisions and gravitational interactions
• Satellite systems
  • Regular satellites orbit in the same direction as the planet's rotation formed from material around the planet
  • Irregular satellites have inclined, eccentric, and/or retrograde orbits captured by the planet's gravity

Extrasolar systems and formation understanding

• Diversity of extrasolar systems challenges traditional formation models
  • Hot Jupiters: gas giants orbiting close to their stars (51 Pegasi b) require migration to explain their positions
  • Super-Earths: planets larger than Earth but smaller than Neptune (Kepler-10b) not found in our solar system
  • Eccentric orbits: some exoplanets have highly elliptical orbits (HD 80606b) indicating dynamic interactions
• Migration explains unexpected orbital configurations
  • Gravitational interactions can cause planets to migrate inward (hot Jupiters) or outward
  • Protoplanetary disk interactions and resonances drive migration (Grand Tack model)
• Disk instability provides an alternative formation mechanism for gas giants
  • Gravitational instabilities in the protoplanetary disk can lead to rapid planet formation
  • May explain some directly imaged giant planets (HR 8799 system)
• Core accretion remains the dominant formation mechanism for gas giants in our solar system
  • Planetary cores form first, then accrete gas from the surrounding disk
  • Explains the structure and compositions of Jupiter and Saturn

Role of collisions in early solar system

• Planetesimal formation begins with collisions
  1. Collisions between dust grains lead to the growth of planetesimals (kilometer-sized objects)
  2. Gravitational interactions cause planetesimals to grow into protoplanets (Moon to Mars-sized)
• Terrestrial planet formation proceeds through collisions
  1. Protoplanets collide and merge to form the terrestrial planets (Mercury, Venus, Earth, Mars)
  2. Collisions result in the formation of the Moon (giant impact hypothesis)
• Late Heavy Bombardment shaped the inner solar system
  • Period of intense asteroid and comet impacts on the inner planets (~4.1 to 3.8 billion years ago)
  • May have been caused by migration of the outer planets (Nice model)
• Asteroid belt structure is influenced by collisions
  • Collisions between asteroids produce smaller fragments and dust (Kirkwood gaps)
  • Jupiter's gravity prevents asteroids from accreting into a single planet
• Kuiper Belt and Oort Cloud are shaped by collisions and gravitational interactions
  • Reservoirs of icy objects beyond Neptune's orbit (Pluto, Eris)
  • Collisions and gravitational interactions shape the structure of these regions

Formation and Evolution Processes

• Gravitational collapse initiates solar system formation
  • Molecular cloud contracts under its own gravity, forming the protosun and protoplanetary disk
• Angular momentum conservation shapes the disk structure
  • As the cloud collapses, it spins faster, flattening into a disk
• Condensation sequence determines planet composition
  • Different materials condense at varying distances from the protosun based on temperature
• Planetary migration alters orbital configurations
  • Planets can move inward or outward due to interactions with the disk or other planets
• Resonances between orbiting bodies influence system dynamics
  • Orbital periods of planets or moons can form integer ratios, affecting their long-term stability
• Differentiation creates layered internal structures
  • As planets heat up, materials separate based on density, forming cores, mantles, and crusts