Lecture 32: The Origin of the Solar System Astronomy 161 – Winter 2004

Key Ideas: The present-day properties of our Solar System hold important clues to its origin. Primordial Solar Nebula: Process of the Sun’s formation Condensation of grains & ices From Planetesimals to Planets: Aggregation of small grains into planetesimals Aggregation of planetesimals into planets Terrestrial vs. Jovian planet formation.

The Birth of the Solar System The present-day properties of the Solar System preserves its formation history. Relevant Observations: Orbits of the planets and asteroids. Rotation of the planets and the Sun. Compositions of the planets

Clues from motions Orbital Motions: Planets all orbit in nearly the same plane. Most planet's orbits are nearly circular. Planets & Asteroids orbit in the same direction Rotation: Axes of the planets tends to align with the sense of their orbits, with notable exceptions. Sun rotates in the same direction as planets orbit. Jovian moon systems mimic the Solar System.

Pluto

Clues from planet composition Inner Planets & Asteroids: Small & rocky (silicates & iron) Few ices or volatiles, no H or He Jovian Planets: Large ice & rock cores Hydrogen atmospheres rich in volatiles. Outer solar system moons & icy bodies: Small ice & rock mixtures with frozen volatiles.

Icy Pluto Giant Gas Planets Mostly H, He, & Ices Rocky Planets

Formation of the Sun Stars form out of interstellar gas clouds: Large cold cloud of H 2 molecules and dust gravitationally collapses and fragments. Rotating fragments collapse further: Rapid collapse along the poles, but centrifugal forces slow the collapse along the equator. Result is collapse into a spinning disk Central core collapses into a rotating proto-Sun surrounded by a “Solar Nebula”.

Cold Interstellar H 2 Cloud

Interstellar Cloud of H 2 and Dust

Stellar-mass fragment

Gas & dust disks observed around young stars

Primordial Solar Nebula The rotating solar nebula is composed of ~75% Hydrogen & 25% Helium Traces of metals and dust grains Starts out at ~2000 K, then cools: As it cools, various elements condense out of the gas into solid form as grains or ices. Which elements condense out when depends on their “condensation temperature”.

Condensation Temperatures

The “Frost Line” Rock & Metals form anywhere the gas cooler than 1300 K. Carbon grains & ices only form when the gas is cooler than 300 K. Inner Solar System: Too hot for ices & carbon grains. Outer Solar System: Carbon grains & ices form beyond the “frost line”.

From Grains to Planetesimals Grains that have low-velocity collisions can stick together, forming bigger grains. Beyond the “frost line”, get additional growth by condensing ices onto the grains. Grow until their mutual gravitation assists in aggregation, accelerating the growth rate: Form km-sized planetesimals after few 1000 years of initial growth.

Terrestrial Planets Only rocky planetesimals inside the frost line: Collide to form small rocky bodies. Hotter closer to the Sun: Inner proto-planets cannot capture or retain H & He gas. Solar wind also disperses the solar nebula from the inside out, removing H & He. Result: Form rocky terrestrial planets with few ices.

Formation of a Terrestrial planet

Jovian Planets Ices augment the masses of the planetesimals. These collide to form large rock and ice cores: Jupiter & Saturn: M Earth rock/ice cores. Uranus & Neptune: 1-2 M Earth rock/ice cores. Larger masses & colder temperatures: Accrete H & He gas from the Solar Nebula. Planets with the biggest cores grow rapidly.

Formation of Jupiter ProtoSun Solar Nebula

Moons & Asteroids Gas gets attracted to the proto-Jovians & forms rotating disks of material: Get mini solar nebulae around the Jovians Rocky/icy moons form in these disks. Later moons added by asteroid/comet capture. Asteroids: Gravity of the proto-Jupiter keeps the planetesimals in the main belt stirred up. Never get to aggregate into a larger bodies.

Icy Bodies & Comets Outer reaches are the coldest and thinnest parts of the Solar Nebula: Ices condense very quickly onto rocky cores. Stay small because of a lack of material. Gravity of the proto-Neptune: Assisted the formation of Pluto-sized bodies in 3:2 resonance orbits (Pluto & Plutinos) Disperses the others into the Kuiper Belt.

Mopping up... The whole planetary assembly process took about 100 Million Years. Followed by ~1 Billion years of heavy bombardment of the planets by the remaining rocky & icy pieces. Sunlight dispersed the remaining gas in the Solar Nebula gas into the interstellar medium.

Planetary motions reflect the history of their formation. Planets formed from a thin rotating gas disk: The disk’s rotation was imprinted on the orbits of the planets. Planets share the same sense of rotation, but were perturbed from perfect alignment by strong collisions during formation. The Sun “remembers” this original rotation: Rotates in the same direction with its axis aligned with the plane of the Solar System.

Planetary compositions reflect the different environments of formation. Terrestrial planets are rock & metal: Formed in the hot inner Solar Nebula. Too hot to capture and retain Hydrogen & Helium. Jovian planets contain ices, H, & He: Formed in the cool outer Solar Nebula Grew large enough to accrete lots of H & He.