APS 105: Formation of the Sun and Planets — The Planets

Vocabulary practice flashcards covering protoplanetary disk cooling, condensation sequences, planetesimal accretion, Jovian gas capture, the snow line, and planetary system evolution.

Edge-On Protoplanetary Disk

A flattened disk of dense gas and dust surrounding a newly formed protostar, observed edge-on obscuring light from the central star, as seen in Hubble Space Telescope observations of the Orion Nebula.

Condensation

The phase transition in the solar nebula where vapor cools and transitions directly into a solid state, with gas molecules attaching to growing, fluffy crystal structures.

Refractory Oxides and Metals

The highest-temperature materials to condense in the solar nebula, including oxides ($Al_2O_3$, $CaO$, $Ti_2O$) condensing at approximately $1700^\ne\text{ K}$ and metals ($Fe$, $Ni$) condensing at approximately $1500^\ne\text{ K}$.

Silicates Condensation Sequence

The condensation of silicate minerals at intermediate temperatures, progressing from silicates ($(Mg,Fe)SiO_4$, $SiO_2$) at approximately $1450^\ne\text{ K}$ down to alkali silicates at approximately $1000^\ne\text{ K}$ and hydrous silicates (serpentine, talc) at approximately $400^\ne\text{ K}$.

Snow Line (Frost Line)

The critical distance from the Sun beyond which temperatures dropped below approximately $273^\ne\text{ K}$, allowing water ice to condense alongside rock and metal, causing an abrupt jump in the surface density of solid material.

Dust Aggregates

Fluffy, microscopic clumps of solid grains formed when tiny condensation particles in the solar nebula collide and adhere to one another in microgravity.

Terrestrial Planets

Rocky, metallic planets that formed within the warm inner solar nebula inside the snow line, where only high-temperature rocks and metals could condense into solid planetesimals.

Runaway Growth

The process by which larger planetesimals grow substantially faster than smaller ones because their greater cross-sectional area and gravitational reach sweep up surrounding material more efficiently.

Accretion

The progressive accumulation of many small solid particles and planetesimals into a few large protoplanetary bodies via mutual gravitational attraction and collisions.

Gas Capture Threshold

The requirement for a growing protoplanet core to reach approximately $10$ Earth masses rapidly enough to gravitationally bind and retain ambient hydrogen ($H_2$) and helium ($He$) before solar winds clear the nebula.

Miniature Disks

Sub-disks of gas and dust that orbited growing Jovian giant planets, within which their regular systems of moons condensed and accreted.

Asteroids

Rocky and metallic leftover planetesimals that remained unaccreted inside the solar nebula's frost line.

Comets

Icy and dust-rich leftover planetesimals that formed and remained unaccreted in the cold outer regions beyond the frost line.

Heavy Bombardment

A period during the late stages of solar system formation when remaining planetesimals intensely impacted the surfaces of planets and large moons.

Origin of Earth's Water

The delivery of water and volatile compounds to Earth's surface via impacts by icy planetesimals originating from beyond the snow line in the outer solar system.

Captured Moons

Irregular planetary satellites, such as Phobos and Deimos, that originated as stray planetesimals and were subsequently trapped by a planet's gravity.

Giant Impact Hypothesis

The prevailing model for the formation of Earth's Moon, where an off-center collision by a Mars-sized body stripped silicate material from early Earth's crust into orbit, which then rapidly accreted into the Moon.

Five Stages of Nebular Theory

The sequence governing solar system evolution: 1. Collapse of the gas cloud, 2. Condensation of solid grains, 3. Accretion of planetesimals, 4. Gas Capture (by rapid, massive cores like Jupiter and Saturn), and 5. Collision and clearing.