Planet Formation and Nucleosynthesis

Key Ideas:

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.

Nucleosynthesis: Origin of the Elements

The Birth of the Solar System

Relevant Observations:

• Orbits of the planets and asteroids.
• Rotation of the planets and the Sun.
• Composition of the planets, especially the strong distinction between Terrestrial, Jovian, and Icy planets.

Clues from planetary motions:

• Planets orbit in nearly the same plane.
• Planet 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 exceptions.
• Sun rotates in the same direction in the same sense.
• Jovian moon systems mimic the Solar System.

Clues from planet composition:

Inner Planets & Asteroids:

• Small rocky bodies
• Few ices or volatiles

Jovian Planets:

• Deep Hydrogen & Helium atmospheres rich in volatiles.
• Large ice & rock cores

Outer solar system moons & icy bodies:

• Small ice & rock mixtures with frozen volatiles.

Formation of the Sun

• Large cold cloud of H₂ 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 rotating "Solar Nebula"

Primordial Solar Nebula

• ~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 materials condense out when depends on their "condensation temperature".

Condensation Temperatures

Temp (K)	Elements	Condensate
>2000 K	All elements are gaseous
1600 K	Al, Ti, Ca	Mineral Oxides
1400 K	Iron & Nickel	Metallic Grains
1300 K	Silicon	Silicate Grains
300 K	Carbon	Carbonaceous grains
300-100 K	H & N	Ices (H2O, CO2, NH3, CH4)

The "Frost Line"

Carbon grains & ices can only form where 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

• Beyond the "frost line", get additional growth by condensing ices onto the grains.
• Grow to where their mutual gravitation assists in the aggregation process, accelerating the growth rate.

Can form km-sized planetesimals after a few 1000 years of initial growth.

Terrestrial Planets

• Collisions between planetesimals form small rocky bodies.
• It is hotter closer to the Sun, so the proto-planets cannot capture H and He gas.
• Solar wind is also dispersing the solar nebula from the inside out, removing H & He.

Result:

• Form rocky terrestrial planets with few ices.

Jovian Planets

These collide to form large rock and ice cores:.

• Jupiter & Saturn: 10-15 M_Earth rock/ice cores.
• Uranus & Neptune: 1-2 M_Earth rock/ice cores.

As a consequence of their larger masses & colder temperatures:

• Can accrete H & He gas from the solar nebula.
• Planets with the biggest cores grow rapidly in size, increasing the amount of gas accretion.

Result:

• The larger Jovian planets had massive rock & ice cores and heavy H and He atmospheres and became Gas Giants.
• The smaller Jovian planets formed further out, and have only shallow H/He atmospheres on ice/rock cores and became Ice Giants.

Moons & Asteroids

• Get mini solar nebula 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

• Ices condense very quickly onto rocky cores.
• Stay small because of a lack of material.

Gravity of the proto-Neptune also plays a role:

• Assisted the formation of Pluto-sized bodies in 3:2 resonance orbits (Pluto and Plutinos)
• Disperses the rest of the icy planetesimals into the Kuiper Belt (30-50AU) to become Kuiper Belt Objects.

Comets and other Trans-Neptunian objects are the leftover icy planetesimals from the formation of the Solar System.

Mopping up...

Followed by a 1 Billion year period during which the planets were subjected to heavy bombardment by the remaining rocky & icy pieces leftover from planet formation.

Light from the Sun 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 have been 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 conditions in which they formed.

Terrestrial planets are rock & metal:

• They formed in the hot inner regions of the Solar Nebula.
• Too hot to capture Hydrogen/Helium gas from the Solar Nebula.

Jovian planets contain ice, H & He:

• They formed in the cool outer regions of the Solar Nebula.
• Grew large enough to accrete lots of H & He.

These are the basic outlines of our current understanding of how our Solar System formed. The basic picture seems sound, but there are many details to be worked out. For example, we are still not sure how Jupiter grew as large as it did, or what influence it had on the formation of the rest of the solar system. The location of the "frost line" is also a matter of some debate, but current thinking holds that it is probably about 4 AU from the Sun. A great deal depends on how much solar radiation can penetrate deep into the outer parts of the primordial Solar Nebula.

The real test will be to observe other planetary systems in the process of formation, and draw information from those to apply to unraveling the history of our own system. Research in this area has begun to take off in exciting new directions in recent years, and we are starting to find support for and against some of the ideas presented here. This will remain an important and exciting area of study for years to come.

Nucleosynthesis

• Fuse Hydrogen into the light elements up to Iron/Nickel
• These accumulate in the core layers of stars.

Supernova Explosion:

• "explosive" nuclear fusion builds more light elements up to Iron & Nickel.
• fast & slow neutron reactions build Iron & Nickel into heavy elements up to ²⁵⁴Cf

Supernova explosions are responsible for creating nearly all of the heavy elements seen in nature, with a few important exceptions. The universe starts out with only Hydrogen (75%), Helium (~25%), and a smattering of light metals like Lithium, Boron, and Beryllium. Most other elements are forged by nuclear reactions occurring inside of stars or in the final moments of supernova explosions.

Top Ten Most Abundant Elements

Supernova Remnants

• Fusion-enriched with metals in the explosion
• Expands at a few x10,000 km/sec

Supernova Blast Wave:

• Plows up the surrounding interstellar gas
• Heats & stirs up the interstellar medium
• Hot enough to shine as ionized nebulae up to a few thousand years after the explosion

Stardust

• Next generation of stars includes these metals.
• Successive generations are more metal rich.

Sun & planets (& us):

• Contain many metals (iron, silicon, etc.)
• Only ~5 Gyr old

The solar system formed from gas enriched by a previous generation of massive stars.