Earth has formed in our solar system

We need to understand planetary formation

Constraints:• Astronomical observations• Study of meteorites• Study of planets• Theoretical modelling

Planetary orbits are nearly circular

Inner or terrestrial planetsSmall,dense, slow rotation, few moons

Giant or jovian planetsLarge, low density, fast rotation, numerous satellites

Compositional classes

Moon Mars

Ancient planetary surfaces are covered with impact craters

Stellar nebula

Solar nebula hypothesis

• Proposed by Kant and Laplace

• Gravitational collapse of a gaseous cloud (supernova?)

• During collapse, faster spin (conservation of angular momentum) and temperature increase

• Competing forces induce flattening

• Temperature depends on location in nebula because of cooling

• Hence location is important for planetary composition (planets closer to the sun should be more refractory)

• Interaction of large body at final stages could change composition considerably

All stars form from clouds of gas and dust which roam our galaxy. Eventually, gravity causes the cloud to collapse; since the cloud is spinning, material falls in along the "poles" faster than it does near the "equator". This flattening results in a disk-like object.

Material slowly makes its way into the center of this disk, forming a new star. While the star continues to grow, lumps form (solar nebula is heterogeneous) in the disk which will ultimately become planets.

The disk eventually thins as more material falls onto the star and the protoplanets. A hole in the disk near the star forms as material is completely incorporated into the star and planets.

Now fully formed planets exist within the hole, even as new planets are still under construction in the outer parts of the disk.

• This stage of planetary formation is very violent: impacts of large objects shape the final planets (e.g. formation of the moon)

• There are many collisions and core formation provide enough heat to melt the planet magma ocean

John Chambers

Ultimately, the remaining dust clears completely, leaving a fully formed solar system like our own.

Time frame for the formation of our solar system

Stevenson, Nature, 2008

Stevenson 2008

Problems with solar nebula model

• Too little angular momentum in the centre (sun)

• Planets in the wrong place

• Deviations from ecliptic plane

Unknowns

• How dense was the solar nebula• Time scales • (fast homogeneous differentiation)

(slow differentiated bodies)• Temperature• Planetary rearrangement after initial

formation• Volatiles