Key Concepts: Lecture 18: Formation of the Solar System

Origin of the Solar System: the Nebular Hypothesis

Conservation of Momentum

Conservation of Angular Momentum

Origin of the Solar System

• Observations of the different members of the Solar System place constraints on theories of its formation–motions–chemical composition–age

Origin of Solar System• Planets orbit the Sun in same direction (counter-clockwise if

viewed from above the Earth’s north pole) and in nearly the same plane.

• Most planets & Sun also rotate in roughly this same direction, although planets can be tilted, occasionally by large angles.

• Most moons orbit around their parent planet in this same direction

• Solar system is differentiated – Terrestrial planets: high densities, small to moderate

atmospheres, slow rotation, 0 to few moons– Jovians: low densities, thick atmospheres, fast rotation, many

moons• Asteroids are very old – primitive unevolved material - common

age of oldest solar system material: 4.6 billion years.• Comets are also very old: primitive icy fragments from Oort

Cloud or Kuiper Belt

Origin of the Solar System: Nebular Hypothesis (originally proposed by Kant & Laplace)

• Slowly rotating interstellar cloud of gas & dust• Gravitational contraction

– disk flattens– conservation of angular momentum

• Sun forms at center of nebula

Conservation of MomentumMomentum = mass × velocitywrite this as: p = m v

Review Newton’s Laws of Motion (Lecture 9).

Newton’s 1st Law implies momentum is conserved (i.e., is unchanging) if there are no forces acting on an object.

Newton’s 2nd & 3rd Laws imply that if a force is applied the overall momentum is also conserved (i.e., unchanged) since there is always an opposite force of equal strength.

Conservation of Angular MomentumAngular momentum ∝ mass × rotation rate × radius2

An object of constant mass will spin faster as it contracts

Demo....

Other examples of conservation of angular momentum

Movie of solar system formation

Origin of the Solar System• All cold interstellar gas we see today

has some small dust grains. We think the pre-solar nebula would have started with these also.

• Growth of these grains to larger sizes and eventually to form Planetesimals

• Gravity collects planetesimals to form Protoplanets

• Formation of Jovians: Large protoplanets sweep up gas

• Formation of Terrestrials: Smaller protoplanets, in a warmer environment, not able to retain large amounts of Hydrogen gas

Origin of the Solar System• Planets form in disk of

gas and dust near center of nebula.! - Orbits are all in same direction and

plane. They are approximately circular.

! - Change in composition of planets is due to changes in local temperature in disk: planets are forming via condensation and accretion and disk is warmer near the Sun, e.g. no solid water ice inside about 4 AU.

Origin of the Solar System• Asteroids– Remaining rocky planetesimals in the

inner solar system– Failed to become protoplanets due to

gravity of Jupiter• Comets– Remaining icy planetesimals in outer

solar system -> Kuiper belt– Gravitational interactions with Jovians

flung planetesimals• Into inner solar system (delivering some

water to Terrestrial planets)• To outer solar system – Oort cloud

We can see other solar systems forming!

• See star formation section later in course