Chapter 19 The Origin of the Solar System

Early Hypotheses catastrophic hypotheses, e.g., passing star hypothesis: Star passing closely to the the sun tore material out of the sun, from which planets could form (no longer considered) Catastrophic hypotheses predict: Only few stars should have planets! evolutionary hypotheses, e.g., Laplace s nebular hypothesis: Evolutionary hypotheses predict: Most stars should have planets! Rings of material separate from the spinning cloud, carrying away angular momentum of the cloud cloud could contract further (forming the sun)

The Solar Nebula Theory Basis of modern theory of planet formation. The Solar system was formed from a giant, swirling interstellar cloud of gas and dust (the solar nebula) Perturb the cloud to begin its collapse Sit back and let physics take over.

Gravity vs. Gas Pressure Constant struggle to form stellar/planetary systems

Protosolar nebula Slowly rotating System initially in pressure balance no collapse

Gravity seeks to collapse cloud System initially in pressure balance no collapse

Gasinitially pressure seeks to expand System in pressure balance no collapse cloud

gas pressure gravity System initially in balance no collapse

gas pressure gravity Now, whack the cloud

Perturbation triggers collapse gravity is winning As collapse proceeds, rotation rate increases

As collapse continues, the rotation rate increases while nebula flattens

Collapse of the Solar Nebula As the solar nebula collapsed to a diameter of 200 AU (1 Ly = 63,240 AU), the following happened: The temperature increased as it collapsed (conservation of energy; gravitational potential energy becomes thermal energy) The rotation rate increased (conservation of angular momentum) The nebula flattened into a disk (protoplanetary disk, natural consequence) Motions of material in the disk became circularized

According to our theory of solar system formation, what three major changes occurred in the solar nebula as it shrank in size? (blue) It got hotter, its rate of rotation increased, and it flattened into a disk. (red) It gained energy, it gained angular momentum, and it flattened into a disk. (yellow) Its mass, temperature, and density all increased. (green) I have no idea

Which law best explains why the solar nebula spun faster as it shrank in size? (blue) Law of universal gravitation. (red) Einstein's law that E = mc2. (yellow) Conservation of angular momentum. (green) Conservation of energy.

Why did the solar nebula ended up with a disk shape as it collapsed? (blue) The force of gravity pulled the material downward into a flat disk. (red) It flattened as a natural consequence of collisions between particles in the nebula, changing random motions into more orderly ones. (yellow) The law of conservation of energy. (green) It was fairly flat to begin with, and retained this flat shape as it collapsed.

Which law best explains why the central regions of the solar nebula got hotter as the nebula shrank in size? (blue) Newton's third law. (red) Law of conservation of energy. (yellow) Law of conservation of angular momentum (green) The two laws of thermal radiation.

Ingredients of the Solar Nebula There was a range of temperatures in the proto-solar disk, decreasing outwards. Condensation the formation of solid or liquid particles from a cloud of gas (from gas to solid or liquid phase) Different kinds of planets and satellites were formed out of different condensates.

Ingredients of the Solar Nebula Metals : Condense into solid form at 1000 1600 K iron, nickel, aluminum, etc. ; 0.2% of the solar nebula s mass Rocks : Condense at 500 1300 K primarily silicon-based minerals; 0.4% of the mass Hydrogen compounds : condense into ices below ~ 150 K water (H2O), methane (CH4), ammonia (NH3), along with carbon dioxide (CO2), 1.4% of the mass Light gases (H & He): Never condense in solar nebula hydrogen and helium.; 98% of the mass

Ingredients of the Solar Nebula The Frost Line Situated near Jupiter Rock & metals can form anywhere the gas is cooler than about 1300 K. Carbon grains and ices can only form where the gas is cooler than 300 K. Inner Solar System: Too hot for ices and carbon grains. Outer Solar System: Carbon grains & ice grains form beyond the frost line.

Formation and Growth of Planetesimals Planet formation starts with clumping together of grains of solid matter: Planetesimals Planetesimal growth through condensation and accretion. Planetesimals (few cm to km in size) collide to form planets. Large planetesimals (>100 km across) become spherical due to the force of gravity.

The Growth of Protoplanets Simplest form of planet growth: Unchanged composition of accreted matter over time As rocks melted, heavier elements sink to the center differentiation This also produces a secondary atmosphere outgassing

The Story of Planet Building Inner Solar System: Outer Solar System: Low mass planets because they're interior to the frost line. Less building material available. Planets are not massive enough to grow by gravitational collapse. Planets grow by accretion. Mass of more than ~ 15 Earth masses: Planets can grow by gravitationally attracting material from the protostellar cloud (nebular capture). Jovian planets (gas giants)

Clearing the Nebula Remains of the protostellar nebula were cleared away by: Radiation pressure of the sun Ejection by close encounters with planets Solar wind Sweeping-up of space debris by planets Surfaces of the Moon and Mercury show evidence for heavy bombardment by asteroids.

Period of Heavy Bombardment Planetesimals remaining after the clearing of the solar nebula became comets and asteriods. Rocky leftovers Icy leftovers astersoids comets Many of them impacted on objects within the solar system during the few first 100 million years. (Creation of ubiquitous craters.)

Formation Review

Evidence for Ongoing Planet Formation Many young stars in the Orion Nebula are surrounded by dust disks: Probably sites of ongoing planet formation right now!

Extrasolar Planets Modern theory of planet formation is evolutionary Many stars should have planets! planets orbiting around other stars = Extrasolar planets Extrasolar planets can rarely be imaged directly. Detection using same methods as in binary star systems: Look for wobbling motion of the star around the common center of mass.

Indirect Detection of Extrasolar Planets Observing periodic Doppler shifts of stars with no visible companion: Evidence for the wobbling motion of the star around the common center of mass of a planetary system Over 100 extrasolar planets detected so far.

Direct Detection of Extrasolar Planets Only in exceptional cases can extrasolar planets be observed directly. Preferentially in the infrared: Planets may still be warm and emit infrared light; stars tend to be less bright in the infrared than in the optical

Is There a Jovian Problem? Two problems for the theory of planet formation: 1) Observations of extrasolar planets indicate that Jovian planets are common (and extremely close to their Sun ). 2) Protoplanetary disks tend to be evaporated quickly (typically within ~ 100,000 years) by the radiation of nearby massive stars. Too short for Jovian planets to grow! Solution: Computer simulations show that Jovian planets can grow by direct gas accretion without forming rocky planetesimals.