Chapter 19: Origin of the Solar System

Any GOOD hypothesis about the origin of the solar system must explain most - if not all - of its characteristics: All of the planets orbit the sun in the same direction, and in the same plane The planets closest to the sun are small and rocky, have few moons The planets further from the sun are large and contain more gas and icy materials Most of the Moons orbit their planets in the same direction as the planets orbit the sun Oldest meteorites are about 4.566 billion years old Planetary surfaces are all younger than the oldest meteorites

Relative sizes of the planets

Sun-planet distance (relative to Earth: AU) Mercury 0.4 AU Venus 0.7 Earth 1.0 Mars 1.5 Jupiter 5.2 Saturn 9.5 Uranus 19 Neptune 30 Sun = 1 mm on this scale, of 1 m = 1 AU 1 AU = 150 million km

Other residents of the solar system: 1. Dwarf planets diameter = 1000-3000 km, smaller than Moon, orbit the sun

Other residents of the solar system 2. Asteroids - rocky, d < 1000 km, orbit the sun

Other residents of the solar system 3. Comets - rock & ice, wide range of sizes (~10 m to 100 km)

Other residents of the solar system 4. Moons - orbit planets, some are larger than Mercury

Other residents of the solar system 5. Meteoroids - small fragments of asteroids that enter earth’s atmosphere (dust to boulder sized)

How many of the planets orbit the Sun in the same direction? About half All except 1 or 2 All of them, no exceptions

What fraction of moons orbit their planet in the same direction as the orbit around the Sun? None About half Most - a few exceptions All of them, no exceptions

Any GOOD hypothesis about the origin of the solar system must explain most - if not all - of its characteristics: All of the planets orbit the sun in the same direction, and in the same plane The planets closest to the sun are small and rocky, have few moons The planets further from the sun are large and contain more gas and icy materials Most of the Moons orbit their planets in the same direction as the planets orbit the sun Oldest meteorites are about 4.566 billion years old Planetary surfaces are all younger than the oldest meteorites

Interstellar cloud of gas & dust collapsed under its own gravity Predicted that protoplanetary nebulae would be observed Explains all of the major features of solar system, and also the exceptions Observations continue to support this theory

Protoplanetary disks last for only about 1-10 million years

What are the planets made of? Element how many atoms gas or solid at (total) Earth Jupiter ________________________________________________ Hydrogen 705,700 gas gas Helium 275,200 gas gas Carbon 3,032 gas soot (solid) Nitrogen 1,105 gas ice Oxygen 5,920 H2O gas H2O ice Silicon 653 rock rock Iron 1,169 metal metal

Planet formation: Terrerstrial vs. giant planets Giant (“jovian”) Lots of solids in the disk (cold > 5 AU) Cores form from ice, rock and metal Grow large, quickly (~1 million years) Big enough to trap H and He gas from disk Terrestrial (“earth like”) Very little solid material in disk at 1 AU Form from rock and metal only Grow slowly (~100 million years) Too small to trap any gas from disk

The next billion years: Debris disks Gas and fine dust blows away after ~ 10 million years Older stars have ‘debris disks’ around them Need a supply of larger objects to regenerate the dust that gets blown away evidence of planets forming around other stars Debris disks are analogous to the Oort cloud and Kuiper belt of comets, and the asteroid belt

Asteroid belt

The Nebular theory predicts most other sun-like stars should have planets Do they?

358 planets have been found around other stars!!! http://www.exoplanets.org

Detecting planets around other stars: Doppler method

Transit method (Kepler Mission)