Stars spew out 1/2 their mass as gas & dust as they die
In the interstellar medium, dust and gas coalesces into clouds
New generations of stars (and their planets, if any) form in these clouds
Interstellar cloud of gas & dust collapsed under its own gravity Prediction: protoplanetary nebulae should be observed Explains all of the major features of solar system, and also the exceptions Observations continue to support this theory Nebular theory
The next billion years: Debris disks Gas and fine dust blows away after ~ 10 million years Jupiter must have formed by then 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
Debris disks around stars > 100 million years old are very common!
Any GOOD hypothesis about the origin of the solar system must explain most - if not all - of its characteristics: 1.All of the planets orbit the sun in the same direction, and in the same plane 2.The planets closest to the sun are small and rocky, have few moons 3.The planets further from the sun are large and contain more gas and icy materials 4.Most of the Moons orbit their planets in the same direction as the planets orbit the sun 5.Oldest meteorites are about 4.566 billion years old 6.Planetary surfaces are all younger than the oldest meteorites
Sun-planet distance (relative to Earth: AU) Mercury0.4 AU Venus0.7 Earth1.0 Mars1.5 Jupiter5.2 Saturn9.5 Uranus19 Neptune30 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
What are the planets made of? Element how many atoms gas or solid at (total) EarthJupiter ________________________________________________ Hydrogen 705,700 gasgas Helium 275,200 gasgas Carbon 3,032 gassoot (solid) Nitrogen 1,105 gasice Oxygen 5,920 H 2 O gas H 2 O ice Silicon 653rockrock Iron 1,169metalmetal
Planet formation: Terrerstrial vs. giant planets Giant (“jovian”) 1.Lots of solids in the disk (cold > 5 AU) 2.Cores form from ice, rock and metal 3.Grow large, quickly (~1 million years) 4.Big enough to trap H and He gas from disk Terrestrial (“earth like”) 1.Very little solid material in disk at 1 AU 2.Form from rock and metal only 3.Grow slowly (~100 million years) 4.Too small to trap any gas from disk
Terrestrial planets form by accretion of solids Dust >rocks >planetesimals >embryos >planets Terrestrial planets form by accretion of solids Dust >rocks >planetesimals >embryos >planets
Formation of Jovian Planets: Fast! (< 10 Myr) Core accretion: icy planetesimals clump together first Gravitational instability: dense clump of nebular gas forms first
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