Origin of the Solar System

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

Protoplanetary disks

Protoplanetary disks last for only about 1-10 million years

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!

(artist’s drawing of a debris disk)

Zodiacal light

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 billion years old 6.Planetary surfaces are all younger than the oldest meteorites

Relative sizes of the planets

Sizes of the planets relative to Sun

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 = km, smaller than Moon, orbit the sun

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

Asteroid belt

Asteroids are really quite rare…

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

Asteroids and comets are leftover planetesimals Some moons are captured planetesimals

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

Meteor!

Zodiacal light

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 billion years old 6.Planetary surfaces are all younger than the oldest meteorites

Protoplanetary disks last for only about 1-10 million years

H, He gas is present throughout the disk Icy compounds and rock/metal Rock & metal ice line Condensation: gas becomes solid

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

Connecting the dots: From planet formation to early Earth Computational astrophysics meets field geology!

1 million years 10 million years >100 million years, 3.8 billion years ago Hot+Dry (H 2 O gas)H 2 O ice Jupiter habitable zone

Terrestrial planets form by accretion of solids Dust >rocks >planetesimals >embryos >planets Terrestrial planets form by accretion of solids Dust >rocks >planetesimals >embryos >planets

The Moon-Forming Event A protoplanet the size of Mars (1/10 Earth’s mass) struck Earth, forming the Moon 4.5 billion years ago Oceans boiled away, silicate-vapor atmosphere for at least 1 Myr Earth had already differentiated into core & mantle structure by this time t=0 : IMPACT!6 minutes20 minutes32 minutes

But what if you don’t know: the initial number of parent & daughter atoms? how much of the P & D’s have entered or left the rock?

Solution: Isochron dating, requires a 4th measurement (the amount of a stable isotope of one of the elements) 48.8 Gyr Slope = D(now)/P(now)

melt solid

Make measurements for different minerals in rock. If data are linear, there is a strong correlation between: The amount of P in each sample The extent to which the sample has been enriched in D

Stable isotope geochronology 87 Sr/ 86 Sr 87 Rb/ 86 Sr

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!!!

Detecting planets around other stars: Doppler method

Transit method (Kepler Mission)