Formation of the Solar System

• Uncovering the origin of the Solar system

• Early days of the formation

• Building the planets and other stuff

• Other planetary systems

Comparative Planetology

• Studying planets as worlds and compare them with each other is called comparative planetology

• Planetology is applied to any noticeably large object in the system (planets, moons, asteroids, comets)

To start we need to seek clues to the origin of the Solar system

Four Challenges1. Pattern of MotionAll planets orbit the Sun in the same direction (counterclockwise as seen from the Earth’s North Pole) Planet orbits are nearly circular and co-planarPlanets rotate in the same direction which they orbitAlmost all moons orbit their planets in the direction of the planet rotationThe Sun rotates in the direction planets orbit it

Explain: Why is this order so good?

Four Challenges

2. Different types of planetsTwo distinct groups of planets:Terrestrial planets (Mercury, Venus, Earth, Mars)Small, rocky, abundant in metals, few moons

Jovian planets (Jupiter, Saturn, Uranus, Neptune)Large, gaseous (made of hydrogen and its compounds), no solid surfaces, have rings, a lot of moons (made of low-density ices and rocks)

Explain: Why is the inner and outer Solar system divided so neatly?

Four Challenges

3. Asteroids and Comets

Asteroids are small, rocky bodies that orbit the Sun mostly between Mars and Jupiter (the asteroid belt)Almost 9,000 asteroids have been discovered

Comets are small and icy bodies that spend most of their lives beyond the orbit of PlutoThey occupy 2 regions: Kuiper belt and Oort cloud

Explain: The existence and general properties of the large number of these small bodies

Four Challenges

4. Exception to the Rules

Mercury and Pluto have larger orbital eccentricitiesUranus and Pluto have tilted rotational axesVenus rotates backwards (clockwise)Earth has a large moonPluto has a moon almost as big as itself

Allow for these exceptions

The Nebular Theory

The Solar system was formed from a giant, swirling interstellar cloud of gas and dust

A cloud is called nebula - nebular hypothesis

The collapsed piece of cloud that formed our own solar system is called the solar nebula

The hypothesis was originally suggested by Immanuel Kant (1755) and Pierre-Simon Laplas (~1790)

Collapse of the Solar Nebula

Three important processes gave form to our system, when it collapsed to a diameter of 200 A.U.

1. The temperature increased as it collapsed2. The rotation rate increased3. The nebula flattened into a disk (protoplanetary

disk)

Evolution of the Solar System

Building the Planets

Initial composition: 98% hydrogen and helium, and 2% heavier elements (carbon, nitrogen, oxygen, silicon, iron)

Condensation: the formation of solid or liquid particles from a cloud of gas

Different kinds of planets and satellites were formed out of different condensates

Ingredients of the Solar SystemMetals : iron, nickel, aluminum, etc.Condense into solid form at 1000 – 1600 K0.2% of the solar nebula’s mass

Rocks : primarily silicon-based mineralsCondense at 500 – 1300 K, 0.4% of the mass

Hydrogen compounds : methane (CH4), ammonia (HN3), water (H2O)Condense into ices below 150 K, 1.4% of the massLight gases: hydrogen and heliumNever condense in solar nebula; 98% of the mass

Condensation

Accretion

Accretion is growing by colliding and stickingThe growing objects formed by accretion – planetesimals (pieces of planets)

Small planetesimals came in a variety of shapes, reflected in many small asteroidsLarge planetesimals (>100 km across) became spherical due to the force of gravity

Inner solar system: only rocks and metals condensed and only small bodies formed

Nebular Capture

Nebular capture – growth of icy planetesimals by capturing larger amounts of hydrogen and heliumIt led to the formation of the Jovian planets

Numerous moons were formed by the same processes that formed the protoplanetary diskCondensation and accretion created mini solar systems around each Jovian planet

The Solar Wind

Solar wind is a flow of charged particles ejected by the Sun in all directionsIt was stronger when the Sun was young

The wind swept out a lot of remaining gas and interrupted the cooling of the nebula

If the wind were weak, the ices could have condensed in the inner solar system

Leftover Planetesimals

Planetesimals remained from the clearing became comets and asteroids

They were tugged by the strong gravity of the jovian planets and got more elliptical orbits

Rocky leftovers became asteroidsIcy leftovers became comets

Planetary Evolution - Geological

Internal heating leads to geological activity: volcanism, tectonics

As core cools and solidifies, activity slows, and eventually stops (Moon)

Earth and Venus are large enough to be active

Planet Activity

Planetary Evolution - Atmosphere

Atmospheres are formed by:- gases escaping from interior- impacts of comets (volatile-rich debris)

Fate of water depends on temperature (distance from the Sun)

Atmospheres changed chemically over time

Life on Earth substantially changed the atmosphere

Other Planetary Systems

Over 100 extrasolar planets have been discovered since 1995 The Extrasolar Planet Encyclopedia

Stars are too far away from the Sun, and direct imaging cannot detect planets near them

Current strategy involves watching for the small gravitational tag the planet exerts on its star

The tag can be detected using the Doppler effect

Extrasolar Planets in the Sky

Planet Transits

The Nature of Extrasolar Planets

The discovery of extrasolar planets gives us an opportunity to test the solar system formation theory

Most of the discovered planets are different from those of our systemThey are mostly Jupiter-size and located closer to their stars But: possible planet migration discovered planets are exceptions

Summary

All the planets were formed from the same cloud of dust and gas

Chance events may have played a large role in the formation and evolution of individual planets

Planet-forming processes are apparently universal