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 Challenges

1. Pattern of Motion All planets orbit the Sun in the same direction (counterclockwise as seen from the Earth’s North Pole) Planet orbits are nearly circular and co-planar Planets rotate in the same direction which they orbit Almost all moons orbit their planets in the direction of the planet rotation The Sun rotates in the direction planets orbit it Explain: Why is this order so good?

Four Challenges

2. Different types of planets Two 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 Pluto They 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 eccentricities Uranus and Pluto have tilted rotational axes Venus rotates backwards (clockwise) Earth has a large moon Pluto 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 The hypothesis was originally suggested by Immanuel Kant (1755) and Pierre-Simon Laplas (~1790) A cloud is called nebula - nebular hypothesis The collapsed piece of cloud that formed our own solar system is called the solar nebula

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 collapsed 2. The rotation rate increased 3. 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 System

Metals : iron, nickel, aluminum, etc.

Condense into solid form at 1000 – 1600 K 0.2% of the solar nebula’s mass Rocks : primarily silicon-based minerals Condense at 500 – 1300 K, 0.4% of the mass Hydrogen compounds : methane (CH 4 ), ammonia (HN 3 ), water (H 2 O) Condense into ices below 150 K, 1.4% of the mass Light gases : hydrogen and helium Never condense in solar nebula; 98% of the mass

Condensation

Accretion

Accretion is growing by colliding and sticking The growing objects formed by accretion – planetesimals (pieces of planets) Small planetesimals came in a variety of shapes, reflected in many small asteroids Large 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 helium It led to the formation of the Jovian planets Numerous moons were formed by the same processes that formed the protoplanetary disk Condensation 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 directions It 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 asteroids Icy 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 system They 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