Transcript Formation of the Solar System

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
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)
Four Challenges
3. Asteroids and Comets
Asteroids are small, rocky bodies that orbit the Sun
mostly between Mars and Jupiter (the asteroid belt)
Almost 10,000 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
Four Challenges
4. Exception to the Rules
• Mercury has larger orbital eccentricities
• Uranus has tilted rotational axes
• Venus rotates backwards (clockwise)
• Earth has a large moon
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)
Formation of Solar System
Summarized
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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 (CH4), ammonia
(HN3), water (H2O)
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