The Origin of the Solar System

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Transcript The Origin of the Solar System

The Origin of the
Solar System
The Great Chain of Origins
—a review of the origin of matter
John Dobson said, “Three ingredients of the universe are
hydrogen, helium, and dust of exploded stars.”
The Big Bang created protons, neutrons, and electrons, which later
combined to form hydrogen and helium.
Nucleosynthesis inside stars accounts for the elements heavier than
helium up to iron; elements heavier than iron were created in
supermassive stars and in supernovae. Supernovae are the primary
means by which heavy elements are dispersed into interstellar space,
making possible the formation of rocky planets like Earth.
Recently, dark matter and dark energy have become the mysterious
ingredients of the universe that seem to far outweigh the visible matter.
The Origin of the Solar System
Early Hypotheses
1) Catastrophic hypotheses
Example: passing star hypothesis:
Star passing near the Sun tore material out of
the Sun, from which planets could form
Prediction: Only few stars should have planets!
2) Evolutionary hypotheses
Example: Laplace’s nebular hypothesis:
Rings of material separate from the spinning cloud,
carrying away angular momentum of the cloud
 cloud could contract further (forming the sun)
Prediction: Most stars should have planets!
The Solar Nebula
Basis of modern theory of
planet formation.
Planets form at about
the same time from the
same cloud as the star.
Planet formation sites
observed today as dust
Sun and our solar system
formed ~ 5 billion years ago.
Evidence for Ongoing Planet Formation
Many young stars
in the Orion
Nebula are
surrounded by
dust disks:
Probably sites of
planet formation
right now!
Dust Disks
Dust disks around
some T Tauri stars
can be imaged
directly (HST).
The Story of Planet Building
Planets formed from the same protostellar material
as the Sun, still found in the Sun’s atmosphere.
Rocky planet material formed from clumping together
of dust grains in the protostellar cloud.
Mass of less than ~ 15
Earth masses:
Mass of more than ~ 15
Earth masses:
Planets can not grow by
gravitational collapse
Planets can grow by
gravitationally attracting material
from the protostellar cloud
Earthlike planets
Jovian planets (gas giants)
The Condensation of Solids
To compare densities of planets,
compensate for compression due
to the planet’s gravity:
Only condensed materials could
stick together to form planets
Temperature in the protostellar
cloud decreased outward.
Further out
 Protostellar cloud cooler
 Metals with lower melting point
 Variation of chemical composition
throughout solar system
Formation and Growth of Planetesimals
Planet formation starts with clumping
together of grains of solid matter:
Planetesimals (few cm to km in size)
Planetesimal growth through
condensation and accretion
Gravitational instabilities may
have helped in the growth of
planetesimals into protoplanets
The Growth of
Simplest form of planet growth:
Unchanged composition of
accreted matter over time
As rocks melted, heavier elements
sink to the center  differentiation
This also produces a secondary
atmosphere  outgassing
Solar Nebula Theory wrap-up
Dust grains
(< a few cm)
Violent event
(high pressures & Metallic core?
Dwarf Planets
Small SolarSystem Bodies
– Meteoroids
– Meteorites
– Meteors
Extrasolar Planets = planets orbiting around
other stars
Extrasolar planets can
not be imaged directly
Detection using same
methods as in binary star
systems (spectroscopic
Look for “wobbling”
motion of the star
around the common
center of mass.
(Indirect) Detection of Extrasolar Planets
Observing periodic
Doppler shifts of
stars with no visible
Evidence for the
wobbling motion of
the star around the
common center of
mass of a planetary
Over 150 extrasolar
planets detected so
Survey of the Solar System
Relative Sizes of the Planets
Planets and their orbits
Note: orbits and bodies are not to the same scale
Planetary Orbits
All planets in almost
circular (elliptical)
orbits around the
sun, in approx. the
same plane (ecliptic).
Sense of revolution:
Sense of rotation:
(Exceptions: Venus and
(Distances and times reproduced roughly to scale)
Orbits generally
inclined by no
more than 3.4o
Mercury (7o)
Two Kinds
of Planets
Terrestrial planets
Jovian planets
Mercury, Venus, Earth, Mars
Jupiter, Saturn, Uranus, Neptune
Four inner planets
of the solar system
Relatively small in
size and mass (Earth
is the largest and most
Rocky (solid) surface
Relatively dense: 3.3 – 5.5 g/cm3
Surface of Venus can not be seen
directly from Earth because of its
dense cloud cover.
Craters on Planets’ Surfaces
Craters (like on our
moon’s surface) are
common throughout
the solar system
Not seen on
Jovian planets
because they
don’t have a
solid surface
The Jovian Planets
Much larger in mass and size
than terrestrial planets
Low average density:
0.7 – 1.7 g/cm3
Dense atmosphere
Mostly gas; no
solid surface
All have rings (not
just Saturn!)
Space Debris
In addition to planets, small bodies orbit the sun:
asteroids, comets, meteoroids
imaged by
the NEAR
• Asteroids are small
generally rocky bodies
that orbit the Sun,
mostly in the asteroid
• They range
tremendously in size,
from Ceres (~1000 km)
down to kilometer-sized
bodies and even
• Probably billions are
house-sized rocks.
Asteroid Belt
of 21,785
Note: Making the plotted points large enough to see causes them to appear far more
closely packed than they really are.
Comets (“icy mud balls”)
Visible as streaks
of light: meteors
The Oort Cloud
and the
Kuiper Belt
Approx. 100 AU
Orbits and bodies are not to scale
• The number of planetary
changes frequently as more
– Jupiter 62
– Saturn 31
– Uranus 27
– Neptune 13
– Mars 2
– Earth 1
– Mercury and Venus are
Dating the Solar System
Sun and planets should have
about the same age.
Ages of rocks are measured
by radioactive dating:
Measure abundance of a
radioactively decaying
element to find the time
since formation of the rock.
Dating of rocks on Earth,
on the moon, and
meteorites all give ages of
~ 4.6 billion years.
Dwarf Planets
• Pluto and similar objects fail to fit into
either family
• Recently, scientists have discovered more
than 200 similar objects orbiting the Sun
at the same distance as Pluto
• In 2006, a new family was introduced –
the dwarf planets
– Massive enough to pull themselves spherical
– Orbits have not been swept clear of debris