The ancient Greeks observed that objects in the sky seemed to move in circles around the Earth, leading to the geocentric model where the Earth is at the center of the universe. Ptolemy refined earlier Greek astronomical models, proposing that planets moved in smaller circles within larger circles, called the Ptolemaic model. This view persisted for over 1500 years until Copernicus and Galileo proposed the heliocentric model, which placed the Sun at the center. Today we recognize our Solar System formed from a collapsing cloud of gas and dust around the Sun, resulting in terrestrial, rocky planets near the Sun and larger gaseous Jovian planets farther out.

1. Solar System

2. Ancient Observations
• The ancient Greeks observed
the sky and noticed that the
moon, sun, and stars seemed
to move in a circle around
the Earth.
• It seemed that the Earth was
not moving and everything in
the heavens revolved around
the Earth.
• As it turned out, it was very
difficult to prove that the
planets did not revolve
around the Earth without
leaving the planet.

3. • Geocentric Model

4. Geocentric Theory
• Ancient Greeks such as
Aristotle believed that the
universe was perfect and
finite, with the Earth at
the exact center.
• This is the geocentric
theory, which stated, the
planets, moon,
sun, and stars revolve
around the Earth.

5. Claudius Ptolemy
•Ptolemaic Model
•Earth-centered
•His model of the solar system and
heavenly sphere was a refinement of
previous models developed by Greek
astronomers.

6. • His model had the planets move in little circles
that also moved in bigger circles.
• This belief persisted for about 1500 years.
• Earth was the center of not only the solar
system, but the entire universe.

8. Heliocentric Model
• “Helio” means sun

9. Heliocentric Theory Prevails
• Galileo supported
Copernicus’ theory which
clashed with the religious
views of the time, he
underwent many trials and
tribulations, and was even
sentenced to house arrest
for his remaining years.
• His view has withstood the
test of time.
• Today we talk about our
Solar System, not our Earth
system

11. ORIGIN OF THE SOLAR SYSTEM
• All the building material necessary to make our Solar
System was assembled in the process of
nucleosynthesis in the early Universe, in stellar cores
and during supernova explosions
• Protons, neutrons and electrons were made in the first
3 min. of the existence of the Universe
• Hydrogen, deuterium, helium, lithium and beryllium
were made in the first 30 min.
• Heavier elements all the way up to iron were
synthesized by nuclear fusion in stars
• All elements heavier than iron were made in supernova
explosions
• We are made out of that stuff!!!

12. The origin of planets
• The most likely explanation of the creation of our Solar
System involves the familiar process of star formation
out of a collapsing molecular cloud of gas and dust.
This is called the solar nebula theory
• A rotating cloud, compressed by a passing shock wave,
collapses into a flat, spinning disk with a protostar at its
center
• Planets are made out of the disk material in the process
of condensation and coagulation of dust particles
• When nuclear fusion begins in the central star and it
becomes luminous enough, most of the remaining
debris is cleared away (this process is seen in T Tauri
stars)

13. Protoplanetary and debris disks
around young stars

14. • Properties of the Solar System indicate that the
solar nebula theory is the most plausible one
• The solar nebula theory predicts that planets
should form as a by-product of star formation
• This means that protoplanetary disks should be
common around other stars and so should be the
planets
• Observations show that ~50% of young, solar-
mass stars have such disks and that some older
stars have dusty debris disks around them that
could hide planetary companions

15. Two kinds of planets
• Planets are clearly divided into two types:
terrestrial (Earthlike) planets and Jovian planets
(gas giants)
• Terrestrial planets, Mercury, Venus, Earth and
Mars, lie in the inner Solar System, are small,
rocky and dense, and have less dense
atmospheres than the Jovian planets
• Jovian planets, Jupiter, Saturn, Uranus and
Neptune, lie in the outer Solar System, are large,
gaseous and low-density objects
• Pluto does not fit either of these two categories

16. Two Kinds of Planets
Planets of our solar system can be divided into
two very different kinds:
Terrestrial (earthlike) planets:
Mercury, Venus, Earth, Mars
Jovian (Jupiter-like) planets: Jupiter,
Saturn, Uranus, Neptune

17. Terrestrial
Planets
Four inner planets of
the solar system
Relatively small in size
and mass (Earth is the
largest and most
massive)
Rocky surface
Surface of Venus can not be seen
directly from Earth because of its
dense cloud cover.

18. 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.

19. The Jovian Planets
Much larger in mass
and size than
terrestrial planets
Much lower
average density
All have rings (not
only Saturn!)
Mostly gas; no
solid surface

20. Planetary Orbits
Earth
Venus
Mercury
All planets in almost
circular (elliptical)
orbits around the sun,
in approx. the same
plane (ecliptic).
Sense of revolution:
counter-clockwise
Sense of rotation:
counter-clockwise
(with exception of
Venus, Uranus, and
Pluto)
Orbits generally
inclined by no
more than 3.4o
Exceptions:
Mercury (7o)
Pluto (17.2o)
(Distances and times reproduced to scale)