Solar System

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.

• Geocentric Model

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.

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.

• 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.

Heliocentric Model

• “Helio” means sun

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

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!!!

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 Tauristars)

Protoplanetary and debris disks around young stars

• 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

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

Two Kinds of PlanetsPlanets 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

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.

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 PlanetsMuch larger in mass

and size than terrestrial planets

Much lower average density

All have rings (not only Saturn!)

Mostly gas; no solid surface

Planetary Orbits

Earth

VenusMercury

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)