OUR SOLAR SYSTEM

Chapter 29

29.1 Overview of our Solar SystemThe Earth is one of nine (9)

planets that orbit the SunAll planets, as well as their

moons, orbit in the same direction

All planets except Pluto lie near the same plane

Each of our planets have various sizes, surface conditions, and internal structures

Early ObservationsEarly ideas about the solar system were

developed solely on the basis of Earth-based observations of the sky

Early astronomers assumed the Sun, planets, and stars orbited a stationary Earth (Earth centered model)

Problem with Earth centered model is that some aspects of planetary motion were difficult to explain. For instance, occasionally a planet would move in the opposite direction across the sky, retrograde motion.

Retrograde motion caused astronomers to continue searching for a better model of our solar system

The Sun-centered modelNicolaus Copernicus suggested the Sun to

be the center of the solar system in 1953.The sun-centered model is referred to as

the heliocentric model, suggesting that the Earth and other planets orbited a stationary Sun.

Provided a simple explanation for retrograde motion, noting that the inner planets move faster so when they bypass the slower-moving outer planets it appears that the outer planets are moving backward

KEPLER’S FIRST LAW Johannes Kepler demonstrated that each planet orbits the Sun in

a shape called an ellipse, rather than a circle An ellipse is an oval shape that is centered on two, rather than

one, points The two points are called the foci The major axis is the line that goes between the two foci

Astronomical UnitEach planet’s elliptical orbit is a different

shape and size, with the Sun always being at one focus

For each Sun – planet pair, half of the length of the major axis is called the semi-major axis

The average distance between the Sun and the planet is measured in astronomical units (1.496 x 108)

Earth is 1 AU from the Sun

Eccentricity A planet in an elliptical orbit is not at a constant distance

from the Sun

When a planet is closest to the Sun in its orbit it is said to be at perihelion

When a planet is furthest from the Sun in its orbit it is said to be at aphelion

The shape of a planet’s elliptical orbit is defined by eccentricity, which is the ratio of the distance between the foci to the length of the major axis

The length of time it takes a planet or other body to travel a complete elliptical orbit around the Sun is referred to as the orbital period

KEPLER’S SECOND AND THIRD LAWS• Kepler’s second law states that an imaginary line between the

Sun and a planet sweeps out equal amounts of area in equal amounts of time.

• Kepler derived a mathematical relationship between the size of a planet’s ellipse and its orbital period. He found that the square of the orbital period (P) equals the cube of the semmi-major axis of the orbital ellipse (a).

• This mathematical relationship (P2 = a3) is Kepler’s third law. (P refers to a unit of time measured in Earth years and a is a unit of length measured in astronomical units.)

Gravity and OrbitsIsaac Newton developed an understanding

of gravity by observing the Moon’s motion, the orbits of the planets, and the acceleration of falling objects on Earth.

Newton’s statement of the relationship among the masses of two bodies and the force and distance between them is known as the Law of Universal Gravitation.

This law is stated as follows:◦Every pair of bodies in the universe attract each

other with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them

29.2 The Terrestrial PlanetsThe inner four planets

Close to the size of Earth

Each have rocky, solid surfaces

Mercury, Venus, Earth, and Mars

Mercury

•Closest planet to the Sun•No moons•Approximately 1/3 the size of Earth•In 2 of Mercury’s years, 3 of Mercury’s days have passed•Essentially no atmosphere but what does exist is composed of primarily oxygen and sodium•Largest day / night temperature difference (427 C daytime / -173 C at night)•Surface is covered with craters and plains with a planet-wide system of cliffs•Extensive nickel-iron core

Venus

Second planet from the Sun No moons, highest albedo of all

planets Thick clouds in atmosphere,

composed of sulfuric acid Extremely hot on surface One day is 243 Earth days in length

(slow rotation) Atmosphere is primarily carbon

dioxide and nitrogen Greenhouse Effect causes Venus to

be the hottest planet (although it is not closest to the Sun)

Surface has been smoothed by volcanic lava flows, only a few impact craters

Surface is relatively young with little evidence of tectonic activity

Internal structure is similar to Earth. Considered to be Earth’s “twin”

EarthThird planet from SunUnique properties in

comparison to other planetsWater exists in all three

states (solid, liquid, and gas)

Life is abundant on EarthModerately dense

atmosphere comprised primarily of nitrogen and oxygen

Axis is tilted, which causes seasonal changes

Wobble in rotation on axis, called precession

Only has one moon

Mars

Fourth planet from Sun, outermost terrestrial planet

Often referred to as Red Planet, which is caused by a high iron content in the soil

Smaller, less dense than Earth. Has 2 irregularly-shaped moons – Phobos and Deimos

Atmosphere is thin and turbulent, constant wind and dust storms

Southern hemisphere is heavily cratered and northern hemisphere is dominated by plains

Four gigantic shield volcanoes, largest is Olympus Mons. (Also largest mountain in solar system)

Martian surfaces suggest that liquid water once existed on the surface as there are dried river and lake beds present

Polar ice caps on both poles Hypothesize that the core of Mars is

iron and nickel, possibly mixed with sulfur. Mars has no magnetic field, so it is believed that the core is solid

There is no evidence of tectonic activity on the surface or crust of Mars

29.3 The Gas Giant PlanetsRemaining planets (minus Pluto)

in solar system

Larger than Earth in size

Gaseous, lack solid surface

Jupiter, Saturn, Uranus, and Neptune

Jupiter Largest planet, 11 times larger than

Earth, fifth from Sun Banded appearance, due to a result of

flow patterns in the atmosphere Four major satellites Low density, atmosphere is made up

mostly of hydrogen and helium – which exist in a liquid form

Magnetic field is present Rotation is rapid, spinning on axis in a

little less than 10 hours, making shortest day in solar system

Belts (low, warm, dark-colored clouds that sink) and zones (high, cool, light-colored clouds that rise) are present throughout Jupiter

Great Red Spot – atmospheric storm that has been rotating around Jupiter for more than 300 years

Four major moons composed of ice and rock mixtures.

Discovery of one large ring, 6400km wide, that first proved Saturn is not the only gas giant with rings

Saturn

•Sixth planet from Sun, second largest in solar system•Density is lower than that of water•Rotates rapidly for its size, has several zones and belts (as Jupiter)•Atmosphere is dominated by hydrogen and helium, also contains ammonia ice•Internal structure is likely fluid, with solid core and strong magnetic field•Most intricate ring system in solar system. Rings are composed of rocks and ice that ranges from microscopic to the size of an average house!!•Many satellites, including its largest moon, Titan, and a number of smaller moons•Titan is larger than Earth’s moon and its atmosphere is made of nitrogen and methane

URANUS•Seventh planet from the Sun•Discovered accidentally in 1781•Titania and Oberon are two of larger moons; however, there are several that orbit Uranus’ equatorial plane•Exact moon count is constantly changing•Dark, black rings•4 times as large and 15 times as massive as Earth•Atmospheric composition causes a blue, velvety appearance •Methane gas reflects blue light back into space although most of the atmosphere is composed of hydrogen and helium•Very few clouds present on Uranus and they are difficult to detect due to their similar brightness and color to the surrounding atmosphere•No distinct zones or belts like those observed on Jupiter and Saturn•Internal structure is similar to Jupiter – it is completely fluid with exception to a small, solid core•Strong magnetic field•Rotational axis is tipped so far that the North Pole nearly lies in its orbital plane•It is believed that Uranus was knocked on its side by a massive collision with a passing object

Neptune

• Discovered in 1846• Smaller and more dense than Uranus, but

four times as large as Earth• Bluish color caused by methane in the

atmosphere, atmospheric compositions, temperatures, magnetic fields, interiors, and particle belts

• Distinctive clouds and atmospheric belts and zones similar to Jupiter and Saturn

• Had a persistent storm (The Great Dark Spot) that disappeared in 1994

• Many moons, largest being Triton• Triton has a retrograde orbit (goes

backwards) and a thin atmosphere with nitrogen geysers.

• Six rings composed of microscopic-sized dust particles

Pluto Discovered in 1930 Solid surface, but not classified as

terrestrial due to low density and small size Made of half ice and half rock Pluto is smaller than the Earth’s moon, at only about 1,400

miles wide (That is about half the width of the United States) It takes Pluto 248 years to make one revolution around the

sun. Pluto takes 6 1/2 days to rotate Atmosphere is composed of methane and nitrogen Orbit is eccentric – at aphelion it is 50 AU from the Sun and

at perihelion is it almost 30 AU from the Sun, which means at times it is closer to the Sun than Neptune

Rotational axis is tipped, making its North Pole point “south” of its orbital plane

Satellite – Charon – orbits in Pluto’s equatorial plane

Kicked out…Today, Pluto is called a dwarf planet,

which is round and orbits the sun just like the eight major planets

But unlike a planet, a dwarf planet does not have enough gravity to attract all of the space dust and tiny objects in its path

A dwarf planet also is much smaller than a planet, but it is not a moon

Pluto is in a region called the Kuiper BeltThe Kuiper Belt is a large band of thousands of small, icy objects that orbit the sun beyond Neptune

In 2003, U.S. astronomer Mike Brown discovered a new object beyond Pluto. He thought he had discovered a new planet because the object, which he named Eris, was larger than Pluto.

The discovery of Eris caused other astronomers to talk about what makes a planet a "planet,” and it was determined that Pluto and objects like it were not really planets due to their size and location in the solar system. It was determined that these objects should now be referred to as dwarf planets.

Pluto also is called a plutoid, which is a dwarf planet located further out in space than Neptune.

CURRENT PLUTOIDS

29.4 Formation of our Solar SystemWe use Earth based observations

and data from probes to derive theories about how our solar system formed

Significant observations include the shape of our solar system, the differences among the planets, and the oldest planetary surfaces, asteroids, meteorites, and comets

A Collapsing Interstellar CloudStars and planets form from clouds of gas and

dust, called interstellar cloudsInterstellar clouds consist primarily of helium

and hydrogenThey often appear as blotches of light and dark

spots, due to the dust particles being partially illuminated by the reflection from stars

The density of an interstellar cloud is very low (due to gas content) but they can start to condense due to gravity.

The cloud then becomes concentrated enough to form a star and possibly planets.

Astronomers agree that our solar system began this way.

ORION’S NEBULA IS ONE OF THE MOST BRIGHT AND BEAUTIFUL INTERSTELLAR CLOUDS VISIBLE TO THE NAKED EYE WITHIN THE NIGHT SKY

Sun and Planet FormationThe disk of dust and gas that formed the

Sun and planets is called a solar nebulaThe condensed concentration at the center

eventually became the Sun As the condensing slowed, tiny bits of

condensed materials accumulated and merged together to form larger bodies called planetesimals.

Further growth of planetesimals continued, through mergers and collisions, sometimes destroying them and making smaller bodies (still relatively large) we now call planets

Order of Formation In the outer solar system, the first large planet to develop

was Jupiter. As Jupiter increased in size through the merger of icy planetesimals, gravity began to attract additional gas, dust, and planetesimals… making it grow even larger.

Saturn and other gas giants then formed in similar patterns, but could not grow as large because Jupiter had collected so much material in the vicinity.

As each gas giant gathered material from its surroundings, a disk formed within the equatorial plane – much like that of the early solar system – which then formed satellites

Inner planets then began forming in the same fashion; however, the gravitational pull of the Sun swept up much of the gas in the area and prevented them from acquiring much additional materials. This is believed to be the reasoning why they are so much smaller in size than the gas giants.

Asteroids Thousands of bodies that orbit the Sun

within the planetary orbit are called asteroids

Range in size from a few km to about 1000 km in diameter

Pitted, irregular surfaces Most are located in the asteroid belt,

between Mars and Jupiter Asteroids are thought to be leftover

planetesimal pieces from the time of the solar system’s formation

As they orbit, they occasionally collide and cause broken fragments to fall toward other planets (such as Earth)

When interplanetary material falls toward Earth and enters Earth’s atmosphere it is called a meteoroid

When the meteoroid falls toward Earth but burns up in the atmosphere it is called a meteor

If the meteoroid does not completely burn up and part of it collides with the ground it is called a meteorite

Comets Small, icy bodies that have highly eccentric orbits around the Sun

Made of ice and rock, ranging from 1 to 10km in diameter

Head of comet is called the coma and the center, solid core is called the nucleus

Sometimes, a comet intersects with Earth and a meteor shower occurs as result of particles burning up upon entering the Earth’s atmosphere

Two clusters, or clouds, of comets:1. Kuiper Belt – close to Pluto,

30-50 AU from the Sun2. Oort Cloud – more than

100,000AU from the Sun