Chapter 7 The Solar System. Components of the Solar System

Chapter 7

The Solar System

Components of the Solar System

The Sun

• The Sun is a star, a ball of incandescent gas whose light and heat are generated by nuclear reactions in the core. It’s mass is more than 700 times the mass of everything else in the solar system put together.

The Sun-2

The Sun’s gravitational force holds the planets other bodies in the solar system in their orbital patterns.

The Sun is mostly hydrogen, 71%, and helium, 27%.

It also contains very small proportions of all of the other elements.

The Planets

• The planets are much smaller than the Sun and orbit around it.

• They emit no visible light of their own but shine by reflected sunlight, a property known as albedo.

• The planets move around the Sun in approximately circular orbits, all lying on nearly the same plane.

The Planets-2

• The Solar System is like a spinning disc with the planets traveling around the sun in the same counterclockwise direction.

• Because of this, the planets appear to lie in a line in the sky.

• As the planets orbit the Sun, each spins on its rotation axis.

• The spin is generally in the same direction.

The Planets-3

• The tilt of the rotational axes relative to the planetary orbit is not far from perpendicular.

• There are 2 exceptions, Venus and Uranus. Their tilts are extremely large.

• The flattened structure and the orderly orbital and spin properties of the Solar System are 2 of its most fundamental features and any theory of the Solar System must explain them.

• A third, but equally important feature is that the planets fall into 2 families called inner and outer planets.

The Planets-4

• The inner and outer planets are classified based on their size, composition and location in the Solar System.

Two Types of Planets

• Inner Planets

• Mercury

• Venus

• Earth

• Mars

• Outer Planets

• Jupiter

• Saturn

• Uranus

• Neptune

Two Types of Planets-2

• The inner planets are small rocky bodies with relatively thin or no atmospheres.

• The outer planets are gaseous, liquid or icy. They have deep, hydrogen-rich atmospheres.

• The term “rocky” for the inner planets means material composed of silicon and oxygen (SiO2-sand) with a mixture of other elements such Al, Mg, S and Fe.


• By ice, we mean frozen liquids and gases, not just frozen water.

• This would include CO2, NH3, CH4 etc.

• Rock is rare in the Solar System by percentage because of the abundance of Hydrogen compared to Silicon.

• The inner part of the system is mostly rock because of the heat of the sun.


• The gases and liquids cannot condense to mix with the Silicon in the heat.

• The outer planets generally have no true surface.

• The atmospheres of the outer planets thicken with depth and eventually liquefy.

• There is no distinct boundary between atmosphere and crust.


• Deep in the interior, the liquid may compress enough to form a solid.

• The transition from liquid to solid is no sharply defined.

• We probably will never land on Jupiter, we would simply sink deeper and deeper into its interior.


• The inner planets are sometimes referred to as the “Terrestrial” or Earth-like planets.

• The outer planets are sometimes referred to as the “Jovian” or Jupiter-like planets.

Satellites

• Many of the planets have satellites themselves.• Only Mercury and Venus do not have moons.• The moons usually move in a roughly circular

path around the equator of the planet.• Only the moons of Uranus and Pluto are not

near the equatorial plane, an important clue to the origin of the moons.

• Jupiter, Saturn and Neptune have large families of moons, 62,31 and 27 respectively.

Asteroids and Comets

• Asteroids and comets are much smaller than planetary bodies.

• Asteroids are rocky or metallic bodies with diameters that range from a few meters up to about 1000 km.

• Comets are icy bodies about 10 km or less in diameter.

• Comets grow huge tails of gas and dust as they approach the sun.

Asteroids and Comets-2

• The comets are partially vaporized by the flow of energy from the sun.

• Their composition puts them into 2 families, much the same as the planets.

• Asteroids and comets also differ in their location within the Solar System.

• Most asteroids are found in a large gap between Mars and Jupiter called the “asteroid belt”.


• It may be material that failed to aggregate into a planet as a result of disturbance by the gravity of Jupiter.

• Most comets are found in an orbit far beyond Pluto in an area called the Oort cloud.

• It is named after the Dutch Astronomer who proposed its existence.


• It completely surrounds the Solar System in a spherical region 40,000 to 100,000 astronomical units from the sun.

• Even though most comets originate in the Oort cloud, some may come from a disk-like swarm of icy objects that lies just beyond the orbit of Neptune and extends to about 60 au from the Sun.

• This area is called the “Kuiper Belt”.• Together the cloud and belt may hold 1012 comet

nuclei.

Composition Differences

• Astronomers can deduce a planet’s composition in several ways.

• From the planet’s spectrum we can measure its atmospheric composition and get some information about the nature of its surface rocks.

• The spectrum does not give a clue about internal composition.

Composition Differences-2

• Earthquake waves could give us information, but to date we do not have any working detectors on the inner planets.

• The outer planets provide a different problem because of the lack of surface.

• Density is another way to give us an idea of composition.

Density as a Measure of a Planet’s Composition

• By using Newton’s modification of Kepler’s 3rd law, we can determine the mass of the planet by observing the effect of the mass of the planet has on an orbiting body such as a moon.

• Volume can be determined by measuring the planets radius.

• (V=4πR3/3; R is the planets radius)

Density as a Measure of a Planet’s Composition-2

• Radius can be measured by angular size and distance.

• With both mass and volume, we can determine density of the planet. (D=M/V)

• Once the planet’s average density is known, we can compare it with the density of abundant candidate materials.

• Earth has a density of about 5.5, about intermediate between silicate rock (3 g/cm3) and iron (7.8 g/cm3).


• Because of this data, we can deduce that the Earth has a silicate rock surface with and iron core.

• This has also been confirmed using earthquake information.

• Density comparison is a powerful tool to use to study planetary composition, but it also has drawbacks.


• There may be similar substances that might match the given density.

• The density of a material can be affected by a planet’s gravitational force.

• A massive planet may crush rock whose normal density is 3 to a density of 7 or 8.

• All of the terrestrial planets have a density similar to that of the Earth (3.9-5.5).


• The jovian planets have a density that is much smaller (0.71-1.67), about that of ice.

• After correcting for gravitational compression, we can conclude that all of the inner planets contain large amounts of rock and iron and that the iron has sunk to the core.


• The outer planets contain mainly light materials such as hydrogen, helium, methane, ammonia and water.

• The outer planets probably have core of iron about the size of the Earth, beneath their deep atmosphere.

• Astronomers deduce the existence of the cores in 2 ways.


• If the outer planets have the same relative amounts of heavy elements as the Sun, they should contain several Earth masses of iron and silicates.

• Because they are much more dense than the gases that make up the majority of the planet’s mass, they sink to form the core.

• Analysis of rotational data shows that the equatorial bulges can best be explained if they have small dense core.


• Composition studies show the differences between families but also furnishes astronomers with another clue to the origin of the planets.

• The Sun and the planets were made from the same material.

• Because Jupiter and Saturn have a composition nearly identical to the Sun, and the inner planets have a similar composition if you remove the hydrogen and helium component, we can conclude that the process must keep the inner planets from capturing the light gases.

Bode’s Law

(0+4)/10

(3+4)/10

(6+4)/10

(12+4)/10

0.4

0.7

1.0

1.6

Mercury

Venus

Earth

Mars

0.39

0.72

1.00

1.52

(24+4)/10

(48+4)/10

(96+4)/10

(192+4)/10

2.8

5.2

10.0

19.6

?

Jupiter

Saturn

Uranus

?

5.2

9.5

19.2

Bode’s Law

• Bode’s Law is a curious relationship, that is stilled unexplained.

• The gap in the prediction corresponds nicely with the location of the asteroid Ceres, first discovered by Giuseppi Piazzi.

• Verification of Bodes Law will come when we can establish the same relationship in other solar systems.

Age of the Solar System

• In spite of the differences in size, structure and composition, the planets, asteroids and comets all seem to have formed at nearly the same time.

• We can directly measure the Earth, Moon and some asteroids from the radioactivity of their rocks (about 4.6 billion years old).

• The Sun’s age is similar based on it’s current brightness and it’s presumed rate nuclear fuel consumption.

Origin of the Solar System

Origin of the Solar System

• Observations that have to be accounted for when formulating a theory about the origin of the solar system.

1. The Solar System is flat, with all of the planets orbiting in the same direction.

2. There are 2 types of planets, inner and outer, with the rocky ones near the Sun and the gaseous or liquid ones farther out.

Origin of the Solar System-2

3. The composition of the outer planets is similar to the Sun’s while that of the inner planets is like the Sun’s minus the gases that condense only at low temperatures.

4. All of the bodies in the Solar System whose ages have so far been determined to be 4.6 billion years old.


• Other details that could also be explained are structure of asteroids, number of craters on planetary and satellite surfaces as well as detailed chemical composition of surface rocks and atmosphere.

• The currently favored theory for the origin of the Solar System derives from the theories proposed in the eighteenth century by Immanuel Kant and Pierre Simon LaPlace.


• They independently proposed the “Solar Nebula Hypothesis”.

• The Solar System originated from a rotating flat disk of gas and dust, with the outer part of the disk becoming the planets and the center becoming the Sun.

• This theory offers a natural explanation for the flattened shape of the system and the common direction of motion of planets around the Sun.

Interstellar Clouds

• Interstellar clouds are the raw material of the Solar System.

• The clouds are found in many sizes and shapes .

• The one that formed the Solar System was probably a few light years in diameter and contained about twice the present mass of the Sun.

Interstellar Clouds-2

• If the cloud was typical of today’s clouds, it contained about 71% hydrogen and 27% helium with tiny traces of other elements such as carbon, oxygen and silicon.

• In addition to the gases, interstellar clouds also contain tiny dust particles called interstellar grains.


• Interstellar grains range in size from large molecules to micrometers or larger.

• They are believed to be made of a mixture of silicates, iron compounds, carbon compounds and water frozen into ice.

• This is determined by analyzing the spectrum of light that passes through the cloud.


• The cloud began its transformation into the Sun and planets when the gravitational attraction between the particles in the densest part of the cloud caused it to collapse inward.

• The collapse may have been triggered by a star exploding nearby or by a collision with another cloud.


• The infall was not directly to the center.• Because the cloud was rotating, it

flattened.• Flattening occurred because rotation

retarded the collapse perpendicular to the rotation.

• A similar effect occurs when pizza dough is flattened by tossing it into the air with a spin.

Formation of the Solar Nebula

• It probably took a few million years for the cloud to collapse and to become the rotating disk with a bulge in the center.

• The disk is called a solar nebula.• The disk eventually condensed into the

planets and the bulge into the Sun.• This helps to explain the property of the

solar system with the planets on the same plane.

Formation of the Solar Nebula-2

• The solar nebula was probably about 200 AU in diameter and 10 AU thick.

• These measurements seem consistent with stars and disks around them.

• The stars in the center are not yet hot enough to emit visible light

Condensation of the Solar Nebula

• Condensation occurs when a gas cools and its molecules begin to stick together to form liquid or solid particles.

• The gas must cool below a critical temperature.

• If we cool a cloud of vaporized iron (2000 K) to 1300 K, tiny flakes of iron will condense from it.

Condensation of the Solar Nebula-2

• If we condense a cloud of silicates to about 1200 K, it will begin to condense.

• At lower temperatures, other substances will condense.

• As the vaporized material cools, its molecules move more slowly, so that when they collide, they bond.

• They first bond into pairs, then clumps and eventually into droplets.


• As the droplets cool at different temperatures, they begin to solidify.

• If the temperature does not drop low enough, the material fails to condense.

• If the temperature does not drop below 500 K, water will not condense and only the silicates and iron form.

• This kind of condensation occurred in the solar nebula.


• The Sun was so hot in the inner disk that water could not condense nearly out to the orbit of Jupiter.

• Iron and silicate on the other hand were able to condense throughout the system.

• The nebula became divided into 2 regions; the inner region with iron-silicate particles and the outer region with similar particles plus ice.


• Water, hydrogen and other easily vaporized materials were present as gases in the inner solar system.

• These gases chemically combined with the silicates so that the rocky materials contained the gases in small amounts.

Accretion and Planetesimals

• In the next stage of planet formation, the tiny particles condensed from the nebula must have begun to stick together into bigger pieces. This process is called accretion.

• The process is similar to building a snowman.• In the solar nebula, the tiny grains stuck together

and formed bigger grains that grew into clumps.• Subsequent collisions, if not too violent, allowed

the objects to grow in size from millimeters to kilometers.

Accretion and Planetesimals-2

• The larger objects are called planetesimals.

• Because some of the planetesimals formed near the Sun and some far enough away to also include ice in their structure.

• There are 2 forms; the rocky iron-silicates and the icy-iron-silicates.

Formation of the Planets

• As planetsimals moved within the disk and collided with one another, planets formed.

• Some collisions led to the shattering of the 2.• Some collisions led to a merging of the 2 bodies.• There orbits gradually became nearly circular.• These orbits are similar to the orbits given by

Bode’s Law.• Merging of the planetesimals increased their

mass and thus their gravitational attraction.

Formation of the Planets-2

• This increased their attraction for other particles.• The planet forming process probably took place

over about a 100,000 year span of time.• The planet growth occurred more rapidly in the

outer part of the solar system.• There was more material to work with because

ice was about 10 times more abundant than the other materials.


• The planetsimals in the outer solar system could be 10 times as large as the inner bodies.

• At some point, the planet was massive enough to attract and retain gas by its own gravity.

• Since hydrogen and helium were very abundant in the solar nebula, planets that were large enough could tap the reservoir and increase their size even more.


• Jupiter, Saturn, Uranus and Neptune may have begun as Earth-sized and then grew due to the attraction of the additional gases.

• As planetesimals struck the growing planets, their impact released gravitational energy that heated both of the bodies.

• Planetesimals hitting the crust give energy to the particles that appears as heating.


• This heat in combination with radioactive heating served to melt the planet and allowed the material with a higher density to sink to their cores.

• Lower denser material, such as silicate rock floated.

Direct Formation of Giant Planets

Formation of Moons

• The moons of the outer planets probably formed from planetesimals orbiting the growing planets.

• Once a body became massive enough that it’s gravitational force could draw in additional material, it became ringed with debris.

• Moon formation in the outer planets is probably a scaled-down version of planet formation.

• The outer moons have the same regularities as the planets around the Sun.

Formation of Moons-2

• All 4 outer planets have a flattened satellite system, in which, with few exceptions, the satellites orbit in the same direction.

• Many of these satellites are about as large as Mercury and would be considered planets if they were orbiting the Sun.

• Some of them even have atmospheres.

Formation of Moons-3

• They do not have enough mass to maintain the gases of the solar nebula (i.e. Hydrogen and Helium), so they are mostly composed of rock and ice.

• Because of the solid surface, these moons can become cratered and show signs of volcanic activity.

Final Stages of Planet Formation

• The last stage of planet formation was a bombardment of planetesimals that created the huge craters we see throughout the solar system where we find solid surfaces.

• Sometimes the incoming planetesimals was so large that it did more than leave a crater.

Final Stages of Planet Formation-2

• Our Moon may have been created as a result of a collision with a Mars-size body.

• Mercury may have lost its crust due to a massive collision.

• Uranus may be atilt because of a collision as well.

• Most of the planetesimals were consumed in the process, but some survived to form small moons, comets and asteroids.

Final Stages of Planet Formation-3

• Many of the remaining bodies and their fragments remained between Mars and Jupiter, unable to accrete because of the huge impact of Jupiter’s gravity.

• The icy planetesimals have their orbits disrupted by the giant planets are flung in towards the Sun and some outwards forming the Oort cloud.

Formation of Atmospheres

• Atmospheres are the last part of the planet-forming process.

• The 2 types of planets form atmospheres differently.

• The outer planets probably captured most of their atmosphere from the solar nebula.

• The inner planets were not massive enough or too hot to capture gas from the solar nebula.

Formation of Atmospheres-2

• Venus, Earth and Mars probably created their original atmospheres from volcanic eruptions and retaining gases from infalling comets and icy planetesimals that vaporized on impact.

• Bodies too small to small to have captured atmospheres directly, but show clear signs of volcanic activity have atmospheres.

Formation of Atmospheres-3

• More quiescent ones do not.

• Small bodies such as Mercury and our Moon essentially show no atmosphere because of weak gravitational force being unable to hold an atmosphere.

Cleaning Up the Solar System

• The solar system assembled the planets over a relatively short period of time (hundreds of thousands to a few million years).

• The rain of planetesimals continued for a much longer period.

• The last thing that happened in the process was the removal of the residual gas and dust.

Cleaning Up the Solar System-2

• The intense heat of the Sun swept the gas and debris with the solar “gusts” to the fringes of the solar system.

• The gusts were super-heated blasts of gas escaping from the young, less-structured Sun.

• Gas flows like this are seen mostly in young stars.

• Similar occurrences still happen today, but less often than the early days.

Cleaning Up the Solar System-3

• There are still questions to be answered (and asked) about the origin of the Solar System.

• How can we confirm the theory?

• The best way would be confirmation of the theory by observing other stars just beginning the process.

Other Planetary Systems


• We are very interested in searching for extra-solar planets.

• The hope is that in finding these far-flung planets we will be better able to understand how our own planet formed.

• Directly seeing these planets is currently impossible.

• We do have other methods that will allow us to “see” their presence.

Other Planetary Systems-2

• The search is on to find systems that are just beginning to form, according to the solar nebula hypothesis.

• They should be surrounded by a disk of dust and gas many AU across.

• Recently, disks like these have been found.• Some of them even have “lumps” which may

represent the first clustering of planetesimals.


• In some cases the disks show a strong ring, perhaps caused by planets that have already formed.

• The disks exist, but do the planets?

• Most present evidence is from the effect of the planets on the star they orbit.

• Newton’s 3rd Law explains the “wobble” that we see when we observe some stars.


• The wobble of the star creates a Doppler shift in the star’s light that astronomers can measure.

• From the shift and its change in time, astronomers can deduce the planet’s orbital period, mass and distance from the star.

• Based on current data collected, the systems that have been discovered are very different from ours.

• Most of these planets are very large and orbit close to their star.


• Does this disprove our model of solar system formation?

• Maybe. But more likely it is simply a by-product of how we are searching.





Migrating Planets

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Chapter 7 The Solar System. Components of the Solar System