Astonishing Astronomy 101

With Doctor Bones (Don R. Mueller, Ph.D.)

EducatorEntertainer

J

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Scientist

ScienceExplorer

Chapter 6The Structure of the Solar System

Components of the Solar System

• The vast majority of the Solar System’s mass resides in the Sun.

• All the planets, asteroids and comets make up less than 1/700 of the mass of the Solar System.

• The rocky inner planets: Mercury, Venus, Earth and Mars are called terrestrial planets.

• The gaseous outer planets: Jupiter, Saturn, Uranus and Neptune are the Jovian planets.

• An asteroid belt separates the inner and outer planets.

• Pluto, once a planet, has been reclassified as a dwarf planet.

Please insert figure 32.1

The Role of Mass and Radius

• Mass and size of a planet help determine its environment.

• Small planets cool quickly, leading to dead worlds.

• Small planets also have trouble holding an atmosphere.

• Larger planets cool slower, and have active interiors and surfaces.

• Mars is right in the middle – not too large, and not too small.

Interiors, Atmospheres and Surfaces of Terrestrial Planets

The Role of Water and Biological Processes

• The presence or absence of water helps determine the nature of the atmosphere.

• Water acts as a sink for CO2, removing the greenhouse gas from the atmosphere.

• Water helps lock CO2 into rock.

• Too much CO2 can lead to a runaway greenhouse effect (e.g., Venus).

• Too little CO2 can lead to cooling (e.g., Mars).

• Biological activity impacts the environment as well.

• Burning fossil fuels releases CO2 into the air.

• Animals remove oxygen from the system and release CO2 and methane.

• Our planet’s O2 comes from the breakdown of water and CO2 by plants.

The Role of Sunlight

• A planet’s distance from the Sun determines how much sunlight it receives.

• Venus receives ¼ of the

energy per square meter that Mercury does.

• Planets in eccentric orbits receive varying amounts of sunlight.

• The axial tilt of a planet determines its seasons.

• Sunlight warms, however the atmosphere has an impact too:– Venus’s atmosphere warms the

surface to 750 K, but it would be very warm even without the CO2.

– Mercury is closer to the Sun, but still cooler than Venus.

– The Moon is cooler than the Earth, even though they are at the same distance from the Sun.

• Sunlight also determines the makeup of the planets:– Inner planets are rock and iron

bodies.

– Outer planets are gaseous.

The Outer Planets

• Far from the Sun: cold enough that water vapor condenses into ice.

• Beyond this frost line, planets are primarily composed of hydrogen and ices.

• The low temperatures allowed the inner planets to capture hydrogen and helium gas and to grow to immense sizes.

• The outer planets have no surfaces:

– Pressures steadily climb, turning gases into liquids and eventually metals.

Equatorial Bulges

• Jovian planets rotate much faster than terrestrial planets.

• From the principle of conservation of angular momentum.

• Faster rotational speeds make the outer planets much wider at the equator. This leads to the so-called equatorial bulge.

Other Interesting Differences

• Each gas giant has a set of rings:

• Easy to see: Saturn

• Hard to see: Neptune

• Gas giants generate more internal heat than they receive from the Sun.

• The gas giants have many more moons.

Differences Among the Giants

• Strong color differences between the giants are related to their distances from the Sun.

• Ammonia and methane condense at lower temperatures than water, so the chemistry of the outer giants differs from the inner giants.

• The least massive of the giants (Uranus) also seems to generate the least internal heat, again similar to the terrestrial planets.

The Kuiper Belt

Outside the orbit of Neptune lies the Kuiper Belt.

Located around 40 AU from the Sun:

Trans-Neptunian Objects (TNOs) such as Pluto are found here.

Bodies smaller and larger than Pluto are in this region, including the dwarf planet, Eris.

Pluto’s Reclassification: Will the “Real” Pluto please stand up.

• In 1920, Pluto was discovered and classified as a planet.

• Is Pluto a planet? The debate:

A planet must be massive enough:

(1) for its gravity to pull it into a roughly spherical shape, and

(2) for it to have cleared out the neighborhood of its orbit of comparable mass objects.

• This means that the objects lying in both the asteroid and Kuiperbelts are not planets.

• Alas, in 2006, Pluto was reclassified as a dwarf planet.

Pluto?

1920

versus

Pluto?

1930

Opik – Oort Cloud

Ernst Julius Öpik (1893 –1985) was an Estonian astronomer. Jan Hendrik Oort (1900 – 1992) was a Dutch astronomer .

• The Solar System is surrounded by a cloud of cometarybodies:– Located around

50,000 AU from the Sun.

– Gravitational influences from passing stars occasionally send comets into the Solar System.

Please insert figure 32.3

Rotation and Revolution in the Solar System

http://www.youtube.com/watch?v=9R5P9Y9gRYY&feature=related

• Due to the conservation of angular momentum, all planets revolve around the Sun in the same direction and nearly the same plane:

– Mercury’s orbit is tipped by 7 degrees.

• Most of the planets rotate in the same direction:

– Counterclockwise as viewed from above

– Venus rotates clockwise as viewed from above

– Uranus’ rotational axis is tipped significantly

Orbits of all the planets (Including Comets)

http://www.youtube.com/watch?v=NrODEmei-wA&feature=related

The comet Shoemaker-Levy, discovered in 1993, was important because it was the first comet humans witnessed impacting a planet.

Planetary Tilt Angles

Calculating a Planet’s Density

• Calculate the planet’s mass (M) by observing its satellite’s orbital distance (d) and period (P)

• Use Newton’s modified form of Kepler’s 3rd Law:

• If we know the distance to the planet, we can measure its angular diameter and calculate its linear diameter (radius) and then its volume:

• The planet’s average density is then:

2

3

GP

πd4M

3πR3

4V

V

Mρ

Average Densities of the Planets in our Solar SystemInner planets have high average densities (~5 kg/liter): Small bodies of rock and iron.

Outer planets have lower densities (~1 kg/liter): Large bodies of gas and ice.

The Age of the Solar SystemExample: Potassium–Argon dating or K-Ar dating

• A number of naturally occurring atoms undergo radioactive decay.

The time it takes for half of the atoms in a given sample to decay is called the material’s half-life.

After n half-lives, the fraction of original material is:

• We can then use radioactive dating to determine the age of rocks.

The oldest Earth rocks: 4 billion years old.

Older samples have been found on the Moon and in meteorites.

• Bodies in the Solar System whose ages have so far been determined are consistent with having formed about 4.5 billion years ago.

n

2

1Fraction

Formation of the Solar System: Solar Nebula Theory

• The most successful model of Solar System formation is the Solar Nebula Theory:

– The Solar System originated from a rotating, disk-shaped cloud of gas and dust, with the outer part of the disk becoming the planets, and the inner part becoming the Sun.

• 4.5 billion years ago, the cloud of gas and dust that would become our Solar System began to contract.– Contracting and flattening into a disk

that began to spin faster: (Conservation of Angular Momentum)

– Most of the material in the cloud moved to the center to become the Sun.

Planetesimal Formation: From the hypothesis of Viktor Safronov: Stating that planets formed out of dust grains, colliding and sticking together to form larger and larger bodies.

• The inner solar system: silicate crystals and metal grains accreted over time, to form rocky planetesimals: The terrestrial planets.

• In the outer solar system, icy planetesimals formed.

Condensation Temperatures of Major Elements

Element Condensation

Temperature (K)Percent by

Mass in Sun

Percent by Mass

in Earth

Hydrogen 180 (H2O) 70.6 0.0033

Helium 3 27.4 0.00000002

Carbon 80 (CH4) 0.31 0.045

Nitrogen 130 (NH3) 0.11 0.0004

Oxygen 1300 (silicates),

180 (H2O)0.96 30.1

Neon 9 0.18 0.0000000004

Silicon 1300 (silicates) 0.07 15.1

Iron 1400 0.18 32.1

Protoplanets and differentiation

• Planetesimals grew into protoplanets: heated by collisions and by radioactive decay.

• Denser material sank toward the center of the bodies and lighter material floated toward the surface.

• This separation process is called differentiation.

Atmospheric Retention

• Retaining an atmosphere can be a problem.

• Small planets will have low escape velocities.

• Atmospheres around planets close to the Sun will be very warm, giving the gas atoms a high thermal velocity.

• If the thermal velocity of atmospheric gases is close to the escape speed for the planet, the atmosphere can escape into space.

We are Stardust• A supernova or stellar explosion creates anincredibly luminous burst of radiation thatcan outshine an entire galaxy before fadingfrom view. In this short time interval, thesupernova can radiate as much energy asour Sun is expected to emit during its lifespan. The explosion expels the stellarmaterial at velocities approaching that of10% of the speed of light. The shock wavecreated sweeps out an expanding shell ofgas and dust called a supernova remnant.Supernovae, play a critical role in enrichinginterstellar media with higher mass

elements. The heavy elements greaterthan iron that you are made of wereformed in a supernova.

We truly are “stardust.”

The Asteroid Belt: Most asteroids can be found between

the orbits of Mars and Jupiter.

• Using Bode’s Rule (a simple mathematical formula) the asteroid Ceres was discovered between the orbits of Jupiter and Mars

The Shapes and Sizes of Asteroids

• Asteroids come in all shapes and sizes: Big and small.

• Ceres is massive: Large enough to pull itself into a sphere and therefore be classified as a Dwarf planet.

• Most asteroids are small: tens of kilometers across.

• Still large enough to cause tremendous damage if impacting the Earth.

• Spacecraft have only recently visited asteroids.

Vesta

ErosCeres

Asteroid Eros: Potato-shaped

Visiting Comets

Comet Halley visited by

GiottoComet Wild 2:

visited by Stardust

Comet Tempel 1: visited by

Deep Impact

The Origin of Comets• Comets may originate in

either the Oort Cloud or the Kuiper Belt.

• Oort cloud: a cloud of comet-like planetesimalsmore than 100,000 AU from the Sun.

• Oort cloud objects may have formed near the giant planets and then were tossed outwards by gravitational forces.

• Passing stars or other gravitational influences nudge the comets into the inner Solar System.

Please insert figure 47.5