Exploring the Cosmos: Solar System, Part I

Question 1

Name all the planets in order from the sun.

Answer 1

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Question 2

Where is the asteroid belt located?

Answer 2

Between the terrestrial and Jovian planets, between the orbits of Mars and Jupiter.

Question 3

Three things that define a planet

Answer 3

1- Must be in orbit around the Sun
2- Must be large enough to take on nearly
spherical shape
3- Must have “cleared its orbit”

Question 4

Name the terrestrial Planets and describe their densities and surfaces

Answer 4

Mercury, Venus, Earth, Mars:
•have high density
•have solid rocky surfaces and dense metal cores

Question 5

Name the Jovian planets and describe their densities and surfaces

Answer 5

Jupiter, Saturn, Uranus, Neptune:
•Large, gaseous, low density, no well-defined surface

Question 6

Name the five dwarf planets in our solar system and where are they found.

Answer 6

1- Ceres: largest object in Asteroid Belt (diam = 975 km)

2- Pluto: largest object in Kuiper Belt (diam = 2306 ± 20 km)

3- Eris (2003 UB313): largest object in Scattered Disk
(diam = 2326 ± 12 km)

4- Haumea

5- Makemake

Question 7

Where is the Kuiper belt located?

Answer 7

just beyond the orbit of Neptune

Question 8

What forces shape the solar system?

Answer 8

Gravity:
Dominant force over long distances (by a big margin!)

•Electrical and Magnetic Fields:
Very weak effect on planets etc:
affect only very small electrically charged particles

•Radiation Pressure and Solar Wind:
Affect only atoms, molecules and small dust particles

Question 9

Which direction are orbits when seen from north?

Answer 9

All orbits are anti-clockwise about the Sun when seen from the north.

Question 10

What shape are orbits and what is meant by eccentricities?

Answer 10

Most orbits are almost circular with eccentricities less than 0.1. Exceptions are Mercury (0.2056) and Pluto (0.2484).

Orbit shape classified by eccentricity (e). If circular, eccentricity is zero. If very elongated, eccentricity is close to one.

Question 11

What planetary object has the most anomalous orbit?

Answer 11

Pluto has the most anomalous orbit. Its orbital plane is rotated 17 degrees from the plane of Earth’s orbit.

Question 12

What are Kepler’s three laws?

Answer 12

1- Planets move in an ellipse with the Sun at a focus.

2- The line between the planet and Sun sweeps out an area at a constant rate.

3- The period of the orbit is proportional to a3/2, where a is the semi-major axis of the ellipse

Question 13

According to Kepler’s 2nd law, when would a planet travel faster?

Answer 13

A line between the planet and Sun sweeps out an area at a constant rate :

•Close to aphelion (furthest from Sun), planets travel slower

•Close to perihelion (closest to Sun), planets travel faster

Question 14

Which directions do planets rotate? And what are the exceptions?

Answer 14

When viewed from the north most planets rotate in an anti-clockwise direction about their axes

In the same direction as the general motion around the Sun and the rotation of the Sun.

The rotation axes of the planets are approximately at right angles to the plane of the orbit (obliquity).

Exceptions – Venus, Uranus and Pluto

Question 15

What is obliquity? And how large are the planets obliquity angles, exceptions?

Answer 15

Obliquity: This is the angle that the axis of rotation makes with the normal to the plane of the orbit

Most planets have small obliquity angles.
•Exceptions: Venus, Uranus (and dwarf planet Pluto)
•Venus: retrograde rotation (opposite to other planets)

Question 16

What is De Buffon’s Passing Star Model?

Answer 16

Planets formed from material pulled from the Sun by another star passing close by.

Question 17

Is De Buffon’s Passing star model a catastrophic or evolutionary model?

Answer 17

An example of a catastrophic model – the result of a singular, peculiar event.

Question 18

Why was De Buffon’s model unlikely and what problems would arise from it?

Answer 18

Stars are very far apart: likely to be a very rare event, though this does not mean that it could never happen.

Suggests very few stars have planets.

Problems-
The material pulled from the Sun would be very hot and unlikely to form planets.

Stable, almost circular, orbits would be unlikely.

It is difficult to explain the varying properties of the different planets.

Question 19

Is the Solar Nebula Hypothesis an catastrophic or evolutionary model?

Answer 19

An example of an evolutionary model in which process occurred slowly without requiring an unusual event

Question 20

Describe stage 1 of The Solar Nebula Hypothesis

Answer 20

Stage 1: Cloud of Gas and Dust

The Sun and the planets formed at the same time from a contracting rotating cloud of gas

Question 21

Describe stage 2 of The Solar Nebula Hypothesis

Answer 21

Stage 2: Disc Rotation

As the cloud rotated it became smaller. The disc speeded up and flattened.

Gravity caused a concentration at the centre of the disc which became the Sun.

Further from the centre, grains of dust and molecules of gas start to form other bodies by a process of accretion.

Question 22

Describe stage 3 of The Solar Nebula Hypothesis

Answer 22

Stage 3: Formation of Planetesimals

Initially, accreting bodies too small for gravity to be effective.

Clumps of matter acted as condensation nuclei.

Through electrostatic forces and collisions, the clumps started to coalesce and grow (like a growing snowball)!

Accretion of matter made clumps grow into bodies, ranging in size from a few centimetres to several kilometres - planetesimals.

Gravity began to play an important role.

Question 23

Describe stage 4 of The Solar Nebula Hypothesis

Answer 23

Stage 4: Formation of Protoplanets

Clumps coalesced into larger bodies, about 100 km in size.

These continued to grow to form protoplanets, more massive objects that were the next stage in the formation of the planets.

Largest protoplanets had greatest gravitational attraction: collected other bodies more effectively.

They grew fastest by sweeping up smaller bodies.

Planet sized bodies formed.

Question 24

What does The Solar Nebula Hypothesis successfully explain?

Answer 24

Disc shape of solar system

Why almost everything rotates in the same direction

Question 25

Where did the dust and gas originally come from? For the solar nebula hypothesis

Answer 25

Planets made of many chemical elements.

The Big Bang gave roughly 73% hydrogen, 25% helium and about 2% heavier elements. The Sun still has this composition (almost).

Stars produced heavier elements but only up to about iron from nuclear reactions (fusion)

Supernovae produced heavy elements up to uranium

The nebula was mainly hydrogen and helium with much smaller amounts of the heavier elements.

Question 26

How were the bodies in the solar system formed?

Answer 26

As disc coalesced, central parts would be hottest.

Most central region formed the Sun, hot enough to start nuclear reactions.

Next region formed inner, rocky planets. Still too hot for volatile compounds (e.g. water, methane and ammonia ice).

Cooler outer regions contained outer gaseous planets made of more volatile compounds.

Question 27

How much mass is required for a planet to start capturing gas?

Answer 27

Need about 15 Earth masses to start capturing gas, small planets are rocky and large planets are gaseous.

Question 28

Why do the terrestrial and Jovian planets have different compositions?

Answer 28

Materials with low condensation temperatures were cleared from the inner solar system before it was cool enough for them to condense.

Question 29

What are the Stages in the Formation of a Planet?

Answer 29

1.Accretion and differentiation
2.Period of intense bombardment
3.Flooding by lava and/or water
4.Subsequent slow evolution

Although evidence for all the above are not present on all the planets

Question 30

What happens in the Accretion and Differentiation stage of the formation of planets

Answer 30

Accretion forms protoplanets

Differentiation is the sinking of heavier material to the planet core

Due to gravity (causes heating as potential energy is lost)

Question 31

What happens during the period of intense bombardment stage of the formation of planets?

Answer 31

The planets collect the remaining debris

Heavy cratering on planets

Question 32

How was the solar system cleared of debris?

Answer 32

Debris left over from planet formation cleared by:

•Solar wind and radiation pressure
⁃Ejected gas and fine dust

•Planets - gravity removed larger bodies by
⁃Ejecting them from the Solar System
⁃Sending them close to the Sun
⁃Striking one of the planets

Jupiter was particularly effective at this.

Question 33

What is needed to measure the age of the solar system? How is this worked out?

Answer 33

Age of rocks measured using radioactive elements in them.

The ratio of the amount of the original radioactive material to the amount of its decay products is used.

The sample rock must be undisturbed since it was formed.

The half life (characteristic decay time of a radioactive element) must be long enough, i.e. comparable with the age of the Solar System.

The best estimate for the Solar System is
about 4.6109 years