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Phys 1830: Lecture 22
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test marks posted outside my office
other marks posted as available
Previous Class:
Visualization: Computer Simulations
Planetary Systems
This Class: Took up term test #2
Solar System
Next Class:
Solar System Formation
Tour of the Solar System
ALL NOTES COPYRIGHT JAYANNE ENGLISH

2. Solar System Overview: Planet Definitions
Classical Planet
Orbits the sun.
Massive enough that is own gravity has caused its shape to be nearly spherical.
It has “cleared the neighbourhood” around its orbit of other bodies.
i.e. either by colliding with (sweeping up) the debris in the disk or by gravitationally kicking the debris out of its path (slingshot effect).
This leaves us with 8 planets.

3. Solar System Overview: Planet Definitions
Dwarf Planet
Orbits the sun.
Massive enough that is own gravity has caused its shape to be spherical.
Is not a satellite of another body.
(Has not cleared its neighbourhood.)
Exact wording is at . See International Astronomical Union Resolution 5A.
Examples:
Pluto
Eris (1.3 * Pluto’s mass)
Ceres (in the asteroid belt)
Objects at Neptune and beyond are called Trans-Neptune Objects (TNO) and those TNO that are similar to Pluto are called plutoids.

4. revolve & rotate in the same direction as other planets?
Solar System Overview: What does the class already know about the classical planets?
For each planet:
revolve & rotate in the same direction as other planets?
primarily composed of rock or of gas? # Earth Masses, # Earth radii
small or large? (i.e. closer to Earth size or Jupiter size?)
in outer region or inner region of solar system?
hot or cold? surface T in Kelvin
Lots of moons?
Any other details are welcome  (eg. Does it have rings? B field?)
If a planet spins in the same direction as its orbit, its spin is called “prograde”. If a planet spins in the opposite direction to its orbital motion, that spin is called “retrograde”.
Note about T:
-273 C = 0 K

5. Solar System Overview: Material for our contest!
How do we know these things?
The first 8 are planets.
Note the second column.

6. Keplerian Rotation Curve.
Solar System Overview
Keplerian Rotation Curve.

7. Solar System Overview The density in kg/m
If the density of an object is less than that of water, then it will float.
The density in kg/m
1000 for water; anything less than this floats in water.
for rocks and 8000 for iron
Note the 2nd last column. Note the density of Earth.
Which planets have densities like rocks/iron? Float on water? 3

8. Pluto, because it is the smallest planet.
Solar System Overview
This classical planet would float if there was a big enough bathtub to put it in.
Pluto, because it is the smallest planet.
Earth, because it has so much water anyway.
Europa, because it is icey.
Saturn, because its density is less than water.

9. Solar System Overview:
How do we know what we know about our solar system?
Distances
Diameters
Masses
Densities
You actually have the tools to measure these things on your own.

10. Distances from the Sun:
Solar System Overview
Distances from the Sun:
Radar
Kepler’s Third Law (empirical)
Orbits of planets are nearly circular  use Newton’s Laws for a circular orbit of radius “r”. M is mass of sun.
Recall:
Empirical == a law determined from observations rather than theory. Newton provided the theory.
The mass of the sun is known also from the v squared equation.
Note: F_inertia is the same as F_orbit.
Rearrange:
Need velocity to get radius.

11. Solar System Overview: Distances
Velocity of Planets:
v = distance/time
Distance: The length of the path of the orbit is the circumference of a circle.
Time: The time to travel the full orbit is the Period “P”.
Substitute in for distance and time:

12. Solar System Overview: Distances
Substitute v squared into the equation for radius:
Just need to observe the Period to get the distance!

13. Solar System Overview: We have the distance!
Kepler’s Third Law
-- for ellipses
Observe the Period!
a == semi-major axis
Using a in au and P in Earth years.
Orbit of planet
Kepler derived this empirically – that is, he didn’t have a theory about why it happened but he observed that the planets:
a) travelled in ellipses
“a” is the semimajor axis of an ellipse with the sun at one focus.
b) there was a relationship between “a” and “P”
sun a

14. Keplerian Rotation Curve.
Solar System Overview
Using P he could calculate a velocity and plot the rotation curve for the Solar System.
Keplerian Rotation Curve.

15. 2. Diameters - from lecture 4
Solar System Overview
2. Diameters - from lecture 4
Linear diameter angular diameter =
2 pi * Distance degrees
2 pi * Distance
linear diameter = * angular diameter
360 degrees
(This could be on the test.)
This can be used to measure the radius of an orbit of a moon around another planet too.
We can get the distance between a planet and Earth by using step 1 to get its distance to the sun and then using geometry to get the distance to the Earth. (In contemporary times, we can use radar.) Then we just need to measure the angular diameter and we have the size of the planet.
Also for radius of orbits.

16. 3. Masses Solar System Overview Rearrange:  use step 2 procedure.
Of course this is a way to get the mass of the sun as well, using “a” = “r” and v from the orbit of a planet.
There are man-made satellites that have visited Venus and Mercury.
Do this with a satellite around the planet. For example the moon around the Earth. Then “r” is the Earth-Moon distance and M is the mass of the Earth. Velocity v is determined from the Period of the moon’s orbit (e.g. 1/12 of a year).

17. 4. Density Solar System Overview And for a sphere: So
(Where R is radius.) So
Density is often represented by the Greek letter “rho” .
Planets are almost spherical. Slightly flattened at the poles.
We know the planet’s mass from step 3. and the diameter from step 2
 Mass from step 3.
diameter
from step 2.

18. Solar System Overview: That is how do we know some of these values
You could actually calculate these things!
Note that a table like this shows us that Jupiter and the outer planets have lower density than the inner planets.

19. Does it revolve in the same direction as the other planets?
Solar System Overview: What does the class already know about the classical planets?
For each planet:
Does it revolve in the same direction as the other planets?
Is it primarily composed of rock or of gas?
Is it small or large? (i.e. closer to Earth size or Jupiter size?)
Is it in the outer region or inner region of the solar system?
Is it hot or cold?
Lots of moons or few?
Any other details are welcome  (eg. Does it have rings?) B field?
Mass and Radius only relative to Earth.
Temperature only in Kelvin.
If a planet spins in the same direction as its orbit, its spin is called “prograde”. If a planet spins in the opposite direction to its orbital motion, that spin is called “retrograde”.

20. Mercury Impact craters
Messenger: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Impact craters
Evidence of lava flows (volcanic activity) in smooth parts
Surface Temperature K

21. Venus: Venus Express/European Space Agency Ultraviolet Image
White regions are sulfuric acid clouds.
Surface Temperature 730K
spin backwards

22. Earth Notable that it has a moon, life (possibly intelligent), etc.
Mean Surface Temperature 290K (290 – 273 = 17C)
1 moon

23. Mars Mars Express/European Space Agency Hebes Chasma
High Resolution Stereo Camera aboard Mars Express. Four colour filters.
Mean Surface Temp 210K … about -50C around the equator.
2 moons

24. Jupiter New Horizons/NASA IR image.
The moon in the image is IO – this is a montage. 3 IR bands  white Red Spot.
Temperature at cloud tops: 124K
At least 61 moons

25. Saturn
Cassini/NASA
97K at cloud tops
At least 31 moons

26. Uranus “True” Colour False Colour Voyager2/NASA Rolls in its orbit
Mean Surface Temperature 58K
At least 27 moons

27. Neptune Voyager2/NASA Blue since methane absorbs red light.
Mean surface temperature 59K
At least 13 moons.

28. Which is the hottest planet in the solar system?
Question:
Which is the hottest planet in the solar system?
Mercury because it is closest to the sun.
Venus because its clouds cause a runaway greenhouse effect.
Jupiter because it is like a mini-star.
Venus is roughly 750K