Download PHYS 185 Chapter 5 Highlights 1. Definition of a planet a. Who: The

PHYS 185
Chapter 5 Highlights
1. Definition of a planet
a. Who: The International Astronomical Society (IAU) governs the definition and
naming of planets. Individual members are professional astronomers from all over
the world, at the Ph.D. level or beyond, and active in professional research and
education in astronomy.
b. Why: There was no formal definition! Then came the discovery of other large
objects orbiting the sun, out beyond Pluto:
i. Quaoar (“kwah-oh-are”) in 2002 - 1/2 size of Pluto; kicked off the whole
debate about a planet really was
ii. Eris in 2003 – larger than Pluto and has a moon Dysnomia. So you can’t
define a planet based on it having a moon, unless you make Eris a planet.
iii. Sedna in 2004 - 3X further out than Pluto, bigger than Quaoar
iv. more than 1000 objects found that are at least 1/2 the size of Pluto, some
of them with moons. Are these planets? Kuiper belt objects?
c. Official definition voted on Aug 24, 2006:
A "planet"1 is a celestial body that: (a) is in orbit around the Sun, (b) has
sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the
neighbourhood around its orbit.
1
The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and
Neptune.
d. Pluto was designated as a “dwarf planet”. In June 2008, the IAU established a
sub-category of dwarf planets called “plutoids” – based on size, brightness, and
having an orbit beyond Neptune. Total to date is four: Pluto, Eris, Haumea, and
Makemake.
2. Mercury
a. Has the most extreme temperature changes in the Solar System
i. Very hot days, since close to the sun
ii. Very cold nights, since very slow rotation (nights are long) and very little
atmosphere (not much to hold heat in)
3. Venus
a. Intense cloud cover prevented any pictures of surface for a long time
i. Clouds are made of sulfuric acid!
ii. High pressures (~90 atm, equivalent to swimming 2700 feet underwater
on Earth) crushed early probes
b. Good model for studying Greenhouse Effect
i. CO2 in atmosphere lets sunlight in (transparent to visible wavelengths) but
traps heat in (opaque to infrared wavelengths, “heat” = IR wavelengths)
ii. Has the hottest temperatures of the Solar System: average temperature
hotter than Mercury’s high temperatures
4. Mars
a. Olympus Mons – largest volcano in the Solar System
b. Mars has two moons: Phobos (“Fear”) and Deimos (“Panic”), that look like
potatoes – indicates that their origin was that they were likely captured (irregular
space debris wandering by and caught my Mars gravity)
c. Spirit and Opportunity – rovers on Mars collecting data.
5. Earth
a. atmosphere mostly of nitrogen, with some carbon dioxide (CO2) and oxygen
b. abundant life
c. etc… see chapter 4 for details
6. Jupiter
a. Big ball of gas, mostly H & He, surrounding rocky core
b. Deep inside has liquid metallic H (pressures large enough to squeeze H gas into a
liquid form). “Metal” means electrons are free to roam from atom to atom.
Moving charged particles create magnetic fields, so Jupiter has a very large,
strong magnetic field. If Jupiter’s magnetic field were visible it would fill an area
in our sky about 16x the size of a full moon!
c. 4 Galilean moons, visible with small telescope:
i. Io - orbits in about 1.8 days! Lots of active volcanoes due to strong tidal
forces since so close to a large planet (near the Roche limit; see Saturn)
ii. Europa - may have water – may be a place where life can be found
iii. Ganymede - largest moon in the solar system
iv. Callisto – bears scars of huge asteroid impact
d. Latest count = 63 moons!
7. Saturn
a. Rings are chunks of ice and ice coated rock, from dust to 10 meter size
b. Rings are 89,000 km wide, but only 2 km thick (that’s very thin!)
c. Put all the ring material together and it would make a moon only 250 km in
diameter.
d. “Roche Limit” = closest to a planet a moon can orbit without being ripped apart
by tidal forces. If planet and moon have same density, Roche limit is about 2.4X
the radius of a planet. (Note: Io is getting darned close to its Roche limit around
Jupiter – tidal forces aren’t ripping it apart, but that’s what causes the volcanic
activity.)
e. Origin of rings, some current models
i. Was a moon that strayed inside of the Roche limit
ii. Was a moon destroyed by a collision near the Roche limit
iii. Was material left over from the formation of the planet, but since inside
the Roche limit it could never get together to form a moon
8. Uranus
a. Discovered 1781 by Herschel – was charting faint stars and noticed this one
moved against the background of stars
b. Appears blue-green due to a methane atmosphere – methane absorbs long
wavelength light (red), leaving short wavelengths (blue) to reflect
c. Appears “tipped over” – axis of rotation is over 90° to ecliptic plane – maybe a
violent collision in the early days of the solar system??
9. Neptune
a. Most moons are irregular = probably captured. Triton shows retrograde motion =
orbits around the planet backwards = probably captured. Triton is slowly spiraling
inward so will eventually get ripped apart by tidal forces.
b. Discovery was based on applying mathematics and Newton’s Laws of Motion
i. Uranus’ orbit didn’t seem quite right
ii. 1840’s, mathematicians Adams & Leverrier independently calculated
where another planet should be, beyond Uranus, to affect Uranus’ orbit
iii. 1846, Astronomer Galle found Neptune, right where the mathematicians
said it should be, on the first try!
10. Pluto
a. Discovered 1930 by Tombaugh, really by luck. It was thought that there were
discrepancies in Neptune’s orbit, so people tried to do calculations to find another
planet. There really were NOT major discrepancies, but it got people out looking
for a new planet. Of course, now it’s not a planet!
b. Pluto is made up of ice & rock
c. Charon, discovered 1978. It’s relatively close to same size as Pluto, they are close
= Pluto-Charon are 1/20th of the distance from Earth to Moon.
d. Pluto and Charon orbit around a point between them, their center of mass
e. Current debate: Is Charon a moon? Are Pluto & Charon a binary system?