Download Summary from last lecture

Summary from last lecture
What is a planet? (solar system only for now!)
(a) a body that is in orbit about the Sun
(b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes
a shape under hydrostatic equilibrium
(c) has effectively cleared the neighbourhood
around its orbit.
What is a planet? (solar system only for now!)
(b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes a
shape under hydrostatic equilibrium
=> must be “round” (and not potato-shaped!)
An irregular solid body will deform under
self-gravity to hydrostatic equilibrium when:
internal pressure ~ tensile strength
c.f. Titan + Mercury or Pluto + Moon
Solar system = 8 planets that satisfy a, b, and c
+ 5 dwarf planets: satisfies (a) and (b), but not (c) + not
a satellite
8 planets + 5 dwarf planets
2003 EL61
“Haumea”
2009 Illinois
“Easterbunny”
“Makemake”
What is a planet?
(c) “Has effectively cleared the
neigbourhood around its orbit”
>1
Haumea
0.00067
Makemake
0.00067
Soter 2006; Astrophysical Journal
2.68x10-4
2.22x10-4
>100
0.02
0.02
Mercury from Mariner 10
First messenger flyby in January this year
Happy Birthday NASA!
mercury
• 0.39 AU from the Sun: Kepler’s laws break
down (General relativity)
• e = 0.205
• Large Fe core, no atmosphere
• Huge day -- night temperature variations
• Some evidence of (water) ice near polar caps
• Rotation period 58.6462 (Earth) days = 2/3
orbital period of 87.95 days. 3:2 resonance
mercury
• Rotation period 58.6462 (Earth) days = 2/3
orbital period of 87.95 days. 3:2 resonance
• This 3:2 resonance “spin-orbit resonance” not
due to chance! Simple relation between 2
timescales: day and year.
• Mercury rotates only through 180 deg. between
one perihelion and the next!
• Tidal forces act to synchronise the orbit.
Remember r-3 dependence => at perihelion,
orbital and rotational motion are synchronised
Venus: Earth’s sister(?)
a = 0.73 AU
e = 0.007
P = 225 (Earth)d
M = 0.82 M_E
R = 0.95 R_E
Radar imaging Venus
Venus
Venus from Venera
Venus’ retrograde orbit
• Venus moves around the
Sun in a retrograde
orbit, with its surface
moving about 60x
slower than its upper
atmosphere
Venus’ retrograde orbit
• retrograde orbit: puzzle. All other planets in prograde
orbits!
• Probably due to gravitational perturbations
• Tilt b/w equatorial and orbital planes ~ 177deg!
• As atmosphere developed, it could be significantly
affected by tidal forces, perhaps leading to a damping
effect