Telescopic Discoveries

• 1659 Saturn’s rings• 1758 Uranus• 1846 Neptune• 1801 Largest

Asteroid ‘Ceres’• 1930 Pluto• Many Planetary

Moons.Spectacular planetary

alignment in April 2002

Our Solar System

• One Star: The Sun• 9 Planets (including Pluto)• 165 Moons (to date)• 8 Asteroids >100 Kuiper Belt

objects > 300km in size• 10,000’s smaller asteroids• Numerous comets• Countless Meteoroids (<100m across)

Hale-Bopp

Sun and Planets to Scale

What is a Planet?

• New rules to define a “planet”:

(a) It must orbit the Sun (obviously!),(b) It must be big enough for gravity to

squash it into a (nearly) ‘round’ ball, (c) It must have cleared other things out of

the way in its orbital neighbourhood.

Pluto is now a “Dwarf Planet”

• On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a “dwarf planet”.

• A ‘size-challenged’ planet satisfies rules (a) & (b) , not not (c)!

• All other objects are called "Small Solar-System Bodies".

Quaoar

• Discovered 2004• Kuiper Belt object• Half the size of Pluto….many more ‘KBOs’

will be discovered, hence clarification over meaning of a ‘planet.

Another Unusual KBO Discovered!

Planetary Orbits to Scale

Our Solar System

• Size: The Sun - Pluto distance is ~40AU.This is ~15,000 x Earth-Moon distance.Still < 1/2000th of a Light Year!

• Most planetary orbits are near the plane of the ecliptic. Approximate inclinations are:

Mercury 7.0o; Venus 3.4o ; Mars 1.9o ; Jupiter 1.3o

Saturn 2.5o; Uranus 0.8o ; Neptune 1.8o ; Pluto 17.1o

• All rotate counter clockwise about the Sun.(As seen from above Earth’s North Pole.)

The Mysterious Titius-Bode “Law”

• Is there a pattern to the planets’ spacing?• 1766 Johann Titius devised an empirical

relationship to give the orbits:

• n = - ∞ (Mercury); 2 (Venus); 3 (Earth); 4 (Mars)…• “Predicts” n = 8, 9, 10 at 19.8, 38.8 and 77.2 AU• Uranus (1781) 19.2AU! Neptune only 30AU…• But Asteroid Ceres fits with n = 5 at 2.8AU!

223.04.0 −+= nn xa

Titius-Bode “Law”

No physicalbasis for the equation.

It is weird that it works so well!

Kepler’sMysterium

Geometrical Solar System!

• ‘Discovered’ in 1595 at age 25.

• 5 ‘Perfect’ solids circumscribed by spheres.

• Pythagorean!• Only 6 planets…

Kepler’s GeometricalSolar System

Solar System Exploration: Telescopes

• Mt Palmar (Ca, USA) 5m• VLT (Chile) 4x8.2m – effective area 16m

Hubble Space Telescope

Solar System Exploration: Probes

Some which orbit, or flyby……some that land!

Extrasolar’ Planets!

• Planets orbiting about other stars.• ~70 (to date) • Not observable directly (too small).• Presence deduced from Doppler shifts

(fluctuations) in the observed starlight due to gravitational effects from large (Jovial-type) planets.

Planets Revealed (CM-15.5)

Star 51 Pegesi(40LY away)

has planet of at least half of

Jupiter’s mass with an orbital period of just

4.2 days!

Planet Triplet Revealed!

Star Upsilon Andromedae has 3 planets of masses: ~0.7,2.1 and 4.3 MassJupiter, with semi-major axes of ~0.06, 0.83, 2.6AU.

Planetary Properties

• Distance: Kepler’s laws• Sidereal Orbital Periods• Planetary radius – Angular size + distance• Planetary Masses: from Newton’s laws of

motion & gravity applied to their moons.Mercury and Venus (+ our moon and Ceres)are harder as they have no ‘moons’. Found via small gravity perturbations in orbits.

Planetary Properties II

• Satellites and space probes also used to determine planet masses via their gravitational interactions.

• Planet’s Rotation Period. (a) Watch surface features move. (hard)(b) Doppler techniques.

• Average Planet Density. Mass/Volume

Table of Basic Planetary Properties

Two ‘Classes’ of Planets

• The “Terrestrial” Planets:Mercury, Venus, Earth and Mars

• Small, Dense and Rocky…. Solid Surfaces• Weak magnetic fields

• The “Jovial” Planets:Jupiter, Saturn, Uranus and Neptune

• Large, Low Density, Gaseous….Liquid Surfaces• Strong magnetic fields, many moons and Rings

Interplanetary Debris

• Countless chunks of rocks andice orbiting the Sun on highly eccentric paths…

• From large asteroids to ‘dust’ grains.• “Asteroid” >100m in diameter.• “Meteoroid” is anything smaller…• Net mass in solar system is < Moon’s.

Together with comets, understanding the origin of debris is important for evolutionary models of the

solar system.

Appendix IThe Inner Planets:

Mercury, Venus, Earth, Mars

Appendix IIThe Outer Planets:

Jupiter, Saturn, Neptune and Pluto

Appendix IIIThe Major Moons

Appendix IVComparison of the Terrestrial and

Jovial Planetary Properties