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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