The Solar System. Dr. Ken Rice. Discovering Astronomy S

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1 The Solar System Dr. Ken Rice

2 The Solar System Is the region around the Sun comprising 8 planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. 3 Dwarf planets Ceres, Pluto, Eris Asteroids Asteroid belt between Mars and Jupiter Satellites/moons and Rings All planets except Mercury and Venus have satellites All Jovian planets have rings Comets Short period : Kuiper belt Long peroid : Oort cloud

3 What is a solar system planet? Planet vs Star Star : Generates energy via nuclear fusion emits predominantly blackbody radiation that depends on its temperature Planet : Shines largely because of reflected light. International Astronomical Union (IAU) definition of a Solar System planet Orbits the Sun. Is massive enough that its gravity causes it to be spherical (hydrostatic equilibrium) Has largely cleared its neighbourhood of other bodies. By this definition, Pluto is no longer a planet since it has not cleared its neighbourhood of other bodies. Pluto, and other small spherical solar system bodies in orbit around the Sun are now known as dwarf planets. Spherical bodies in the solar system are in hydrostatic equilibrium. The inward force of gravity is balanced by an outward pressure force.

4 Solar System Properties All the planets (and most other bodies) orbit in the same direction. have orbital planes very close to the ecliptic. All planets have small eccentricities (e < 0.1). Comets, Kuiper belt objects and Oort cloud objects can have very large eccentricities. Titius-Bode Law The planets semimajor axes (orbital radius) generally satisfy the following law n + 4 a( AU) = where n = 0, 3, 6,12, 24, 48 n 10 Missing planet when n = 24 (Asteroid belt?) No strong theoretical reason why this Titius-Bode law should hold Orbital resonances? ( n > 3, n = 2[ 1] )

5 Planetary orbits

6 Planetary orbits and rotation All of the planets orbit in the same direction. All of the planetary orbits lie very close to the ecliptic. Apart from Venus and Uranus, all of the planets rotate in the same direction. Collision (either after or during the formation process) probably caused the retrograde rotation of Uranus and Venus.

7 Types of planets Planets in the Solar System can be divided into two basic types 4 inner rocky/terrestrial planets Mercury, Venus, Earth Mars 4 outer gaseous/icy planets (Jovian) Jupiter, Saturn, Uranus, Neptune

8 The Scale of the Solar System By observing Venus from different locations on the Earth when it transits the Sun, we can measure its parallax angle. d = r sinθ rθ (radians) r = θ d ( radians) Path seen from the south Using Kepler s third law 2 Venus 3 ( years) a ( AU) P = Venus d θ Can determine the distance from Venus to the Sun in AU (AU not yet known). Path seen from the north Now know the distance from Venus to the Sun in AU, and the distance from the Earth to Venus in metres r metres r(metres) = ( 1 avenus ) AU AU = 1 a ( ) ( ) Venus

9 The Scale of the Solar System 2 We can now use radar to determine the distance to Venus. When Venus is at its greatest elongation (the furthest from the Sun when viewed from Earth), we can use trigonometry to determine the distance from the Sun to the Earth Radar gives distance, d, to Venus. AU = d/cos(e) From this : Mass of the Sun Stellar parallax

10 Determining planetary properties Distance determined using Kepler s third law 2 3 ( years) ( AU) P = a Period can be determined observationally Remember synodic and sidereal periods 1 P syn 1 = P E 1 P S or 1 P syn = 1 P S 1 P E Apply Kepler s laws to orbiting satellites (man-made or natural) to determine planet s masses Radii determined by occultations of stars, moons or satellites Densities P 2 2 4π a = G M ( + m ) pl 3 sat M ρ = 4/3πR 3 sat Terrestrial planets : kg/m3 Jovian planets : kg/m3 Water : 1000 kg/m3 (Saturn could float on water) 3 sat 2 4π a GM pl M pl 2 π a = GP 3 sat 2

11 Dwarf planets Spherical bodies orbiting the Sun that do not dominate their neighbourhoods! Currently 3 dwarf planets Pluto, Ceres, Eris Almost certainly more that have yet to be discovered.

12 Satellites/Moons All the planets except Mercury and Venus have satellites Earth Moon Jupiter and Saturn (Jovian) 63 and 47 respectively Galilean moons 4 satellites of Jupiter discovered by Galileo Io, Europa, Ganymede, Callisto Some of the Jovian satellites are larger than Mercury e.g., Ganymede, Callisto, Titan Most of the solar system moons orbit in the same plane as the planets and in the same direction.

13 Rings All 4 Jovian planets have ring systems. Made of small particles, mainly water ice. Regular collisions cause Saturn s rings to be very thin Only a few metres thick. Small moons in the rings open gaps Other gaps formed by resonances with outer moons.

14 Saturn s new ring New ring of dust found around Saturn! It would appear larger than the Moon if we could see it from Earth. May be material ejected from Saturn s moon Phoebus. May explain the dark regions on one side of Iapetus Dark material is dust from the ring.

15 Asteroids Small rocky bodies generally a few km in size. Most orbit the Sun in the Asteroid belt Region between Mars and Jupiter. Irregular shapes Largest may be a few hundred km in size Asteroid belt also contains the dwarf planet Ceres.

16 Comets Small ball of dust and ice ( dirty snowballs ) that orbit the Sun. Highly elliptical orbits. Short period Coplanar Kuiper belt Long period All directions Oort cloud Long tails

17 Meteorites A rocky object that can reach the ground is called a meteorite. Primitive meteorites Remnants from the birth of the solar system. Processed meteorites Once part of a larger object that has processed the primitive material. Some appear to come from the Moon or Mars Ejected by collisions. Panspermia!

18 Kuiper belt and Oort cloud Kuiper belt Ring of material AU from the Sun. ~ bodies bigger than 100 km. Short period comets. Pluto and Eris. Oort cloud Cloud of material extending to about AU from the Sun. May contain a trillion objects/comets Long period comets. Sedna?

19 Exploration of the Solar System There are four main ways to explore the solar system directly Flyby Orbiter Lander or probe Sample return mission

20 Mars Landers Two rovers landed on Mars in early 2004 Opportunity Spirit Carry various instruments for determining conditions on Mars Cameras Spectrometers Magnetometers

21 Combination missions Combines various different strategies Flybys before arriving at destination Orbiter Lander or probe Galileo Jupiter Orbiter plus a probe Cassini Saturn Flybys if Venus, Earth and Jupiter Orbiter Huygens probe that landed on Saturn s moon Titan

22 Sample return missions First sample return mission were the Apollo missions to the moon (manned). More recently there have been unmanned sample return missions Genesis collected solar wind particles Crash landed in Utah when parachute didn t open Stardust collected dust sample from comet Wild-2 Safely returned in January 2006

23 Basic properties of solar system bodies

24 Time to think Asteroids have irregular shapes because a. they suffer many collisions b. there hasn t been enough time for them to settle into spherical shapes. c. Jupiter s gravity prevents them from becoming spherical d. they are not massive enough for their own gravity to make them spherical. e. they spin very fast.

Our Planetary System. Earth, as viewed by the Voyager spacecraft. 2014 Pearson Education, Inc.

Our Planetary System. Earth, as viewed by the Voyager spacecraft. 2014 Pearson Education, Inc. Our Planetary System Earth, as viewed by the Voyager spacecraft 7.1 Studying the Solar System Our goals for learning: What does the solar system look like? What can we learn by comparing the planets to