The Solar System. Order of the Planets. Planetary Orbits. A mnemonic to help you remember this! 10/5/2010

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1 The Solar System solar system = Sun + 8 planets + satellites + asteroids + comets Pluto is no longer considered a planet! Why study? understand the Earth better and our impact on it understand how planets form so we can search for Earth-like planets around other stars Order of the Planets Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune (Pluto) A mnemonic to help you remember this! My Very Erotic Mate Joyfully Satisfies Unusual Needs (Passionately) Planetary Orbits 1

2 Characteristics of Planetary Orbits Low eccentricity ellipses (except Mer, Mar, Plu) Revolve about Sun in same direction Revolve about Sun in same plane (except Plu) Rotate in same direction as revolution (except Ven, Ura, Plu) The Building Blocks of Planets Mean Density (ρ) ρ = m / V where: ρ = mean density (kg/m 3 ) m = total mass (kg) V = total volume (m 3 ) ρ is a measure of the overall or bulk composition of a planet Common Building Block Mean Densities Material Density (kg/m 3 ) Iron 8000 Rock 3000 Water Ice 1000 Hydrogen Gas 1 Determining the Bulk Properties of the Planets Compare the mean density of a planet or moon with the densities of the main building blocks Examples 2

3 A planet has a mean density of 3100 kg/m 3 What is it made of? A planet has a mean density of 3100 kg/m 3 What is it made of? Mostly rock! A moon has a mean density of 2000 kg/m 3 What is it made of? A moon has a mean density of 2000 kg/m 3 What is it made of? A mixture of rock and ice! Main difference between stars and planets Stars generate their own light while planets reflect light from the Sun! Stars are Blackbodies The color of a star is a measure of its temperature! Example: A blue star is hot while a red star is cool 3

4 Chemical compositions are determined from the spectroscopy of reflected sunlight from a body Planets are NOT Blackbodies The color of a planet is determined by its chemical composition! Example: Mars is red because of iron oxides on its surface The Spectrum of Europa By comparing the spectrum the object with the spectrum of materials in a lab we can determine the chemical composition of surface or atmosphere of a planet or moon! Europa has an icy surface! There are two kinds of planet! Terrestrial Planets Earth-like Small, rocky, close to the Sun (< 1.5 AU) High mean densities Mercury, Venus, Earth and Mars 4

5 Sizes of Terrestrial and Jovian Planets Jovian Planets Jupiter-like Large, gaseous, far from Sun (> 5 AU) Low mean densities Jupiter, Saturn, Uranus, Neptune Planetary Satellites (Moons) There are 169 currently known (Sept 2007) All have except Mercury and Venus Terrestrial have few (2 or less) Jovian have many (13 or more) 7 giant satellites (size similar to Mercury) The Seven Giant Satellites of the Solar System Terrestrial and Jovian Planets Compared Asteroids rocky debris 5

6 Comets icy debris Location of Asteroids and Comets Robotic Space Probes enable direct measurements radio data back to Earth all planets visited except Pluto (New Horizons mission launched Jan 2006) Types of Space Mission Flybys quick look Orbiters extended study from orbit Landers direct measurements Sample return return samples to Earth Human visits using astronauts The Voyager Flyby Missions Made Flybys of Jovian Planets in 1980 s 6

7 The Magellan Venus Orbiter The Mars Exploration Rovers Mapped the surface of Venus with radar in the 1990 s Currently searching for evidence of water on the surface of Mars in the past The Stardust Mission to Comet Wild 2 Jan 15 th 2006: Samples safely returned to Earth Jan 2004: collected dust from comet The only human visit the Apollo missions to the Moon! Lunar Reconnaissance Orbiter (2009) Apollo 12 Landing Site 7

8 Apollo 12 astronaut visiting Surveyor 3 The Formation of the Solar System Condensation (Nebular) Theory Stars form from huge clouds (nebulae) of gas and dust Interstellar clouds are normally stable! Because the inward force of gravity is balanced by the outward force of pressure! 8

9 This can happen when clouds are compressed externally! To form a star and planetary system we need the cloud to become unstable: gravity > pressure causing the cloud to collapse under gravity size temp spin This can be caused by a nearby supernova explosion! The Conservation of Angular Momentum As a rotating object gets smaller it spins faster! Particles of gas and dust stick together within the disk in a process called accretion Similarly, as a cloud collapses, it spins faster forming a rotating protoplanetary disk (proplyd) around a central protostar 9

10 Leading to the formation. of a family of planets. Explains the characteristics of Planetary Orbits! Low eccentricity ellipses Revolve about Sun in same direction Revolve about Sun in same plane Rotate in same direction as revolution Condensation Temperature Maximum temperature that a given material will stick together (condense): iron > rock > ices > gases high low The type of planet formed depends on the distance from the center of the disk The Frost Line 10

11 In the inner disk within the frost line only rocks and metals could condense forming Terrestrial planets In the outer disk beyond the frost line ices could also condense which then attracted gas from the solar nebula forming Jovian planets This explains why our own solar system has inner rocky Terrestrial planets and outer gaseous Jovian planets Other Evidence? The Orion Nebula Protostars and Protoplanety disks are seen inside the Orion Nebula! A star formation region 1500 ly away Infrared (IR) Image 11

12 Disks of gas and dust are seen around other, mature solar-type stars! 12