What is a planet? Lecture 3: Our Solar System The Planets. Solar System to Scale. Eight Solar System Planets

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1 Lecture 3: Our Slar System The Planets What is a planet? Overview: What is a planet? Finally defined in August 2006 by the Internatinal Astrnmical Unin (IAU)! Why? In 2005, a bdy slightly larger than Plut at 96.7 AU was annunced. A 10 th planet? Planetary prperties Lead t planet definitin: 1. Orbits the Sun Slar System planets NASA Slar Dynamics Observatry 2. Has enugh mass s that gravity vercmes the strength f bdy and becmes rund Chapter 3 f Intrductin t Astrnmy and Csmlgy 3. Has cleared its rbit the nly bdy f its size at that distance frm the Sun Part 3 demted Plut satisfied (1) and (2) but nt (3) => dwarf planet. Eight Slar System Planets Slar System t Scale Inner Terrestrial planets: 1. Mercury 2. Venus 3. Earth 4. Mars Outer Jvian planets: 5. Jupiter 6. Saturn 7. Uranus 8. Neptune

2 Planetary Cmpsitin Planetary Masses Can estimate cmpsitin frm planetary density Can als be fund if the planet has a satellite r flwn by r rbited by a spacecraft. Density f a planet is: Density = Mass Vlume Mercury fly-my by NASA s Mariner 10 and Messenger Venus rbited by NASA Pineer Orbiter and ESA s Venus Express T determine mass frm Kepler s 3 rd Law, need: 1. Mean Sun-planet separatin 2. Orbital perid T determine vlume, need: 1. Distance frm Earth 2. Angular extent f the planet Other planets have natural satellites (mns) Use Newtn s Law f Gravitatin, mass f planet is: M = 4π 2 a 3 GP 2 where a is the semi-majr axis and P is the perid. Planetary Densities Planetary Prperties Vlume determined frm: V = 4/3 π R 3 θ d 2R Frm size and density, can divide the planets int tw categries: 1. Terrestrial Planets where 2R = d (θ 2 π / 360) Small size, high density and in inner slar system Cmpare t densities (g cm -3 ): Crk: 0.2 Wd: 0.5 Water: 1.0 Basalt: 3.3 Lead: 11.0 Gld: 19.0 Cmpsed f metals and rcks Mercury, Venus, Earth, Mars 2. Jvian Planets Large size, lw density and in uter slar system Cmpsed f sme rck, ice and light gasses Jupiter, Saturn, Uranus, Neptune

3 Terrestrial Planets: Mercury Terrestrial Planets Irn-nickel cres surrunded by mantle and crust f rck. Mass: ME Radius: RE Semi-majr axis: AU Cratered surface, like the Mn Old surface, inactive gelgically NASA Messenger image N atmsphere - t small and t ht. Lw reflectivity ( Albed ) It reflects nly 6% f light frm Sun Jhn Lennn Crater NASA Messenger Image f Mercury in March 2011 Further details at Terrestrial Planets: Venus Mass: ME Radius: 0.95 RE Semi-majr axis: AU Rcky planet Atmspheric pressure ~90 times that f Earth. 96% CO2,4% N2,1% O2 Runaway greenhuse effect frm CO2 Surface temperature = 500 C. Cncentrated sulphuric acid rain

4 Terrestrial Planets: Venus Sfd Terrestrial Planets: Earth Mass (ME): kg Radius (RE): 6371 km Semi-majr axis: 149 millin km Rcky surface Mlten irn cre Gelgically active Plate tectnics, vlcanes USSR VENERA 13 (1982) Thin atmsphere 78% N2, 21% O2 See daily images frm NASA/NOAA Deep Space Climate Observatry (DSCOVR) at Further details at Terrestrial Planets: Mars Mass: ME Radius: RE Semi-majr axis: 1.52 AU Rcky planet Red surface due t irn xide Irn cre Thin atmsphere f CO2 Terrestrial Planets: Mars Thin CO2 atmsphere Olympus Mns shield vlcan Evidence fr water n Mars frm NASA's Mars Recnnaissance Orbiter (MRO)

5 Jvian Planets Slid cres f sme rck and ice deep inside surrunded by primrdial gas left ver frm frmatin f Slar System mainly Hydrgen and Helium Jvian Planets: Jupiter Mass: ME Radius: RE Semi-majr axis: 5.2 AU Cre f irn-nickel and rck 71% H, 24% He, 5% heaver elements Cvered in banded cluds f ammnia crystals Orange/brwn bands frm phsphr and sulphur Great Red Spt has wind speed f up t 100 km/s 67 mns! Ice Giants: Uranus Jvian Planets: Saturn Mass: ME Cre f irn-nickel and rck Radius: 9.45 RE Mainly made f H and He Semi-majr axis: 9.58 AU Distinctive rings (~1 km thick) 93% water ice and dust Jupiter s mn I (NASA Galile) Mass: ME Radius: ~4 RE Equatrial: 25,560 km Plar: 24,973 km Semi-majr axis: AU Cre f rck and ice Mainly made f H and He Highly tilted axis f rtatin. Almst parallel t ecliptic plane Distinctive rings

6 Ice Giants: Neptune Dwarf Planets Mass: M E Radius: ~3.8 R E Equatrial: 24,764 km Plar: 24,341 km NASA s Vyager 2 images f rings (left) and cluds (bttm) IAU Dwarf Planet Definitin: 1. Orbits the Sun Semi-majr axis: 30.1 AU 2. Has enugh mass s that gravity vercmes the strength f the bdy and it becmes rund Cre f rck and ice Mainly made f H and He 3. Has nt cleared its rbit ther similar sized bdies will rbit the Sun at a similar distance Red rings f silicn- and carbn-based material 4. Is nt a satellite f anther bdy 13 mns Lecture 3 Practical Task Lcate Jupiter shining brightly in eastern dawn sky all mnth. Planetary cnjunctin: Jupiter will be clsest in sky t Venus n Oct 26. Use binculars t identify 4 mns f Jupiter I, Eurpa, Ganymede and Callist. Read full article at