Life in the Universe (PHY-30025) Spring 2020 Jacco van Loon Contents 1 Introduction 1 1.1 Motivation.................................... 1 1.1.1 Planetdiscoveries............................ 1 1.1.2 Spaceage ................................ 2 1.2 Settingthestage ................................ 2 1.2.1 Whatislife?............................... 2 1.2.2 Extra-terrestrialintelligence . ..... 3 1.2.3 Drake’sequation ............................ 4 2 Life as we know it – basics 5 2.1 Lifeforms..................................... 5 2.1.1 Simpleorganisms ............................ 5 2.1.2 Complexorganisms........................... 5 2.2 Essentials .................................... 6 2.2.1 Carbon ................................. 6 2.2.2 Liquidwater............................... 7 2.3 Chemicalcomponents.............................. 7 2.3.1 Lipids .................................. 7 2.3.2 Carbo-hydrates ............................. 8 2.3.3 Aminoacidsandproteins . 8 2.3.4 Codinggeneticinformation: DNA. 8 2.4 Energy...................................... 10 3 Life as we know it – functioning 13 3.1 Physiology.................................... 13 3.1.1 Thecardio–vascularsystemandlungs. ... 13 3.1.2 Sensors.................................. 14 3.1.3 Thenervoussystemandbrain . 16 3.1.4 Motoricattributes ........................... 17 3.2 Growthanddecline............................... 18 3.3 Extremophiles.................................. 19 4 Mentality 21 4.1 Thebrain .................................... 21 4.2 Mentalstates .................................. 22 4.2.1 Consciousness.............................. 23 4.2.2 Sleep................................... 24 i ii CONTENTS 4.2.3 The rˆole of neurons in memory and consciousness . ..... 25 4.3 Automata .................................... 25 5 The emergence of life on Earth 27 5.1 Howdiditstart? ................................ 27 5.1.1 TreeofLife ............................... 27 5.1.2 TheMiller–Ureyexperiment . 28 5.1.3 Oneoflife’smysteries: chirality . .... 28 5.2 Whenandwherediditstart? . 28 5.2.1 ThechangingconditionsonEarth. 28 5.2.2 The significance of the other planetary bodies . ..... 30 5.3 TheoriginoflifeonEarthinanutshell? . ..... 31 6 HabitableZonesandEarth’satmosphere 33 6.1 Conditionsforaplanettobehabitable . ..... 33 6.2 Thetemperatureofaplanet . 34 6.2.1 Radiativeequilibrium. 34 6.2.2 Albedo.................................. 35 6.2.3 Temperaturevariations. 35 6.2.4 TheGreenhouseeffect . 36 6.3 TheHabitableZone............................... 37 6.4 Theplanet’satmosphere . 38 6.4.1 Hydrostaticequilibrium . 38 6.4.2 Evaporation............................... 40 7 Life elsewhere in the Solar System 41 7.1 Whereit’sdefinitelynot . 41 7.1.1 Mercury ................................. 41 7.1.2 TheMoon................................ 41 7.2 Venus–ourclosetwin ............................. 41 7.3 Mars–ourlittlebrother . 42 7.3.1 Canals? ................................. 42 7.3.2 Water?.................................. 43 7.3.3 Maintaining the CO2 atmosphere ................... 44 7.3.4 Evidenceforlife............................. 44 7.4 TheGiants:JupiterandSaturn . .. 45 7.4.1 Europa.................................. 45 7.4.2 Ganymede&Callisto. 46 7.4.3 Titan–themoonthatoughttobeaplanet . 46 7.4.4 Enceladus ................................ 48 7.5 Minorbodies .................................. 48 7.5.1 Dwarf planets, asteroids and meteorites . ..... 48 7.5.2 MeteoritesfromMars. 49 7.5.3 AsteroidsfromOuterSpace . 49 7.5.4 Comets ................................. 49 7.5.5 Outersolarsystemdwarfplanets . 50 CONTENTS iii 8 Searching for exoplanets 51 8.1 Directdetection................................. 51 8.1.1 Reflectedstarlight ........................... 51 8.1.2 Thermalemission............................ 52 8.1.3 Angularresolution ........................... 53 8.2 Detectionthroughoccultation . .... 53 8.2.1 Planetarytransits............................ 53 8.2.2 Planetaryeclipses............................ 54 8.2.3 TransitTimingVariations . 54 8.3 Detectionthroughthestar’sreflexmotion . ...... 54 8.3.1 Astrometry ............................... 55 8.3.2 Dopplershifts.............................. 55 8.4 Gravitationalmicrolensing . .... 56 8.4.1 Einsteinradius ............................. 57 8.4.2 Amplification .............................. 58 8.5 Pulsartiming .................................. 58 9 Properties of discovered exoplanets 59 9.1 Planetmasses.................................. 59 9.2 Orbitalradii................................... 60 9.3 Planetarysystems................................ 60 9.4 Planethoststars ................................ 61 9.5 Howcompleteisourpicture? . 62 10 Exoplanet atmospheres and exoplanets in Habitable Zones 63 10.1 Erodingatmospheres . 63 10.1.1 Planetdensities ............................. 63 10.1.2 HotJupiters............................... 64 10.1.3 Radiativeheatingandevaporation . ... 64 10.1.4 Radiationpressure . 65 10.1.5 Stellarwindpressure . 66 10.2 Have we found planets in their Habitable Zone? . ..... 66 10.2.1 Moons around massive gas planets in the Habitable Zone ...... 67 10.2.2 Planetsaroundgiantstars . 67 10.2.3 Planetsaroundpulsars . 67 10.3 Orbits and gravitational perturbations . ........ 68 10.3.1 Theimportanceofeccentricity. ... 68 10.3.2 Theimportanceofmultiplicity. ... 68 11 The formation of stars 69 11.1 Theinterstellarmedium . .. 69 11.2 Gravitationalinstability . ..... 69 11.3 Hydrostatic equilibrium and the need to cool . ....... 72 11.3.1 Interstellar chemistry and the building blocks for life ........ 74 11.4 Thenexthurdle: magneticfields. .... 74 iv CONTENTS 12 Proto-planetary discs 77 12.1 Theangularmomentumcatastrophy . ... 77 12.2 Star–discinteraction . ... 77 12.2.1 Accretionandfeedback. 78 12.3 Proto-planetarydiscs . ... 79 12.4 Observeddiscproperties . ... 80 12.4.1 The showcase disc of β Pictoris .................... 80 12.4.2 Anatlasofproto-planetarydiscs . .. 80 13 The formation of planetary systems 83 13.1 Theformationofplanets . .. 83 13.1.1 Discinstability ............................. 83 13.1.2 Planetformation“bottomup”. 83 13.1.3 Planetmigration ............................ 84 13.2 Gravitationalperturbations . ..... 85 13.2.1 TheHillradius ............................. 85 13.2.2 TheRochelimit............................. 85 13.2.3 Lidov–Kozaicycles . 86 13.2.4 TheRossiter–McLaughlineffect . .. 86 14 Finding signatures of life 89 14.1 Planetaryconditionsforlife . ..... 89 14.2 Atmosphericsignaturesofabiosphere. ....... 90 14.2.1 Detectingandprobinganatmosphere . ... 90 14.2.2 Oxygen ................................. 92 14.2.3 Out-of-equilibrium chemistry as a biomarker . ....... 92 14.2.4 Methaneandammonia . 93 14.3 Detectingvegetation . .. 93 14.4 Directimagingofanexoplanet’ssurface . ....... 94 15 The Sun–Earth connection 95 15.1 TheEarth’sorbitandrotation. .... 95 15.2TheSun’sactivity ............................... 96 15.3Spaceweather.................................. 97 15.3.1 Thesolarwind ............................. 97 15.3.2 Solar flares and Coronal Mass Ejections. .... 98 15.3.3 Geo-magneticphenomena . 98 15.3.4 Cosmicrays ............................... 99 15.4 The Sun’s evolution affects the Habitable Zone . .......100 16 Natural and human hazards to life 101 16.1 Are humans causing the next mass extinction? . ......102 16.1.1 Theozonehole .............................102 16.1.2 Carbon–dioxide and the Greenhouse effect . .102 16.2 Circulationpatternsandweather . .103 16.3Ecosystems ...................................105 16.3.1 Feedback.................................105 16.3.2 Carryingcapacity. .106 CONTENTS v 17 Threats from outer space 107 17.1Impacts .....................................107 17.2Supernovæ....................................109 17.2.1 Thelocal interstellar environment . .110 17.2.2 AGalacticHabitableZone? . .111 18 Travel into space 113 18.1 Basicsofrocketpropulsion . .113 18.1.1 Improving efficiency of rocket propulsion . .114 18.2 Gravityassistmanœuvres . 115 18.3 Enteringorbitanddescent . 116 19 Interstellar travel and settlement 119 19.1 Limitssetbyrelativitytheory . .120 19.2 Alternativepropulsionmethods . .120 19.2.1 Beyondinitialspeed . .121 19.3 Challengesforinterstellartravel . ........122 19.4 Challenges for extra-terrestrial settlement . ............123 19.5 Ethicalconsiderations . 124 20 Extra-Terrestrial Intelligence 125 20.1SETI.......................................125 20.1.1 Whatwouldonebelookingfor? . 126 20.2CETI.......................................127 20.2.1 Whatwouldamessagebelike? . 127 20.3Dysonspheres..................................128 20.4 Colonization by intelligent civilizations . ...........128 20.5Closingremarks................................. 129 vi CONTENTS Chapter 1 Introduction 1.1 Motivation Are we alone? This is our prime motivation when we investigate whether life can exist elsewhere in the Universe, and when we try to find it. As part of this quest, we shall explore many facets of life with important bearings on our health, and indeed survival as a species. Apart from our growing understanding of what is life, and how it has evolved on Earth, there are two important recent developments that facilitate the study of life in the cosmos. 1.1.1 Planet discoveries Since the first planets outside our own Solar System were discovered in the 1990s, over 4000 such ‘exoplanets’ are now known. This has revolutionized our knowledge of the potential for life to exist in the Universe besides that found on Earth. We are still at the dawn of