1Habitable Planetsne Number of planets, per planetary system that aresuitable for life ne = np fsplanetary stellarnp = ne for stars like Sun fs = fraction of stars with suitable propertiesnp, ne could be greater than 1 fs ≤ 12Key requirement Liquid for solvent H2O 273 - 373 K at Earth pressure 647 K at higher pressures 330 K protect proteins, membranes smaller range at lower pressure CH4 (methane) 91-109 at Earth pressure NH3 (ammonia) 195 - 240 K at Earth pressure Pressure, Gravity size of planetWater Phase Diagram3What sets the temperature? In space, absorption and emission ofelectromagnetic radiation (light)Energy in = Energy out(µ L/d2) (µ T4 )*LdTfi T µ Ld2( )1/4µ d-1/2µ1÷d4 ¥ as far from star, T is half as high4Planet Temperatures 1st approximation: A blackbody at a distanced from a star of luminosity LMaximum temperatured*starTmaxT = 394 K Ld2( )1/4L in L§d in AU2nd approximation: A fraction of the light isreflected (not absorbed)Call this fraction the albedo (A)e.g. Moon A = 0.07 Tmax = 387 L = 1 L § correct to few %But Earth : A = 0.39 fi Tmax = 342 K predicted Tmax ≤ 313 KT = 394 K(1-A) Ld2 [ ]1/453rd approximation:Account for rapid rotation - less more close to Tavg Tavg = 279 KTmaxTmin(1-A) Ld2 [ ]1/4Earth: A = 0.39 fi Tavg = 246KActual Tavg = 288K4th approximation: Greenhouse effect6Consequences of Greenhouse Effect:Raises Tavg (Earth) by about 40KOtherwise Tavg < Tfreeze fi Frozen PlanetHabitable Zone (HZ)For fixed luminosity, Greenhouse Effecta required temperature rangetranslates to a requiredrange of distances from star7But Greenhouse Effect could have a bigimpact on the size of HZToo hot*StarHabitable ZoneToo ColdContinuously Habitable Zone (CHZ)Need ~ 5 ¥ 109 years for intelligent life?But Sun’s L increases slowlyTEarth constant to few degrees (mostly)(decreasing Greenhouse)HZ moves out as L risesfi CHZ smaller than HZ8Computer ModelsHart CHZ 0.95 - 1.01 AUfi np < 0.1Negative feedback thermostat T Rainfall rock weathering~PessimistMiddle of the RoadT CO2Whitmire et al. CHZ 0.95 - 1.5 AU fi np ~ 1The Carbon Cycle without Life9The Carbon Cycle on Earth Now10Cold Starts?• As Habitable Zone moves out– Can you unfreeze a frozen planet?– Will it become suitable for life?– If not, HZ will shrink– CHZ is smaller11Temperatures for Life on EarthLower limit?Some microbes survive for long periods in Antarctic icee.g. Lake Vostok - 2.5 miles below glacial ice in AntarcticaMicrobes found ~ 400 feet above lake in an ice coreFreeze-dried for ~ 106 yrs?Revive when exposed to liquidLower limit is probably about -20° C (253 K)12We have learned that some microbes can survive inpressurized water at T up to 400 K (120° C)!Such microbes have special adaptations toprotect their heat-sensitive moleculesFor complex life, upper limit seems to be ~ 325K~52° C or 126° FBut is this limit just an accident of evolution onEarth?Upper Limit?Other Habitable ZonesMicrobial Habitable Zone (MHZ)Fixed by Range of T microbes can withstand“Animal” Habitable Zone (AHZ)“Animal” = complex, differentiated, multicellular lifeWard + Brownlee in Rare Earth note AHZ muchsmaller than MHZThey also argue that parts of our Galaxy unsuitable foranimal lifeWe will consider this point under fi13Snowball EarthIncreasing evidence that Earth nearly froze overtwice2.4 billion years ago & 650-800 Myr agoClimate can have dramatic changesApparently - these were ended by volcanic eruptionsthat put much more CO2 in atmosphere1. Sub-surface Water?If you don’t need photosynthesis, no need to beon surfaceT increases with depth into Earthfi liquid water under “ground” e.g. Mars? Europa (Moon of Jupiter)HZ 1.5 AU 5 AUnp ~ 2 ~ 3Other Considerations142. Other Solventse.g. Titan (moon of Saturn) could have liquidmethane (CH4) or ethane (C2H6)HZ 10 AUnp ~ 43. Other planetary systemsJupiter-like planets ~ 1 AU (in HZ)Life on Moons?Other requirements?Pressure? Bacteria on deep sea floor up to 1000atmospheresBut not “animal” lifeNot too salty? - halophilic bacteriaup to 33% salt solutionpH? - LOG [H ions]pH 1 7 ph 14acid normal alkali H2O15Almost all cells regulate pH to 7.71 microbes 13Again microbes have adapted to justabout any environment of EarthImportance of Heavy ElementsPlanetary systems found so farAre found more commonly around stars witha lot of heavy elementsDoes this apply to systems more like ours?16The Importance of the MoonThe Moon makes the tides bigger than if only the Suncaused tidesMay be important in the origin of lifeThe Moon stabilizes the Earth’s obliquityVaries regularly from 22.1 to 24.5 over 41,000 yrs.23°NProtation axisto SunWithout the Moon, tugs from other planetscould make it vary chaoticallyLarge obliquity could cause snowball EarthWard & BrownleeOnly if a large supercontinent at the poles Williams, Kasting, Caldeira17Issues Raised by Discovery of OtherPlanetary Systems1. We know that not all planetary systems are likeoursBut, searches so far could not find systems likeours fi most could be like ours2. Exotic possibilities for lifeEuropa-like moons around giant planets orbiting~ 1 AUStellar Requirements (fs)1. Sufficient Heavy ElementsTerrestrial planets, bioelements1st generation - ruled out Population II - ruled out No significant loss182. Main Sequence(Stable L fi Stable T possible)e.g. Sun will increase L by 103 5 ¥ 109 yr from nowRed Giants - ruled out0.99 OK3. Stellar Mass Not Too High(Main sequence Life ≥ 5 ¥ 109 yr )Roughly, L µ M4Fuel µ M Lifetime µ orFuelL1M319Stellar LifetimesM (M )3010311/31/10Lifetime (yrs)2 ¥ 1063 ¥ 1076 ¥ 1081 ¥ 10102 ¥ 10113 ¥ 1012§If t > 5 ¥ 109 yrsM < 1.25 MM > 1.25 M - ruled out if we require 5 X 109yr for intelligent life to evolveMost stars are low mass, so0.90 OK§§204. Stellar Mass Not Too Lowa) Do terrestrial planets form? “Jupiters” should form closer to low mass star Prevent formation of terrestrial planets?b) Chance of having terrestrial planet in CHZ?CHZ smaller for Low L* *Lower LHigher LFor Logarithmic Spacing, np independent of size21But if planet spacing as in
View Full Document
Unlocking...