Habitable 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 ≤ 1Key requirement Liquid for solvent H2O 273 - 373 K at Earth pressure 647 K at higher pressures smaller range at lower pressure CH4 (methane) 91-109 at Earth pressure NH3 (ammonia) 195 - 240 K at Earth pressure Pressure depends on Gravity, and therefore thesize of planetWater Phase DiagramWhat sets the temperature? In space, absorption and emission ofelectromagnetic radiation (light)For a blackbody (absorbs all light falling on it,emits light proportional to fourth power oftemperature):Energy in = Energy out(∝ L/d2) (∝ T4 )*LdT⇒ T ∝ Ld2( )1/4∝ d-1/2∝1√d4 × as far from star, T is half as highPlanet Temperatures 1st approximation: A blackbody at a distanced from a star of luminosity LMaximum temperatured*starTmaxT = 394 K Ld2( )1/4L in Ld 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 ⇒ Tmax = 342 K predicted Tmax ≤ 313 KT = 394 K(1-A) Ld2 [ ]1/43rd approximation:Account for rapid rotation - less more close to Tavg Tavg = 279 KTmaxTmin(1-A) Ld2 [ ]1/4Earth: A = 0.39 ⇒ Tavg = 246KActual Tavg = 288K4th approximation: Greenhouse effectConsequences of Greenhouse Effect:Raises Tavg (Earth) by about 40KOtherwise Tavg < Tfreeze ⇒ Frozen PlanetHabitable Zone (HZ)For fixed luminosity, Greenhouse Effecta required temperature rangetranslates to a requiredrange of distances from starBut Greenhouse Effect could have a bigimpact on the size and location 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)Generally, HZ moves out as L rises⇒ CHZ smaller than HZComputer ModelsHart CHZ 0.95 - 1.01 AU⇒ np < 0.1Negative feedback thermostat T Rainfall rock weathering~PessimistMiddle of the RoadT CO2Whitmire et al. CHZ 0.95 - 1.5 AU ⇒ np ~ 1The Carbon Cycle without LifeThe Carbon Cycle on Earth NowCold 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 smallerTemperatures 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)We 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 fiSnowball 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 Earth⇒ liquid water under “ground” e.g. Mars? Europa (Moon of Jupiter)HZ 1.5 AU 5 AUnp ~ 2 ~ 3Other Considerations2. Other Solventse.g. Titan (moon of Saturn) has some liquidmethane (CH4)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 H2OAlmost 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 starswith a lot of heavy elementsDoes this apply to systems more like ours?The 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 otherplanets could make it vary chaoticallyLarge obliquity could cause snowball EarthWard & BrownleeOnly if a large supercontinent at the poles Williams, Kasting, CaldeiraIssues Raised by Discovery of OtherPlanetary Systems1. We know that not all planetary systems are likeoursBut, searches so far could not find systems likeours ⇒ 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 loss2. Main Sequence(Stable L ⇒ 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 ∝ M4Fuel ∝ M Lifetime ∝ orFuelL1M3Stellar LifetimesIf 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 OK4. 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 inCHZ?CHZ smaller for Low L**Lower LHigher LFor Logarithmic Spacing, np independent of sizeBut if planet spacing as in Solar SystemandCHZ
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