Lecture 34: Habitable Zones around StarsAstronomy 141 -Winter 20121 .. - PDF document

Lecture 34: Habitable Zones around StarsAstronomy 141 -Winter 20121 Lecture 34Habitable Zones around Stars MKGFA 0.1Astronomy 141 –Winter 2012 This lecture examines the factors affecting the habitability of stars.Brighter stars have wider Habitable Zones further awayfrom the star.A likely place to look for life is on rocky planets in theHabitable Zones of low-mass Main Sequence stars.Planets in Habitable Zones close to their parent starscan become tidally locked.Low-mass M stars experience violent super-flares thatcould have a negative impact on habitability.Stars that emit a lot of UV radiation would possibly sterilizethe surfaces of their planets. Basic Requirements for Life in a Planetary ContextStable, Long-lived Source of EnergyEnergy to fuel chemical reactionsWarmth to permit liquid water Complex ChemistryElements heavier than H and HeCarbon, liquid water, inorganics Benign Environmental ConditionsStable, well-regulated climateProtection from harmful UV radiation Location for life to emergeOceans, land masses (place to swim/stand) Lecture 34: Habitable Zones around StarsAstronomy 141 -Winter 20122 These conditions should occur on rocky planets in the Habitable Zones of low-mass Main Sequence stars.Main Sequence stars stablygenerate energy by coreHydrogen fusionLow-mass stars (sunhave M-S lifetimes of –4M;1 GyrProvide a stable source ofheat for a long time. Long enough to give planets a chance to form,life a chance to emerge, and evolution time to act. The Habitable Zone is the region around a star where liquid water is stable on a planet’s surfacePlanet too close:Runaway greenhouse effectsuperheats the atmosphereand vaporizes all the water.Planet too far:Water freezes out and won’tbe liquid on the surface. The Sun's Habitable Zone Today VenusRegion around the Sun where liquid water is stable on thesurface of a planet at a pressure of 1 atmosphere.Conservative:0.95 –1.4 AUOptimistic:0.84 –1.7 AU Lecture 34: Habitable Zones around StarsAstronomy 141 -Winter 20123 The location of a star’s Habitable Zone depends on its luminosity.Inneredge of Habitable Zone:Outeredge of Habitable Zone: Brighter stars have more distant Habitable Zones. Sun (G Star): L = L, d= 0.95 –1.4 AU M Star: L = 0.008 L, d= 0.08 –0.12 AUA Star: L = 80 L, d= 8.5 –12.5 AUExamples:Brighter stars have Habitable Zones. MKGFA 0.1Habitable Zones for low-mass Main Sequence Stars 8.5 –12.5AU 1.5 –2.2AU 0.08–0.12AU 0.38 –0.56AU0.95 –1.4 AU Lecture 34: Habitable Zones around StarsAstronomy 141 -Winter 20124 It is not enough for a planet to just be in the Habitable Zone of its star…Other factors can influence habitability:Planets in Habitable Zones close to their parent starsrisk becoming tidally locked.Low-mass M stars are subject to stellar flaresthatcould have a potentially negative impact on life.Excess ultraviolet radiationcould produce environmentshazardous to life (likelihood of dangerous mutations) If a small body orbits too close to its parent body, its rotation will become tidally locked. Examples: The Moon’s rotation period is synchronized with its orbital period around the Earth.Always keeps the same face towards the EarthGalilean Moons of Jupiter are tidally locked insynchronous rotation with their orbits.Always keep the same face towards Jupiter. It takes time for a small body to become tidally locked into synchronous rotation.Example: The MoonThe Earth raises bodytides on the Moon.Constant squeezing & stretching of arapidly rotating Moon generates heatEnergy gets taken from the Moon’s rotationThe Moon’ rotation slows down until its rotation & orbit periods are the same, stopping the squeezing. Lecture 34: Habitable Zones around StarsAstronomy 141 -Winter 20125 The Tidal Locking Timescale depends on the size of the orbit and the mass of the parent star How far can a planet be from its parent star such that itbecomes tidally locked after 4 Gyr? One side of the planetgets all the starlightNear-side gets too HotFar-side gets too Cold MKGFA 0.1 Planets in the habitable zones of M main-sequence stars may be tidally locked. M stars are magnetically active and can produce powerful stellar flares.Flaresare enormous outbursts ofhigh-energy (X-ray and UV) Could potentially be sterilizing.But it could also stimulateevolutionby increasing the mutation rate. Lecture 34: Habitable Zones around StarsAstronomy 141 -Winter 20126 Hot stars produce a great deal of UV radiation that can be damaging to life. M starG starA star Could potentially sterilize the surfaces of their planets No StableEnergy Source In our search for starsthat might harborhabitable planets…Exclude short-lived O & Bstars and UV-bright starsExclude Giants andWhite Dwarfs withouta stable source of energy.Maybe exclude low-massstars with strong flaringand tidal-locking in theirHabitable Zones