HABITABLE PLANETSFor every star with planets, how many of these planets (on average)are habitable for life? (ne in Drake equation)There are several main requirements:A. heavy elements (C, N, O, . . .)Nucleosynthesis: see chap. 1 in textbook for more details.1H-present in big bang. 4He--during ~ few minutes of big bang, ~10% (by number) produced.[Note: some helium is made in stars, but only ~1%.]Everything else is made in stars.1st generation stars can produce:12C---triple alpha reaction (3 4He → 12 C in cores of red giants)16O---from He4 + C12 → O16 in cores of red giants. The red giants lose mass by winds or explosions, “seeding" the interstellar gas for the next generation of stars.Heavier elements made by additions of alphas (He), protons, or neutrons(“s-process”) onto these ligher elements, mostly in massive stars whichexplode as supernovae, scattering newly-formed elements throughout theGalaxy.14N---comes from ``CNO cycle" (need C+O to start it). C and O areused as catalysts for H→He, but some C+O are turned into 14N. So 14N can only be made in 2nd generation (or later) stars.→ Implication for SETI: reject oldest (“population II”) stars. Thesecan be recognized by their spectra (e.g. very weak spectral lines of metals).But only a very small fraction ~10–3–10–4 of stars are Population II.[There is now strong evidence that stars with exoplanets have slightlylarger metal abundances than normal. Some people think this might mean(giant) planet formation is very sensitive to metal abundance, but othersthink it just reflects cannibalism of the (metal-rich) planets. ][Another, trickier, aspect: Might need lots of heavier metals for technology.But whether many of these get to the planet’s surface depends on planet’smass, geology,... ]B. Distance from star (determines planet’s surface temperature)Temperature should be in range for liquid water (273 to 373˚ K [0 to100 o C] ---we'll discuss alternatives later). The corresponding distancerange is called the “habitable zone," HZ, or sometimes “continuouslyhabitable zone,” CHZ, referring to a planet that always has liquid water onits surface.Water is considered necessary or optimum for life by most people formany reasons: 1. A liquid offers protection from the parent star’s UVradiation, and a medium in which the earliest organic molecules couldmove around and react; 2. Water is nearly unique in its molecularstructure, which leads to, for example, ice floating instead of sinking(important!), water’s great abilities as a solvent, and several otherproperties. (To be discussed in next part of course) Consider Venus and Mars---runaway greenhouse on Venus (becauseso hot that H2O stayed in atmosphere) and runaway freezing on Mars(because so cold that H2O froze, increasing planets reflectivity [“albedo”] .(However many people think that Mars once had liquid water, mostly fromthe forms of certain surface features but also because its atmosphere wasprobably much thicker in the past.) So maybe this would have occurred tothe Earth if we had been a little closer or more distant from the sun.(Complicated subject—see below and discussion in class.)Also: “faint young sun” paradox. Sun was only70% as bright whenyoung, water should have frozen. With large albedo of ice, hard to seehow it would ever unfreeze. But it’s known that earth was covered withsignificant liquid water 4 billion years ago! (That’s the “paradox”.)Proposed solutions: Large amounts of CO2 or other greenhouse gases,fast earth rotation (only about 14 hr instead of 24 hr, which may havecaused less cloud cover), less land, all could have helped.Michael Hart's early and crude climate calculation: CHZ could bevery narrow (0.95 to 1.05 AU), and ne therefore very small (<<1). In thiscase we are just very lucky, and life should be relatively rare in the Galaxy.But more recent climate models yield a wider CHZ. Most peopleadopt 0.85 to 1.5 AU as CHZ band (based on a seminal 1993 paper). Morerecent calculations including CO 2 ice clouds {Forget et al. 1997, 98; Mischnaet al. 2001} give an outer CHZ radius up to 2.4 AU! Outer limit isextremely uncertain and difficult to calculate (some explanation given inclass). But if the CHZ is this big, could have several habitable planets perstar.It’s important to understand how the location and size of the CHZdepends on the luminosity (and hence mass, for main sequence stars) of theparent star:More massive star → more luminous → CHZ more distant (andwider)Less massive star → less luminous → CHZ less distant (and thinner)[Note: With a high-luminosity star, the CHZ could be in the Oort cloud,which could imply 100s or even 1000s of habitable planets around thesestars! However these stars are rare, and don’t live long (see below).]Digression: Extending the definition of “habitable zone”There is good evidence that water may exist far outside theconventional CHZ: Jupiter’s moon Europa (as photographed by Galileospacecraft) shows evidence for ice flows: tidally-induced geologic activityheats water ice under surface to near-liquid, then gushes through ice crust,“icy volcanism”. Surface is much smoother than other Jovian moons, fewcraters, also suggesting flows. There are speculations that ocean existsbeneath surface. We’ll briefly discuss a speculative ecology for Europa inPart III of this course.Some people have even speculated that most of the habitable objectsin the universe are not planets, but tidally heated moons of planets.{Williams et al. 1997, Nature 385, 234}C. Size of planetToo small → no atmosphere (from volcanic outgassing) to formoceans (if that’s how they formed), block UV, …Too large → outgasses massive CO2 atmosphere, greenhouse effectprevents liquid water (it all stays in the atmosphere and eventually leaksaway as UV photons break up the H2O).Some rough estimates suggest that habitable planets would have tobe within a factor of two or three of Earth’s mass! Since planetaryformation simulations make planets of a variety of masses, this planet sizeconstraint would make habitable planets much less frequent.D. Large moon? (See earlier notes on formation of moon.)Recall that the fact that we have such a large moon (relative to theEarth’s size) is a very fluky occurrence, probably involving the chancecollision of a large planetesimal, or even planet, with the Earth during theearly evolution of the solar system.We’ll
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