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Other Planetary Systems ffiffi PIaffiffi*mffiW Syst€il?'es R.Paul Butler oDERNAsrRoNoMy nEvEALsto us, for the first time in his_ tory, scenesfrom one end ofthe cosmosto theother. 377 We havepicturesque views of planetarysurfaces in our own solarsystem - asthis book amply demonstrates I I - and panoramasof adolescentdeep-field galaxies swarming near the limit of the observableuniverse . Beyond pro- viding pretty pictures, asronomy pracesour worrd and our brief human livesin their true conrexrs:as vanishingly tiny subplotsin a truly enormous cosmicplay. The curtain op*, *iri, a Big Bang synthesisof the chemicalelembnts that evenruallylead to self- replicating, competitivestructures of moleculeswe call *life.,, While we humans play out our brief bit parts, we yearn ro grasp the overall plot. Naturally we wonder whether there are worrds beyond those of our solarsystem. Are they numerousor rare? How many of thcm haveconditions ripe for biologyf These are not new questions. _ In the fourth century BC, the Greek philosopher Epicurus spoke boldly of the infinite worlds that logrcally ,.aroms,, followed from the infinite number of thar he postulated. His contemporary,Aristotle, differed, seeing Earth asthe unique center ofa perfect crystallinesky. Aristotle,s Earth-centered cosmosdominated Westernthought for more than 1,500 years.The notion ofotherworlds took hold "gain on\ after Copernicusyanked Earth from its centralposition and placed it in orbit around rhe Sun with other planets. A computer simulates the birth of a Soon lupitcr-size planet around various thinkers another star. realizedthat the starsmight be diirant sunsand thercfore might haveplanets of their own. For centuries thereafter, detecting these extrasolar plenet! seemed beyond all possibility. Shining by reflected light, such objects should be roughly a billion times (perhap s 22 to 25 magnitudes) fainter than their host stars. Moreover, they would appear separatedby lessthan a few arcseconds,at best, from even the neareststars in our stellar neighborhood. 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