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ce you leave the surface world。
which brings us back （well； nearly） to our earlier observation that earth is not the easiestplace to be an organism； even if it is the only place。 of the small portion of the planet鈥檚surface that is dry enough to stand on； a surprisingly large amount is too hot or cold or dry orsteep or lofty to be of much use to us。 partly； it must be conceded； this is our fault。 in terms ofadaptability； humans are pretty amazingly useless。 like most animals； we don鈥檛 much likereally hot places； but because we sweat so freely and easily stroke； we are especiallyvulnerable。 in the worst circumstances鈥攐n foot without water in a hot desert鈥攎ost peoplewill grow delirious and keel over； possibly never to rise again； in no more than six or sevenhours。 we are no less helpless in the face of cold。 like all mammals； humans are good atgenerating heat but鈥攂ecause we are so nearly hairless鈥攏ot good at keeping it。 even in quitemild weather half the calories you burn go to keep your body warm。 of course； we cancounter these frailties to a large extent by employing clothing and shelter； but even so theportions of earth on which we are prepared or able to live are modest indeed： just 12 percentof the total land area； and only 4 percent of the whole surface if you include the seas。
yet when you consider conditions elsewhere in the known universe； the wonder is not thatwe use so little of our planet but that we have managed to find a planet that we can use even abit of。 you have only to look at our own solar system鈥攐r； e to that； earth at certainperiods in its own history鈥攖o appreciate that most places are much harsher and much lessamenable to life than our mild； blue watery globe。
so far space scientists have discovered about seventy planets outside the solar system； outof the ten billion trillion or so that are thought to be out there； so humans can hardly claim tospeak with authority on the matter； but it appears that if you wish to have a planet suitable forlife； you have to be just awfully lucky； and the more advanced the life； the luckier you have tobe。 various observers have identified about two dozen particularly helpful breaks we havehad on earth； but this is a flying survey so we鈥檒l distill them down to the principal four。 theyare：
excellent location。we are； to an almost uncanny degree； the right distance from the right sortof star； one that is big enough to radiate lots of energy； but not so big as to burn itself outswiftly。 it is a curiosity of physics that the larger a star the more rapidly it burns。 had our sunbeen ten times as massive； it would have exhausted itself after ten million years instead of tenbillion and we wouldn鈥檛 be here now。 we are also fortunate to orbit where we do。 too muchnearer and everything on earth would have boiled away。 much farther away and everythingwould have frozen。
in 1978； an astrophysicist named michael hart made some calculations and concluded thatearth would have been uninhabitable had it been just 1 percent farther from or 5 percent closer to the sun。 that鈥檚 not much； and in fact it wasn鈥檛 enough。 the figures have since beenrefined and made a little more generous鈥�5 percent nearer and 15 percent farther are thoughtto be more accurate assessments for our zone of habitability鈥攂ut that is still a narrow belt。
1to appreciate just how narrow； you have only to look at venus。 venus is only twenty…fivemillion miles closer to the sun than we are。 the sun鈥檚 warmth reaches it just two minutesbefore it touches us。 in size and position； venus is very like earth； but the smalldifference in orbital distance made all the difference to how it turned out。 it appears thatduring the early years of the solar system venus was only slightly warmer than earth andprobably had oceans。 but those few degrees of extra warmth meant that venus could not holdon to its surface water； with disastrous consequences for its climate。 as its water evaporated；the hydrogen atoms escaped into space； and the oxygen atoms bined with carbon to forma dense atmosphere of the greenhouse gas co2。 venus became stifling。 although people ofmy age will recall a time when astronomers hoped that venus might harbor life beneath itspadded clouds； possibly even a kind of tropical verdure； we now know that it is much toofierce an environment for any kind of life that we can reasonably conceive of。 its surfacetemperature is a roasting 470 degrees centigrade （roughly 900 degrees fahrenheit）； which ishot enough to melt lead； and the atmospheric pressure at the surface is ninety times that ofearth； or more than any human body could withstand。 we lack the technology to make suitsor even spaceships that would allow us to visit。 our knowledge of venus鈥檚 surface is based ondistant radar imagery and some startled squawks from an unmanned soviet probe that wasdropped hopefully into the clouds in 1972 and functioned for barely an hour beforepermanently shutting down。
so that鈥檚 what happens when you move two light minutes closer to the sun。 travel fartherout and the problem bees not heat but cold； as mars frigidly attests。 it； too； was once amuch more congenial place； but couldn鈥檛 retain a usable atmosphere and turned into a frozenwaste。
but just being the right distance from the sun cannot be the whole story； for otherwise themoon would be forested and fair； which patently it is not。 for that you need to have：
the right kind of planet。i don鈥檛 imagine even many geophysicists； when asked to counttheir blessings； would include living on a planet with a molten interior； but it鈥檚 a pretty nearcertainty that without all that magma swirling around beneath us we wouldn鈥檛 be here now。
apart from much else； our lively interior created the outgassing that helped to build anatmosphere and provided us with the magnetic field that shields us from cosmic radiation。 italso gave us plate tectonics； which continually renews and rumples the surface。 if earth wereperfectly smooth； it would be covered everywhere with water to a depth of four kilometers。
there might be life in that lonesome ocean； but there certainly wouldn鈥檛 be baseball。
in addition to having a beneficial interior； we also h