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e 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’t 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’s not much； and in fact it wasn’t enough。 the figures have since beenrefined and made a little more generous—5 percent nearer and 15 percent farther are thoughtto be more accurate assessments for our zone of habitability—but 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’s 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’s 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’s 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’t 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’t imagine even many geophysicists； when asked to counttheir blessings； would include living on a planet with a molten interior； but it’s a pretty nearcertainty that without all that magma swirling around beneath us we wouldn’t 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’t be baseball。
in addition to having a beneficial interior； we also have the right elements in the correctproportions。 in the most literal way； we are made of the right stuff。 this is so crucial to ourwell…being that we are going to discuss it more fully in a minute； but first we need to considerthe two remaining factors； beginning with another one that is often overlooked：
1the discovery of extremophiles in the boiling mudpots of yellowstone and similar organisms found elsewheremade scientists realize that actually life of a type could range much farther than that…even； perhaps； beneath theicy skin of pluto。 what we are talking about here are the conditions that would produce reasonably plexsurface creatures。
we’re a twin planet。not many of us normally think of the moon as a panion planet；but that is in effect what it is。 most moons are tiny in relation to their master planet。 themartian satellites of phobos and deimos； for instance； are only about ten kilometers indiameter。 our moon； however； is more than a quarter the diameter of the earth； which makesours the only planet in the solar system with a sizeable moon in parison to itself （exceptpluto； which doesn’t really count because pluto is itself so small）； and what a difference thatmakes to us。
without the moon’s steadying influence； the earth would wobble like a dying top； withgoodness knows what consequences for climate and weather。 the moon’s steady gravitationalinfluence keeps the earth spinning at the right speed and angle to provide the sort of stabilitynecessary for the long and successful development of life。 this won’t go on forever。 themoon is slipping from our grasp at a rate of about 1。5 inches a year。 in another two billionyears it will have receded so far that it won’t keep us steady and we will have to e up withsome other solution； but in the meantime you should think of it as much more than just apleasant feature in the night sky。
for a long time； astronomers assumed that the moon and earth either formed together orthat the earth captured the moon as it drifted by。 we now believe； as you will recall from anearlier chapter； that about 4。5 billion years ago a mars…sized object slammed into earth；blowing out enough material to create the moon from the debris。 this was obviously a verygood thing for us—but especially so as it happened such a long time ago。 if it had happened in1896 or last wednesday clearly we wouldn’t be nearly so pleased about it。 which brings us toour fourth and in many ways most crucial consideration：
timing。the universe is an amazingly fickle and eventful place； and our existence within itis a wonder。 if a long and unimaginably pleequence of events stretching back 4。6billion years or so hadn’t played out in a particular manner at particular times—if； to take justone obvious instance； the dinosaurs hadn’t been wiped out by a meteor when they were—youmight well be six inches long； with whiskers and a tail； and reading this in a burrow。
we don’t really know for sure because we have nothing else to pare our own existenceto； but it seems evident that if you wish to end up as a moderately advanced； thinking society；you need to be at the right end of a very long chain of outes involving reasonable periodsof stability interspersed with just the right amount of stress and challenge （ice ages appear tobe especially helpful in this regard） and marked by a total absence of real cataclysm。 as weshall see in the pages that remain to us； we are very lucky to find ourselves in that position。
and on that note； let us now turn briefly to the elements that made us。
there are ninety…two naturally occur