2 WELCOME TO THE SOLAR SYSTEMAS

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    TROHESE DAYS  do the most amazing things. If someoruck a mat the Moon, they could spot the flare. From the tihrobs and wobbles of distant starsthey  ihe size and character and even potential habitability of plas muote to be seen—plas so distant that it would take us half a million years in a spaceshipto get there. With their radio telescopes they  capture wisps of radiation so preposterouslyfaint that the total amount of energy collected from outside the solar system by all of themtogether since colleg began (in 1951) is “less than the energy of a single snowflakestriking the ground,” in the words of Carl Sagan.

    In short, there isn’t a great deal that goes on in the universe that astronomers ’t fihey have a mind to. Which is why it is all the more remarkable to reflect that until 1978no one had ever noticed that Pluto has a moon. In the summer of that year, a youngastronomer named James Christy at the U.S. Naval Observatory in Flagstaff, Arizona, wasmaking a routine examination of photographic images of Pluto when he saw that there wassomething there—something blurry and uain but definitely other than Pluto. sulting acolleague named Robert Harrington, he cluded that what he was looking at was a moon.

    And it wasn’t just any mooive to the pla, it was the biggest moon in the solarsystem.

    This was actually something of a blow to Pluto’s status as a pla, which had never beenterribly robust anyway. Since previously the space occupied by the moon and the spaceoccupied by Pluto were thought to be one and the same, it meant that Pluto was much smallerthan anyone had supposed—smaller even than Mercury. Indeed, seven moons in the solarsystem, including our own, are larger.

    Now a natural question is why it took so long for ao find a moon in our own solarsystem. The answer is that it is partly a matter of where astronomers point their instrumentsand partly a matter of what their instruments are desigo detect, and partly it’s just Pluto.

    Mostly it’s where they point their instruments. In the words of the astronomer ClarkChapman: “Most people think that astronet out at night in observatories and s theskies. That’s not true. Almost all the telescopes we have in the world are desigo peer atvery tiny little pieces of the sky way off in the distao see a quasar or hunt for black holesor look at a distant galaxy. The only real work of telescopes that ss the skies has beendesigned and built by the military.”

    We have been spoiled by artists’ renderings into imagining a clarity of resolution thatdoes in actual astronomy. Pluto in Christy’s photograph is faint and fuzzy—a piece ofit—and its moon is not the romantically backlit, crisply delied panion orbyou would get in a National Geographic painting, but rather just a tiny aremelyindistinct hint of additional fuzziness. Such was the fuzziness, in fact, that it took seven yearsfor ao spot the moon again and thus indepely firm its existence.

    One ouch about Christy’s discovery was that it happened in Flagstaff, for it was therein 1930 that Pluto had been found in the first place. That semi in astronomy waslargely to the credit of the astronomer Percival Lowell. Lowell, who came from one of theoldest ahiest Boston families (the one in the famous ditty about Boston being thehome of the bean and the cod, where Lowells spoke only to Cabots, while Cabots spoke onlyto God), ehe famous observatory that bears his name, but is most indeliblyremembered for his belief that Mars was covered with als built by industrious Martians forpurposes of veying water from pions to the dry but productive lands heequator.

    Lowell’s other abiding vi was that there existed, somewhere out beyoune,an undiscovered ninth pla, dubbed Pla X. Lowell based this belief ularities hedetected in the orbits of Uranus aune, aed the last years of his life t tofind the gassy giant he was certain was out there. Unfortunately, he died suddenly in 1916, atleast partly exhausted by his quest, and the search fell into abeyance while Lowell’s heirssquabbled over his estate. However, in 1929, partly as a way of defleg attention awayfrom the Mars al saga (which by now had bee a serious embarrassment), the LowellObservatory directors decided to resume the seard to that end hired a young man fromKansas named Clyde Tombaugh.

    Tombaugh had no formal training as an astronomer, but he was diligent and he was astute,and after a year’s patient searg he somehow spotted Pluto, a faint point of light in aglittery firmament. It was a miraculous find, and what made it all the more striking was thatthe observations on which Lowell had predicted the existence of a pla beyouneproved to be prehensively erroneous. Tombaugh could see at ohat the new plawas nothing like the massive gasball Lowell had postulated, but any reservations he or anyoneelse had about the character of the new pla were soo aside in the delirium thatattended almost any big news story in that easily excited age. This was the first Ameri-discovered pla, and no one was going to be distracted by the thought that it was really justa distant icy dot. It was named Pluto at least partly because the first two letters made amonogram from Lowell’s initials. Lowell osthumously hailed everywhere as a genius ofthe first order, and Tombaugh was largely fotten, except among plaary astronomers,who tend to revere him.

    A few astronomers tio think there may be a Pla X out there—a real whopper,perhaps as much as ten times the size of Jupiter, but so far out as to be invisible to us. (Itwould receive so little sunlight that it would have almost o reflect.) The idea is that itwouldn’t be a ventional pla like Jupiter or Saturn—it’s much too far away for that;we’re talking perhaps 4.5 trillion miles—but more like a sun that never quite made it. Moststar systems in the os are binary (double-starred), which makes our solitary sun a slightoddity.

    As for Pluto itself, nobody is quite sure how big it is, or what it is made of, what kind ofatmosphere it has, or even what it really is. A lot of astronomers believe it isn’t a pla all,but merely the largest object so far found in a zone of galactic debris known as the Kuiperbelt. The Kuiper belt was actually theorized by an astronomer named F. C. Leonard in 1930,but the name herard Kuiper, a Dutative w in America, who expaheidea. The Kuiper belt is the source of what are known as short-period ets—those thate past pretty regularly—of which the most famous is Halley’s et. The more reclusivelong-period ets (among them the ret visitors Hale-Bopp and Hyakutake) e fromthe much more distant Oort cloud, about which more presently.

    It is certainly true that Pluto doesn’t act much like the other plas. Not only is it runty andobscure, but it is so variable in its motions that no one  tell you exactly where Pluto will bea tury hence. Whereas the other plas orbit on more or less the same plane, Pluto’sorbital path is tipped (as it were) out of alig at an angle of seventeen degrees, like thebrim of a hat tilted rakishly on someone’s head. Its orbit is sular that for substantialperiods on each of its lonely circuits around the Sun it is closer to us thaune is. Formost of the 1980s and 1990s, une was in fact the solar system’s most far-flung pla.

    Only on February 11, 1999, did Pluto return to the outside lahere to remain for the 228 years.

    So if Pluto really is a pla, it is certainly an odd o is very tiny: just one-quarter of 1pert as massive as Earth. If you set it down on top of the Uates, it would cover notquite half the lower forty-eight states. This alone makes it extremely anomalous; it means thatour plaary system sists of four rocky inner plas, fassy iants, and a tiny,solitary iceball. Moreover, there is every reason to suppose that we may soon begin to findother even larger icy spheres in the same portion of space. Then we will have problems. AfterChristy spotted Pluto’s moon, astronomers began tard that se of the oreattentively and as of early December 2002 had found over six hundred additional Traunian Objects, or Plutinos as they are alternatively called. One, dubbed Varuna, is nearlyas big as Pluto’s moon. Astronomers now think there may be billions of these objects. Thedifficulty is that many of them are awfully dark. Typically they have an albedo, orreflectiveness, of just 4 pert, about the same as a lump of charcoal—and of course theselumps of charcoal are about four billion miles away.

    And how far is that exactly? It’s almost beyond imagining. Space, you see, is justenormous—just enormous. Let’s imagine, for purposes of edification aertai, thatwe are about to go on a journey by rocketship. We won’t go terribly far—just to the edge ofour own solar system—but we o get a fix on how big a place space is and what a smallpart of it we occupy.

    Now the bad news, I’m afraid, is that we won’t be home for supper. Even at the speed oflight, it would take seven hours to get to Pluto. But of course we ’t travel at anything likethat speed. We’ll have to go at the speed of a spaceship, and these are rather more lumbering.

    The best speeds yet achieved by any human object are those of the Voyager 1 and2 spacecraft,which are now flying away from us at about thirty-five thousand miles an hour.

    The reason the Voyager craft were launched when they were (in August aember1977) was that Jupiter, Saturn, Uranus, aune were aligned in a way that happens onlyonce every 175 years. This ehe two Voyagers to use a “gravity assist” teique inwhich the craft were successively flung from one gassy giant to the  in a kind of icversion of “crack the whip.” Even so, it took them nine years to reach Uranus and a dozen tocross the orbit of Pluto. The good news is that if we wait until January 2006 (which is whenNASA’s New Horizons spacecraft is tentatively scheduled to depart for Pluto) we  takeadvantage of favorable Jovian positioning, plus some advances in teology, ahere inonly a decade or so—though getting home again will take rather longer, I’m afraid. At allevents, it’s going to be a long trip.

    Now the first thing you are likely to realize is that space is extremely well named and ratherdismayingly uful. Our solar system may be the liveliest thing for trillions of miles, butall the visible stuff in it—the Sun, the plas and their moons, the billion or so tumblingrocks of the asteroid belt, ets, and other miscellaneous driftius—fills less than atrillionth of the available space. You also quickly realize that none of the maps you have everseen of the solar system were remotely drawn to scale. Most schoolroom charts show theplas ing oer the other at neighborly intervals—the iants actually castshadows over each other in many illustrations—but this is a necessary deceit to get them allon the same piece of paper. une iy isn’t just a little bit beyond Jupiter, it’s waybeyond Jupiter—five times farther from Jupiter than Jupiter is from us, so far out that itreceives only 3 pert as much sunlight as Jupiter.

    Such are the distances, in fact, that it isn’t possible, in any practical terms, to draw the solarsystem to scale. Even if you added lots of fold-out pages to your textbooks or used a reallylong sheet of poster paper, you wouldn’t e close. On a diagram of the solar system toscale, with Earth reduced to about the diameter of a pea, Jupiter would be over a thousaaluto would be a mile and a half distant (and about the size of a bacterium, so youwouldn’t be able to see it anyway). On the same scale, Proxima tauri, our  star,would be almost ten thousand miles away. Even if you shrank dowhing so that Jupiterwas as small as the period at the end of this sentence, and Pluto was no bigger than amolecule, Pluto would still be over thirty-five feet away.

    So the solar system is really quite enormous. By the time we reach Pluto, we have e sofar that the Sun—our dear, warm, skin-tanning, life-giving Sun—has shrunk to the size of apinhead. It is little more than a bright star. In such a lonely void you  begin to uandhow even the most signifit objects—Pluto’s moon, for example—have escaped attention.

    In this respect, Pluto has hardly been alone. Until the Voyager expeditions, une wasthought to have two moons; Voyager found six more. When I was a boy, the solar system wasthought to tain thirty moons. The total now is “at least y,” about a third of which havebeen found in just the last ten years.

    The point to remember, of course, is that when sidering the universe at large we don’tactually know what is in our own solar system.

    Now the other thing you will notice as we speed past Pluto is that we are speeding pastPluto. If you check your itinerary, you will see that this is a trip to the edge of our solarsystem, and I’m afraid we’re not there yet. Pluto may be the last object marked onschoolroom charts, but the system doeshere. In fact, it isn’t even close to endingthere. We won’t get to the solar system’s edge until assed through the Oort cloud, avast celestial realm of drifting ets, and we won’t reach the Oort cloud for another—I’m sosorry about this—ten thousand years. Far from marking the e of the solar system, asthose schoolroom maps so cavalierly imply, Pluto is barely one-fifty-thousandth of the way.

    Of course we have no prospect of such a journey. A trip of 240,000 miles to the Moon stillrepresents a very big uaking for us. A manned mission to Mars, called for by the firstPresident Bush in a moment of passing giddiness, was quietly dropped when someone workedout that it would cost $450 billion and probably result in the deaths of<big>..</big> all the crew (their DNAtorn to tatters by high-energy solar particles from which they could not be shielded).

    Based on what we know now and  reasonably imagihere is absolutely no prospectthat any human being will ever visit the edge of our own solar system—ever. It is just too far.

    As it is, even with the Hubble telescope, we ’t see even into the Oort cloud, so we don’tactually know that it is there. Its existence is probable but entirely hypothetical.

    *About all that  be said with fidence about the Oort cloud is that it starts somewherebeyond Pluto and stretches some two light-years out into the os. The basiit ofmeasure in the solar system is the Astronomical Unit, or AU, representing the distance from*Properly called the Opik-Oort cloud, it is named for the Estonian astronomer Ernst Opik, who hypothesized itsexisten 1932, and for the Dutch astronomer Jan Oort, who refihe calculatioeen years later.

    the Sun to the Earth. Pluto is about forty AUs from us, the heart of the Oort cloud about fiftythousand. In a word, it is remote.

    But let’s pretend again that we have made it to the Oort cloud. The first thing you mightnotice is how very peaceful it is out here. We’re a long way from anywhere now—so far fromour own Sun that it’s not even the brightest star in the sky. It is a remarkable thought that thatdistant tiny twinkle has enough gravity to hold all these ets in orbit. It’s not a very strongbond, so the ets drift in a stately manner, moving at only about 220 miles an hour. Fromtime to time some of these lonely ets are nudged out of their normal orbit by some slightgravitational perturbation—a passing star perhaps. Sometimes they are ejected into theemptiness of spaever to be seen again, but sometimes they fall into a long orbit aroundthe Sun. About three or four of these a year, known as long-period ets, pass through theinner solar system. Just occasionally these stray visitors smato something solid, likeEarth. That’s why we’ve e out here now—because the et we have e to see hasjust begun a long fall toward the ter of the solar system. It is headed for, of all places,Manson, Iowa. It is going to take a long time to get there—three or four million years atleast—so we’ll leave it for now, aurn to it much later iory.

    So that’s your solar system. And what else is out there, beyond the solar system? Well,nothing and a great deal, depending on how you look at it.

    In the short term, it’s nothing. The most perfect vacuum ever created by humans is not asempty as the emptiness of iellar space. And there is a great deal of this nothingness untilyou get to the  bit of something. Our  neighbor in the os, Proxima tauri,which is part of the three-star cluster known as Alpha tauri, is 4.3 light-years away, a sissyskip in galactic terms, but that is still a hundred million times farther than a trip to the Moon.

    To reach it by spaceship would take at least twenty-five thousand years, and even if you madethe trip you still wouldn’t be anywhere except at a lonely clutch of stars in the middle of avast nowhere. To reach the  landmark of sequence, Sirius, would involve another 4.6light-years of travel. And so it would go if you tried to star-hop your way across the os.

    Just reag the ter of our own galaxy would take far lohan we have existed asbeings.

    Space, let me repeat, is enormous. The average distaween stars out there is 20million million miles. Even at speeds approag those of light, these are fantasticallychallenging distances for any traveling individual. Of course, it is possible that alien beingstravel billions of miles to amuse themselves by planting crop circles in Wiltshire htening the daylights out of s<q></q>ome puy in a pickup tru a lonely road in Arizona(they must have teenagers, after all), but it does seem unlikely.

    Still, statistically the probability that there are other thinking beings out there is good.

    Nobody knows how many stars there are in the Milky Way—estimates range from 100 billionor so to perhaps 400 billion—and the Milky Way is just one of 140 billion or so alaxies, many of them even larger than ours. In the 1960s, a professor at ell namedFrank Drake, excited by such whopping numbers, worked out a famous equation desigocalculate the ces of advanced life in the os based on a series of diminishingprobabilities.

    Under Drake’s equation you divide the number of stars in a selected portion of the universeby the number of stars that are likely to have plaary systems; divide that by the number ofplaary systems that could theoretically support life; divide that by the number on whichlife, having arisen, advao a state of intelligence; and so on. At each such division, thenumber shrinks colossally—yet even with the most servative inputs the number ofadvanced civilizations just in the Milky Way always works out to be somewhere in themillions.

    What an iing aing thought. We may be only one of millions of advancedcivilizations. Unfortunately, space being spacious, the average distaween any two ofthese civilizations is reed to be at least two hundred light-years, which is a great dealmore than merely saying it makes it sound. It means for a start that even if these beings knowwe are here and are somehow able to see us ielescopes, they’re watg light that leftEarth two hundred years ago. So they’re not seeing you ahey’re watg the FrenchRevolution and Thomas Jefferson and people in silk stogs and powdered wigs—peoplewho don’t know what an atom is, ene, and who make their electricity by rubbing a rodof amber with a piece of fur and think that’s quite a trick. Any message we receive from themis likely to begin “Dear Sire,” and gratulate us on the handsomeness of our horses and ourmastery of whale oil. Two hundred light-years is a distance so far beyond us as to be, well,just beyond us.

    So even if we are not really alone, in all practical terms we are. Carl Sagan calculated thenumber of probable plas in the universe at large at 10 billion trillion—a number vastlybeyond imagining. But what is equally beyond imagining is the amount of space throughwhich they are lightly scattered. “If we were randomly ied into the universe,” Saganwrote, “the ces that you would be on or near a pla would be less than one in a billiontrillion trillion.” (That’s 1033, or a one followed by thirty-three zeroes.) “Worlds are precious.”

    Which is why perhaps it is good hat in February 1999 the Iional Astronomiion ruled officially that Pluto is a plahe universe is a big and lonely place. We  dowith all the neighbors we  get.

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