2 WELCOME TO THE SOLAR SYSTEMAS
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In short, there isn’t a great deal that goes on in the universe that astronomers can’t findwhen they 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 uncertain but definitely other than Pluto. Consulting acolleague named Robert Harrington, he concluded that what he was looking at was a moon.
And it wasn’t just any moon. Relative to the planet, it was the biggest moon in the solarsystem.
This was actually something of a blow to Pluto’s status as a planet, 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 anyone to 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 designed to 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 astronomers get out at night in observatories and scan theskies. That’s not true. Almost all the telescopes we have in the world are designed to peer atvery tiny little pieces of the sky way off in the distance to see a quasar or hunt for black holesor look at a distant galaxy. The only real network of telescopes that scans the skies has beendesigned and built by the military.”
We have been spoiled by artists’ renderings into imagining a clarity of resolution thatdoesn’t exist in actual astronomy. Pluto in Christy’s photograph is faint and fuzzy—a piece ofcosmic lint—and its moon is not the romantically backlit, crisply delineated companion orbyou would get in a National Geographic painting, but rather just a tiny and extremelyindistinct hint of additional fuzziness. Such was the fuzziness, in fact, that it took seven yearsfor anyone to spot the moon again and thus independently confirm its existence.
One nice touch 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 seminal event in astronomy waslargely to the credit of the astronomer Percival Lowell. Lowell, who came from one of theoldest and wealthiest 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), endowed the famous observatory that bears his name, but is most indeliblyremembered for his belief that Mars was covered with canals built by industrious Martians for purposes of conveying water from polar regions to the dry but productive lands nearer theequator.
Lowell’s other abiding conviction was that there existed, somewhere out beyond Neptune,an undiscovered ninth planet, dubbed Planet X. Lowell based this belief on irregularities hedetected in the orbits of Uranus and Neptune, and devoted the last years of his life to trying 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 deflecting attention awayfrom the Mars canal saga (which by now had become a serious embarrassment), the LowellObservatory directors decided to resume the search and 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 searching 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 planet beyond Neptuneproved to be comprehensively erroneous. Tombaugh could see at once that the new planetwas nothing like the massive gasball Lowell had postulated, but any reservations he or anyoneelse had about the character of the new planet were soon swept aside in the delirium thatattended almost any big news story in that easily excited age. This was the first American-discovered planet, 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 was posthumously hailed everywhere as a genius ofthe first order, and Tombaugh was largely forgotten, except among planetary astronomers,who tend to revere him.
A few astronomers continue to think there may be a Planet 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 none to reflect.) The idea is that itwouldn’t be a conventional planet 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 cosmos 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 planet at 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 honors Gerard Kuiper, a Dutch native working in America, who expanded theidea. The Kuiper belt is the source of what are known as short-period comets—those thatcome past pretty regularly—of which the most famous is Halley’s comet. The more reclusivelong-period comets (among them the recent visitors Hale-Bopp and Hyakutake) come fromthe much more distant Oort cloud, about which more presently.
It is certainly true that Pluto doesn’t act much like the other planets. Not only is it runty andobscure, but it is so variable in its motions that no one can tell you exactly where Pluto will bea century hence. Whereas the other planets orbit on more or less the same plane, Pluto’sorbital path is tipped (as it were) out of alignment at an angle of seventeen degrees, like thebrim of a hat tilted rakishly on someone’s head. Its orbit is so irregular that for substantialperiods on each of its lonely circuits around the Sun it is closer to us than Neptune is. For most of the 1980s and 1990s, Neptune was in fact the solar system’s most far-flung planet.
Only on February 11, 1999, did Pluto return to the outside lane, there to remain for the next228 years.
So if Pluto really is a planet, it is certainly an odd one. It is very tiny: just one-quarter of 1percent as massive as Earth. If you set it down on top of the United States, it would cover notquite half the lower forty-eight states. This alone makes it extremely anomalous; it means thatour planetary system consists of four rocky inner planets, four gassy outer giants, 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 to regard that section of the cosmos moreattentively and as of early December 2002 had found over six hundred additional Trans-Neptunian 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 percent, 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 and entertainment, 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 need to 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 can’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 and September1977) was that Jupiter, Saturn, Uranus, and Neptune were aligned in a way that happens onlyonce every 175 years. This enabled the two Voyagers to use a “gravity assist” technique inwhich the craft were successively flung from one gassy giant to the next in a kind of cosmicversion 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 can takeadvantage of favorable Jovian positioning, plus some advances in technology, and get there 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 uneventful. Our solar system may be the liveliest thing for trillions of miles, butall the visible stuff in it—the Sun, the planets and their moons, the billion or so tumblingrocks of the asteroid belt, comets, and other miscellaneous drifting detritus—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 theplanets coming one after the other at neighborly intervals—the outer giants actually castshadows over each other in many illustrations—but this is a necessary deceit to get them all on the same piece of paper. Neptune in reality 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 percent 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 come close. On a diagram of the solar system toscale, with Earth reduced to about the diameter of a pea, Jupiter would be over a thousand feetaway and Pluto 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 Centauri, our nearest star,would be almost ten thousand miles away. Even if you shrank down everything 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 come 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 can begin to understandhow even the most significant objects—Pluto’s moon, for example—have escaped attention.
In this respect, Pluto has hardly been alone. Until the Voyager expeditions, Neptune wasthought to have two moons; Voyager found six more. When I was a boy, the solar system wasthought to contain thirty moons. The total now is “at least ninety,” about a third of which havebeen found in just the last ten years.
The point to remember, of course, is that when considering 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 doesn’t end there. In fact, it isn’t even close to endingthere. We won’t get to the solar system’s edge until we have passed through the Oort cloud, avast celestial realm of drifting comets, and we won’t reach the Oort cloud for another—I’m sosorry about this—ten thousand years. Far from marking the outer edge 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 undertaking 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 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 can reasonably imagine, there 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 can’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 can be said with confidence about the Oort cloud is that it starts somewherebeyond Pluto and stretches some two light-years out into the cosmos. The basic unit 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 itsexistence in 1932, and for the Dutch astronomer Jan Oort, who refined the calculations eighteen 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 comets in orbit. It’s not a very strongbond, so the comets drift in a stately manner, moving at only about 220 miles an hour. Fromtime to time some of these lonely comets are nudged out of their normal orbit by some slightgravitational perturbation—a passing star perhaps. Sometimes they are ejected into theemptiness of space, never 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 comets, pass through theinner solar system. Just occasionally these stray visitors smack into something solid, likeEarth. That’s why we’ve come out here now—because the comet we have come to see hasjust begun a long fall toward the center 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, and return to it much later in the story.
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 interstellar space. And there is a great deal of this nothingness untilyou get to the next bit of something. Our nearest neighbor in the cosmos, Proxima Centauri,which is part of the three-star cluster known as Alpha Centauri, 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 next landmark of consequence, Sirius, would involve another 4.6light-years of travel. And so it would go if you tried to star-hop your way across the cosmos.
Just reaching the center of our own galaxy would take far longer than we have existed asbeings.
Space, let me repeat, is enormous. The average distance between stars out there is 20million million miles. Even at speeds approaching 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 orfrightening the daylights out of some poor guy in a pickup truck on 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 othergalaxies, many of them even larger than ours. In the 1960s, a professor at Cornell namedFrank Drake, excited by such whopping numbers, worked out a famous equation designed tocalculate the chances of advanced life in the cosmos 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 planetary systems; divide that by the number ofplanetary systems that could theoretically support life; divide that by the number on whichlife, having arisen, advances to a state of intelligence; and so on. At each such division, thenumber shrinks colossally—yet even with the most conservative inputs the number ofadvanced civilizations just in the Milky Way always works out to be somewhere in themillions.
What an interesting and exciting thought. We may be only one of millions of advancedcivilizations. Unfortunately, space being spacious, the average distance between any two ofthese civilizations is reckoned 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 in their telescopes, they’re watching light that leftEarth two hundred years ago. So they’re not seeing you and me. They’re watching the FrenchRevolution and Thomas Jefferson and people in silk stockings and powdered wigs—peoplewho don’t know what an atom is, or a gene, 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 congratulate 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 planets 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 inserted into the universe,” Saganwrote, “the chances that you would be on or near a planet 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 news that in February 1999 the International AstronomicalUnion ruled officially that Pluto is a planet. The universe is a big and lonely pl