Where Comets Come From

They're the mystery tramps of the solar system, streaking crazily through a warm space they don't belong in. Now astronomers have tracked them to their frozen trans-Neptunian home.

By Sam Flamsteed|Wednesday, November 01, 1995
RELATED TAGS: SOLAR SYSTEM, COMETS
Comets die all the time. The televised death last year of Comet Shoemaker-Levy 9 happened to be particularly dramatic: the comet passed too close to Jupiter, got trapped and ripped apart by the giant planet’s gravity, and finally vanished into the swirling Jovian atmosphere in a weeklong staccato of H-bomb-size explosions. But Shoemaker-Levy’s demise was inevitable in any case. Had it given Jupiter a slightly wider berth, it would have been flung right out of the solar system instead. That’s how most of the comets we see now will make their exit. And even if Shoemaker- Levy had managed to dodge the planets, it would still have plunged into the sun eventually. Or, failing that, it would have been slowly vaporized. A comet is just a ball of ice and dust, after all, and every time it ventures anywhere near the sun, some of that dusty ice steams off into vapor, forming a hazy halo and a flowing tail--and robbing the comet of a fraction of its mass. The best estimate is that Shoemaker-Levy could not have survived in the solar system for more than a few hundred thousand years.

Comets die all the time, then--and yet nobody has made any new comets since the solar system was formed about 4.5 billion years ago. And that, astronomers have known for decades, is a problem. Any comets we see today must have arrived in the relatively toasty and planet-rich neighborhood of the sun quite recently from someplace much colder and farther away. The question is, from where?

The answer has been hovering just a bit out of reach, both literally and metaphorically, for a long time. But now, thanks to the Hubble Space Telescope and some fancy image processing, it’s finally within our grasp. Four astronomers have detected 30 or so vanishingly faint objects that are the best evidence yet for the long-sought Kuiper belt--a disk-shaped swarm of cometary bodies that begins beyond the orbit of Neptune and extends well past Pluto. The Kuiper belt is believed to be the source of the 140 comets whose orbits around the sun take 20 years or less. Since the Hubble team saw 30 Kuiper belt ice-balls in just a small patch of sky, they calculate that at least 200 million and possibly far more are waiting in cold storage for a chance to plunge sunward and sprout tails of vapor and dust.

If the observations are independently confirmed--and nobody has found any technical flaws so far--then they’re enormously important on several levels. These objects represent the first newly identified population in the solar system in nearly 200 years, since the asteroids were discovered, says Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colorado, and a member of the Hubble team. Moreover, since the Kuiper belt appears to resemble the dusty disks of matter that surround a number of nearby stars, including Beta Pictoris and Vega, its discovery dramatically bolsters the notion that planetary formation is extremely common. Says Stern: This is an astounding scientific treasure.

Astounding it may be, but it is not entirely unexpected. The notion that a belt of matter must be circling out beyond Neptune is nearly half a century old. It was in 1949 that British astronomer K. E. Edgeworth first observed that the solar system falls off a figurative cliff at Neptune. If you measure all the solid ice and rock in the outer solar system, Edgeworth found, you get a steady decrease in mass for a given volume as you go outward. After Neptune, though, the mass doesn’t just decrease; it immediately plunges almost to zero. (Pluto is so tiny, and the volume of space described by its orbit so huge, that it counts for almost nothing.)

That didn’t make sense to Edgeworth. How could the solar system possibly have a sharp edge? It was more reasonable to imagine that there just wasn’t enough material to form observable planets, but still plenty out there in small bits--much too small to be seen with any telescope then in existence. Two years later Dutch astronomer Gerard Kuiper came up with the same idea. There’s a lot of uncertainty about whether Kuiper knew about Edgeworth, says Hal Levison, also at the Southwest Research Institute and also on the Hubble team. I can’t imagine that he didn’t. But for whatever reason--perhaps because Kuiper was better known--the proposal is usually credited to him.

Kuiper was not proposing a source of comets, though; he was just pondering the distribution of matter in the solar system. At about the same time, however, another Dutchman, Jan Oort, suggested the existence of a roughly spherical reservoir of comets swirling around the solar system. Whereas the Kuiper belt is thought to extend between 35 and a few hundred astronomical units from the sun (1 AU is the distance between Earth and the sun; Neptune lies at around 30 AU, Pluto at around 39), the so-called Oort cloud is much farther out--between 20,000 and 100,000 AU. Oort’s argument was that comets with orbits of 20 or more years--including Halley’s and the recently discovered Hale-Bopp--come swooping in from all directions, which indicates that their home base is a sphere that surrounds us. The comets’ highly elongated orbits suggest that the sphere is very far away.

It wasn’t until 1980 that a Uruguayan astronomer, Julio Fernandez, came up with a corresponding proposal for the less-than-20-year comets. In contrast to their cousins from the Oort cloud, these approach the plane of the solar system at shallow angles--an average of only 11 degrees. That implies that they originate not in a spherical cloud but in a flattened disk. The disk has a hole in the middle, like an LP record, and the hole covers the solar system inside Neptune. According to Fernandez, the source of the short-period comets should be just the sort of trans- Neptunian belt that Edgeworth and Kuiper had proposed for entirely different reasons.

The computers they had back in 1980, says Levison, were terribly primitive, and Fernandez couldn’t really model the Kuiper disk very well. By 1988, though, Scott Tremaine of the University of Toronto and his colleagues Martin Duncan and Thomas Quinn were able to create such a computer model. The model showed that the short-period comets almost had to originate in a Kuiper disk or Kuiper belt (the terms are used interchangeably). But nothing short of actually seeing a resident of the belt would convince astronomers that it really existed. As it turned out, they didn’t have long to wait.

In 1975, just 12 years before she began searching for the Kuiper belt, Jane Luu was huddled with her family at the Saigon airport, hoping to get a spot on one of the last cargo planes out of South Vietnam. The Luus made it out one day before the city fell to the North Vietnamese. They spent some time in refugee camps; then a year living with an aunt in Kentucky, where Luu learned English; and landed finally in Ventura, California, where she went through high school, got interested in science, and began to excel. Physics at Stanford came next, and a summer job at the Jet Propulsion Laboratory got her interested in astronomy. When MIT offered her a place in its graduate program, she snapped it up. Soon she began working with planetary astronomer David Jewitt.

We started back in 1987, says Luu, before the paper by Tremaine and his colleagues made the idea of the Kuiper belt so fashionable. I was looking for a thesis topic, and Dave suggested that we use a CCD--or charge-coupled device, a highly sensitive electronic light detector--to look for objects in the outer solar system, which hadn’t been done before. Our main focus was to see if it was empty out there or not, and why. I read through all the papers by Kuiper and so on, and there seemed to be no reason to think there wouldn’t be objects beyond Neptune.

We did our search on and off for several years. We couldn’t work on it full-time, because we couldn’t get the telescope time. Nobody really had faith in it except us; the project seemed hokey to a lot of people.

In the summer of 1992 Luu and Jewitt, the latter of whom had by then moved to the University of Hawaii, got a five-night observing run on the university’s 2.2-meter telescope at the summit of the dead volcano Mauna Kea. On the second night, while taking repeated pictures of the same tiny patch of sky, they found a dot of light that moved just slightly from one image to the next. It had moved a hair more by the time they took a third image, and again for a fourth. We were very cautious, says Luu, so we looked for it again over the next three nights. It was still there.

Luu and Jewitt calculated the new object’s distance at some 44 AU--well beyond the orbit of Neptune. They reported their find to Brian Marsden, who runs the International Astronomical Union’s new-object clearinghouse--known, quaintly, as the Central Bureau for Astronomical Telegrams--from his office at the Harvard-Smithsonian Center for Astrophysics. Marsden designated the object 1992QB1, according to an arcane numbering system favored by his organization. He also calculated its orbit. The orbit turned out to be approximately circular.

This was no comet: its diameter was something like 125 miles. But its location put it squarely in the middle of the Kuiper belt, and since Kuiper belt objects would almost have to come in a wide range of sizes, astronomers were willing to consider the proposition that the belt had been discovered at last. Any notion that 1992QB1 was purely a fluke was put to rest a few months later, when Luu and Jewitt found a second object of similar size, labeled 1993FW. Since then they’ve found about 20 more, and discoveries by several other groups around the world have boosted the number to 28. Based on the percentage of the sky surveyed so far, Luu estimates there are perhaps 35,000 of these Hawaii-size lumps drifting lazily around the outer precincts of the solar system.

If there are 35,000 big ones, it’s reasonable to project a much larger number of much smaller objects--that is, of comets--in the same region. But reasonable doesn’t equal proven. Although the existence of the Kuiper belt was just about certain after Luu and Jewitt’s discoveries, there was still some room for doubt. That was our motivation for the Hubble search, says Stern. His colleague Anita Cochran, of the University of Texas at Austin, says, Our team has been interested in this problem for quite a while. I myself was searching for the same sort of large Kuiper belt object that Luu and Jewitt found. The seeing is slightly better in Hawaii than in Texas, though, and we were just unlucky.

Even with the best seeing on Earth, atmospheric blurring would make it impossible for anyone to see a future short-period comet--which would be no more than a tenth the diameter of 1992QB1--way out in the Kuiper belt. We’re talking about looking for something the size of Manhattan and the color of coal from 4 billion miles away, says Levison. So Cochran, Stern, Levison, and Martin Duncan, now at Queens University in Ontario, applied for and got a few precious hours on the Hubble’s tight observing schedule.

But when the first pictures came down from space last year, they were not what the researchers had hoped for: they were incredibly noisy-- they were filled with visual static. We were aghast, says Cochran. After the few stars and galaxies in the field of view had been noted and digitally subtracted, each image was dotted with myriad specks of light. It was impossible to tell which of those specks if any were real Kuiper belt objects; they could just as easily have been produced by light scattering off dust closer to Earth or by cosmic rays hitting the Hubble’s camera. The astronomers didn’t know whether they’d found the home swarm of the short- period comets or merely noise. It took us a few months to decide what to do, recalls Cochran. Levison and I sat in our offices and beat on it awhile, and the others offered their own suggestions.

The solution they finally agreed on was ingenious. Astronomers have to deal with visual static all the time; usually they simply figure that if a speck of light appears in a particular spot in one picture but not in the next, it’s noise. In this case, though, the comets themselves, if any were present, were moving, too. So what the observers did was to superimpose a series of images of one patch of sky, but shifted slightly to simulate what would have happened if they’d been able to swing the telescope and track the comets in their orbits. Now if a given spot of light appeared over and over at the same place within the shifted frames, its light would add up. Noise, popping up at random from frame to frame, would be much less likely to do that.

Unfortunately, nobody could tell precisely what orbit to simulate. The Kuiper belt objects all orbit the sun in roughly the same plane and in the same direction as the planets--counterclockwise if you look down on them from above Earth’s North Pole--but there is undoubtedly a lot of individual variation. There were literally hundreds of reasonable orbits, in fact, that intersected the Hubble’s field of view. So the team tried its frame-shifting technique with 154 of these plausible orbits, and again with a control sample of 154 highly implausible ones--orbits that went clockwise around the sun instead of counterclockwise.

The result, says Cochran, was that with the bogus orbits, we got a total of 24 objects persisting when the images were added. With the real orbits, we got a total of 53. The difference of 29, say the astronomers, represents actual objects popping up above the noise. It’s a purely statistical argument, explains Levison. It’s not as though we can point to any one object and say, ‘There it is.’ Any given blip of light could be noise. But we are confident that more than half of them are in fact cometary bodies in the Kuiper disk.

While the Hubble images appear to settle the long-standing question of where short-period comets come from, their implications are potentially even more far-reaching. The Kuiper belt, just about everyone agrees, is the best laboratory available for trying to understand how our solar system formed 4.5 billion years ago, because it contains the most pristine evidence of that process. Just about everything else in the solar system has been drastically altered from its primordial state. The planets were assembled from Kuiper-size objects that whacked into each other and stuck together; the asteroids are the heavily battered debris of planet formation; and the comets we can see from Earth have been cooked by the sun on each approach. The comets in the Oort cloud, which are believed to have started life between Uranus and Neptune and were then flung out to the remotest reaches of the solar system by the gravity of the outer planets, are probably much as they were 4.5 billion years ago--but they’re too far away to observe.

The Kuiper belt comets can be observed now, and like those in the Oort cloud they have been in deep freeze all along. They are like woolly mammoths preserved in ice--frozen relics of a long-lost age. If astronomers can manage to measure the spectrum of light bouncing off these extremely faint objects, they will learn something of the comets’ chemical composition--and that of the cloud of gas dust that gave rise to the whole solar system. When a more powerful camera is installed in the Hubble in 1999, says Stern, we may start getting some answers.

Individual Kuiper belt objects may be something close to primordial, but the population as a whole has probably not remained unchanged all these billions of years. With the current population of widely scattered, comet-size bricks, says Stern, it is hard to see how you could make a house the size of 1992QB1 through a process of random collision. If you take the disk as it exists today, he says, there’s no way you could make things like those Luu and Jewitt first found. Even if you make the most optimistic assumption, that they always stick together with perfect efficiency when they collide, it would take between 10 and 50 times the lifetime of the solar system to build a 1992QB1.

What’s more, 1992QB1 is not the biggest Kuiper belt denizen: icy Pluto is more than ten times as big, at 1,460 miles across, and along with its moon Charon it too was probably assembled from Kuiper comets. Some astronomers also consider Neptune’s large moon Triton, which is not in the Kuiper belt now, a Kuiper object; in the early days of the solar system, the theory goes, the belt extended into the region between Uranus and Neptune. But today only Triton survives there: it got trapped in orbit around Neptune, while over time its fellow travelers were either destroyed by the giant planets or slung into the Oort cloud.

To explain how objects as large as Triton or Pluto could grow in the first place, says Stern, it helps to assume that the Kuiper belt was not only more extensive once but also more densely populated with smaller comets. That would have ensured enough collisions to produce big planetoids. Many of the higher-velocity collisions, though, would have produced a lot of rubble and dust instead. Some of the dust would have fallen into the sun, and some would have been blown out of the solar system by solar radiation. Over time the Kuiper belt population would have been reduced to its present sparseness. The Kuiper belt today doesn’t contain enough mass to eliminate the puzzle that first intrigued Edgeworth and Kuiper--why does mass drop off so precipitously after Neptune?--but it may be a shadow of its former self.

Even today the Kuiper belt may be full of dust. With a dusty belt around it, our solar system would no longer look so different from what appear to be planetary disks around other stars. The disks discovered around Beta Pictoris, Vega, and several other stars in the 1980s are full of dust--it’s the dust, in fact, that makes them visible. We know, says Levison, that the dynamical lifetime of dust in those systems is short. Something must be constantly replenishing the supply. That something could well be Kuiper belt comets.

It’s a very nice scenario, says Stern. We don’t know if it’s right. But the team still has plenty of unanalyzed data left from two Hubble runs, and it’s applying for more Hubble time. So are other astronomers, including Luu and Jewitt. And Luu, for one, is still planning to train ground-based telescopes on the comet belt. All these studies together should in coming years provide a better idea of what the Kuiper belt looks like. It’s clear that this is suddenly a very fertile area of research, says Stern. We’re going to move quickly beyond simply showing the existence of the Kuiper belt to actually explaining it.

There also remains the open question of what jostles comets loose from the belt and sends them angling in toward the sun. One possible explanation, says Levison, is that the orbits are chaotic. A given object might move in a nearly circular orbit for 4 billion years, and then its eccentricity might suddenly get very large. It swings in toward Neptune and then gets slung into the inner solar system. That’s just one possibility; we still don’t know for sure why comets come to visit us occasionally. But at least we know where most of them call home.
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