One, sometimes called the cold classical Kuiper belt (or simply cold Kuiper belt) lies in the same plane as the rest of the Solar System, mostly about four to five billion miles out from the Sun. These objects, which comprise about half of those discovered to date,[2] aren't “cold” because they're more frigid than the rest, but because their sedate, in-the-plane orbits are dynamically calm.
Others zip high out of the plane, like satellites orbiting above the Earth's poles. These are dynamically “hot,” and are referred to as the hot Kuiper belt. They're particularly interesting because, as we shall see, they probably didn't form in their present orbits. “There's currently a lot of debate about what happened in the early history of the Solar System,” Jones says. “We used to think things were relatively quiet, but we can see that this is not the case. They are dynamically much more excited than we would expect.”
* * * *
A Belt that Shouldn't Exist
Figuring out what it is that kicked so many Kuiper belt objects out of the plane of the Solar System isn't the only problem facing theorists. Kuiper's original prediction notwithstanding, modern scientists have struggled to explain why there's anything out there at all.
“Classical planet-formation models have a hard time growing big objects at this distance from the Sun,” says Alex Parker, a graduate student in planetary science at the University of Victoria who spoke at an October 2010 meeting of the American Astronomical Society's Division of Planetary Science (DPS) in Pasadena, California. “Unless you start with a lot more mass in the primordial disk and then get rid of it later, these models can take longer than the age of the Solar System to grow the kinds of objects we see.”
In a moment, we'll see that classical planet-formation models have a lot of other problems, but for the moment, let's look at two basic issues.
First, all planet-formation models postulate that the Solar System began as a disk of dust and gas circling the Sun like the brim of a sombrero. This is a familiar image, not much changed over the years. But this protoplanetary disk didn't extend outward forever. Most likely, it stopped somewhere around 30 to 35 astronomical units (AU) from the Sun, not far beyond the orbit of Neptune (which lies at 30 AU).[3]
Unfortunately, the Kuiper belt doesn't seem to know the theory. Eris swings in an elliptical orbit ranging from 38 to 98 AU. Makemake is between 45 and 55 AU. Haumea ranges from 35 to 52, and Sedna's minimum distance is 76 AU, while its maximum is more than 900.
Perhaps the protoplanetary disk was simply larger than previously thought. But that leads to the problem alluded to by Parker, which is that even with thousands of objects in the Kuiper belt, there just isn't a lot of mass: no more than one-tenth that of Earth by one estimate. Even if continuing discoveries raise that figure, the total mass is still far too small if a protoplanetary disk dense enough for these objects to have formed extended that far. Under traditional models, the Kuiper belt either shouldn't be there or it should be a lot more densely packed.
A few years ago, these problems led theorists to propose that today's Kuiper belt objects must have formed much closer to the Sun, possibly in the same zone as Jupiter, Saturn, Uranus, and Neptune. Once formed, they were flung outward by dramatic shifts in the orbits of these planets, most importantly Neptune.[4]
The theory, known as the Nice model (because it was developed, in part, by scientists from Nice, France) quickly became a leading explanation for the Kuiper belt. “In some of these simulations, they even suggest that Uranus and Neptune switched places,” Parker said.
There's just one problem.
At the DPS meeting, Parker announced he had found at least one class of Kuiper belt objects for which this simply couldn't have happened. These objects are binary pairs, like Pluto and its moon Charon.
Such pairs are common in the Kuiper belt—estimates are that at least 30 % of objects in the cold classical Kuiper belt exist in pairs or larger groupings. But Parker found seven such pairs (probably harbingers of many more) that are so widely separated they circle each other in a very slow waltz, with orbital periods from four to seventeen years. “They're delicate,” he said, “weakly bound.”
Any close gravitational encounter with a giant planet would have pulled them apart and sent them in different directions. “They would not be there today if [they] were ever hassled by Neptune,” Parker said.
Nice-model theorist Alessandro Morbidelli, of the Laboratoire Cassiopee, Nice, France, agrees. “The Parker result calls for at least some tweak in the Nice model,” he says. “Either the cold Kuiper belt (or part of it) formed in situ, or it was pushed to its current location by a mechanism less violent than envisioned.”
Stephen Tegler, a planetary scientist at Northern Arizona University, concurs. “We have to come up with a way to tweak the Nice Model, or they formed in situ,” he says.
* * * *
Collisional Cascade
Enter Mike Brown, a cheery astronomer from California Institute of Technology who looks a bit like a young Bill Gates.
Brown's Twitter handle is “plutokiller” because of his role in kicking Pluto off the list of planets. But he's also codiscoverer of Eris, Sedna, Makemake, Haumea, and nearly a dozen others, and author of How I Killed Pluto and Why It Had It Coming. And like just about everyone studying the Kuiper belt, he too has come up with surprises to upset the theoreticians’ apple carts.
In his case, the problem stems from the newly found worldlets’ densities.
Traditional theories say planets condensed bit by bit out of the primordial disk in an incremental process that saw microscopic particles coalesce stepwise into ever-larger ones that themselves collided and merged, until eventually they formed planets.
But, as we noted before, the solar disk should have been getting rather tenuous out toward the Kuiper belt. “To get something that large, you would have to have had to accrete from a very large swath of the outer Solar System,” Brown says. This should have averaged out any random variations in the nebula's composition. “You would think they would be some of the most uniformly comprised objects in the Solar System.”
Pluto fits this model perfectly. Its density is about 2.0 grams per cubic centimeter, indicating that it's composed of a fifty-fifty mix of rock and ice, just about what theoreticians would expect.
But not everything is like Pluto.
The first clue came from Haumea. It's not merely elongated, it's flattened.
Earlier I called it football-shaped. But that was a simplification. Brown calls it a football “that's a bit deflated and stepped on.” It's also spinning end-over-end so fast it completes a full revolution every four hours. Small asteroids often look, and spin like that but Haumea is long enough that it wouldn't quite fit between San Diego and Vancouver, British Columbia. And it turns out to have a density of about 3.0 grams per cubic centimeter. “That's 100 percent rock,” Brown says.
As far back as 2006, Brown suggested that Haumea (then known as 2003 EL61) might be the core of a larger planetoid that got clobbered early in its early history. A glancing blow would not only have set it spinning, but knocked off its icy outer mantle. Supporting this is the observation that it has two tiny moons that are almost pure ice. Could these be chips off the original ice block?
A year later,[5] his team reported additional evidence: the discovery of five other fragments whose orbits indicate they were blown away from Haumea near the dawn of the Solar System. The fragments (the largest of which might be 400 kilometers across) appear to be made of nearly pure ice—just right to be more of Haumea's missing mantle.
One anomalously dense world created by a mantle-busting collision could be a fluke. But as more and more numbers come in, the densities of large Kuiper belt objects appear to be all over the place, ranging from well below 1 gram per cubic centimeter (giving them densities akin to that of frothy ice) to nearly as dense as Haumea.
Brown suggests that this means these objects didn't form by gradual accretion. Instead, he thinks they originated from larg
e chunks, several hundred kilometers in diameter—large enough to have already differentiated into rocky cores and icy mantles.[6] These primordial objects, sometimes called planetesimals, then bashed together in big collisions, a process he calls “pyramidal growth.”
That way, even the largest bodies could have formed in only a few steps. Each collision would have blasted debris into space—the source of today's smaller Kuiper belt objects, like those associated with Haumea. Sometimes, two ice-depleted objects would have collided, producing a larger ice-depleted object. Other times, icy chunks of one-time mantle would merge into larger, rock-depleted worlds. Still other collisions would produce Pluto-like mixes of rock and ice.
This only works, however, if the main growth came from collisions between large objects, rather than stepwise accretion from small ones. Otherwise the law of averages would make everything come out more or less identical.
Further evidence comes from Parker's binaries. They are so far apart that it's not merely a close encounter with Neptune that would have disrupted them. They would also have been knocked out of each other's orbits if they'd been pelted by too many kilometer-sized objects: a strong indication that there were never many such objects around—or, in other words, that Kuiper belt objects started out big.[7]
* * * *
Rapid Condensation
All of this is interesting, but it begs the question of where the first round of objects came from. Somehow, the ancestral Kuiper belt had to have jumped from solar disk to planetesimals without going though intermediate steps.
And this isn't the only clue that something might be amiss with the stepwise-growth models. It's not that the traditional theory isn't appealing. It's nice, orderly—tidy, in fact. Microscopic bits of dust and ice coalesce, first into dust bunnies, then into tennis balls, beach balls, etc., until you wind up with objects large enough to be called planetesimals.
But there's always been a problem. It's easy to get from dust cloud to dust bunnies. And it's easy to model what happens to planetesimals once they reach kilometer size. But there's a no-man's-land between, Brown says, in which the models just won't let the midsize objects accrete. Instead, they quickly lose orbital speed from gravitational interactions with the other component of the solar disk: gas. You could think of it as a type of gravitational drag whose effect is that tennis-ball-sized objects and the like spiral into the Sun more rapidly than they can merge into larger bodies.
It's one of those things that tends to inspire scientific hand-waving. So there was this disk of gas and dust, people want to say. It formed a bunch of little clumps, and somehow (enter the hand-waving) these stayed around long enough to merge into things the size of football stadiums. These then merged into planets, moons, the Earth, etc.
That step in the middle has been the elephant-in-the-living-room of planet-formation models. “It's a mystery nobody wanted to think too hard about,” Brown says.
The answer, he and Parker believe, lies in an emerging set of new planet-formation models. Like their predecessors, they start with a dusty solar disk, but instead of focusing solely on stepwise accretion, they include the effect of eddies, vortexes, and other turbulent flows in the early disk.
Brown compares these turbulences to brooms sweeping dust particles together. By creating dense zones in the solar disk, they make for a short-cutted process of planet formation in which “big hunks” fall together without need for intermediates. “The first things [to be formed] are 100, 200, and 300 kilometers, instead of millimeters,” he says.
What we then have is a Kuiper belt in which everything originated as rapidly condensing planetesimals. Today's big objects are the result of Brown's collisional cascade. Small ones are fragments blown away by collisions. And the mid-sized ones are survivors from the original population of planetesimals.
But we still have problems.
To begin with, the only collision we have traces of is that which stripped Haumea of its mantle. The ability to trace fragments from it clearly shows that it occurred out in the belt, but that's the only such collision astronomers have been able to trace in that manner. In fact, it's rather startling that there were ever any big collisions in the Kuiper belt. It's a gigantic zone, covering far more volume than all of the inner portions of the Solar System combined. It's hard to imagine how the objects we see out there ever managed to hit each other often enough for Brown's collisional cascade to have occurred.
And we still have an overall missing-mass problem. The turbulence models still require a minimum density for Brown's “big hunks” to condense. As before, there either should be more (or bigger) objects out there . . . or none at all.
Possibly some of the missing objects were pulverized by giant collisions that reduced them to dust that then fell into the Sun by gas drag. But Morbidelli doesn't buy it. “You cannot hope that big objects disappear into dust by collisions,” he says. “They are too big to destroy completely.”
Instead he believes that the Nice model is still the best explanation. In fact, he thinks Brown's collisional cascade adds considerable support for the model by indicating that Kuiper belt objects originated in a more closely packed region, where there was more opportunity for mantle-busting collisions.
Furthermore, the Nice model solves the missing-mass problem, because most encounters with Neptune would not have been gentle. In the process of being flung outward, computer models show that only one object in a thousand objects actually winds up in the Kuiper belt, he says. The rest are ejected into interplanetary space. “That is exactly what we need,” Morbidelli says.
* * * *
Dual Origins
That model gets us all of the big, collision-produced objects and delivers them nicely to their present locations. Parker's binaries and the cold classical Kuiper belt are another story. “His result implies that at least part of the cold Kuiper belt never experienced encounters with Neptune,” Morbidelli admits.
Perhaps, he suggests, the hot and cold Kuiper belts were produced by different mechanisms. This would mean the hot Kuiper belt objects were produced as we've already discussed, by dramatic encounters with Neptune. But if Neptune played a role in moving the binaries outward, it had to have been by a much less violent mechanism.
Morbidelli and Harold Levison, of the Southwest Research Institute, Boulder, Colorado, suggested one such mechanism as far back as 2003: an orbital resonance with Neptune.[8] Such resonances are common in outer-planet moon systems, where, among other things, they create intriguing features in Saturn's rings. In this case, the binaries would have formed farther out than Neptune's original orbit, then been slowly nudged further by a resonance with Neptune, as Neptune itself migrated to its present position. And that's not the only possibility, because we don't know exactly how Neptune reached its modern orbit. “Many variants are possible,” Morbidelli says.
But we again have a missing-mass problem because now we can't use Neptune to toss most of the initial population of objects off toward the stars. “This is a great conundrum,” Morbidelli says.
To date, there is no clear solution. One possibility is that the solar disk did extend into the Kuiper belt, with enough density for planetesimals to form, at least as far out as the locations of Parker's binaries. But that far out, perhaps it contained a lower fraction of dust. That might alter the process of planetesimal formation, blocking the rapid-condensation turbulence process from working.
“It might be that well within the edge, most of the solids are converted into big planetesimals,” Morbidelli says. “Well beyond the edge, no solids are converted into planetesimals, and [dust clumps] simply spiral inward by gas drag.
“Close to the edge, maybe a little fraction of the solids is converted into large planetesimals, while most still spiral inward. If this were the case (and we have no proof that this was) then a few large planetesimals could have formed in the 35 to 45 AU range. [These] would constitute the current cold belt, without need to remove excess mass.”
This also would
have the advantage of suggesting that the existence of Parker's binaries means exactly what it appears to mean: that they formed in situ. But, Morbidelli notes, at present all of this is mere speculation. “To go beyond will take a lot of time.”
* * * *
Rogue Planets
From a science fictional perspective, this provides wonderful fodder for stories. In his classic novels Gateway and Beyond the Blue Event Horizon, Frederick Pohl postulated a CHON (carbon, hydrogen, oxygen, and nitrogen) food factory on a protocomet far out on the edges of the Solar System. It lay in the Oort cloud, a shadowy zone that might be the birthplace of comets, nearly a light-year from the Sun.
The Oort cloud is a long way out, however. The same resources should be findable in the Kuiper belt (which wasn't discovered until 1992, more than a decade after Pohl wrote his stories).
Many stories have envisioned asteroid-belt civilizations. The Kuiper belt might provide something similar, not too impossibly remote, especially for super-loner types who think the asteroid belt is too crowded.
What we know about the Kuiper belt's formation also raises interesting possibilities for interstellar space. In a recent paper in Nature, a team led by Takahiro Sumi of Osaka University, Japan, announced the discovery of free-floating Jupiter-sized worlds in interplanetary space. Using a technique called gravitational lensing, in which otherwise too-faint objects are revealed by their gravitational effect on light passing by them, the scientists spotted ten such planets in a single search—enough that they concluded that rogue Jupiters, unbound to any star, might outnumber stars by a factor of two to one.[9] These giant planets would have gotten tossed away from the stars of their birth by processes similar to those that, under the Nice model, may have hurled myriads of smaller bodies off into interstellar space.
The bottom line is that planets and stars need not go together. And while rogue Jupiters probably wouldn't be particularly habitable, rogue Plutos or Sednas might be—especially because there are arguments that some of these bodies might even have liquid water, generated by internal heating.
Analog SFF, April 2012 Page 5