On the Fringe

Astronomers look to the Kuiper belt  for clues to the solar system’s history

Beyond Neptune lies a reservoir of rejects — icy debris left to roam the solar system’s dim outer limits having never coalesced into planets. But these frozen relics preserve a trove of clues about the earliest history and architecture of the solar system, astronomers are discovering.

ON THE FRINGE Pluto and other icy bodies in the Kuiper belt (illustrated here) may help scientists re-create the solar system’s past. Gary Hincks/Science Photo Library
TODAY’S KUIPER BELT ORBITS | The elongated, highly inclined orbits of many of the denizens in the doughnut-shaped Kuiper belt (shown in blue) suggest that a massive intruder barged into the belt early in the history of the solar system, ejecting bodies and jumbling orbits. Illustration by Anil Rao
HIT-AND-RUN | Quaoar is an oddball in the belt because of its rocklike density. In one formation scenario, Quaoar could have hit a bigger Pluto-sized body at high speeds and ricocheted off instead of sticking. This may have stripped away Quaoar’s outer, fluffy layers. Craig Agnor, E. Asphaug

Named for astronomer Gerard Kuiper, who in 1951 predicted the existence of this 3-billion-kilometer-wide swath of icy chunks, the Kuiper belt didn’t begin to reveal itself to observers until 1992. Since then, researchers have found more than a thousand bodies filling a doughnut-shaped belt, which extends 30 to about 50 astronomical units from the sun. One astronomical unit is the average distance between the Earth and sun.

Pluto may be the most famous resident of this frozen netherworld, but other objects in this sparsely populated region stand out for their bewildering variety of shapes, colors, densities and orbits. Some travel sedately on circular paths that hew closely to the plane in which the planets orbit the sun. Many have wildly elliptical orbits and move on paths at high inclination to that plane.

The puffed-up, elongated orbits and present-day sparseness of the belt all but scream that the region had a close and violent encounter with at least one of the outer planets, says theorist Hal Levison of the Southwest Research Institute in Boulder, Colo. Recent findings are providing new evidence of this long-ago melee, and the details could help scientists reconstruct early conditions in the solar system.

Planetary scientists examining the Kuiper belt today are “like a CSI team going into a room where there was a grisly murder,” Levison says. And sometimes the blood spattered on the wall — the thousands of small bodies in the Kuiper belt — can tell you more about the early solar system than the actual planetary bodies can, he adds.

Astronomers don’t yet have a complete picture of the Kuiper belt, and new riddles —some discussed for the first time in October in Fajardo, Puerto Rico, at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences — are emerging. A comprehensive new survey of the belt, set to begin by early spring, will likely explain some of these mysteries and uncover new ones.

Migration and mayhem

To understand how the Kuiper belt has retained so much information about planet formation, scientists must first understand the planet-building process. All the planets and smaller bodies in the solar system formed from particles of gas and dust that stuck together within a protoplanetary disk surrounding the young sun, Levison notes.

But particles can’t stick unless they collide gently. Careening rocks and ice chunks in elongated, high-inclination orbits — like many of those in the Kuiper belt today — would hit with high velocity, which would break them apart instead of building them up. Only objects in more circular orbits have low enough relative velocities to coalesce.

That means that the belt’s biggest bodies, such as Pluto and Eris (the largest known in the region), would never have formed unless they originally followed more circular, low-inclination orbits. In addition, the belt must have been much more crowded and thousands of times heavier than it is today. Like a ghostly highway with only a few cars, the belt nowadays has such a low density of objects that any collision — whether a high-speed crack-up or a low-speed merger — is improbable.

“You needed a massive Kuiper belt, and you needed relative velocities to be low” for large bodies to have formed during the first few millions years of the solar system, Levison notes. But then, “you need to perturb the objects and [tip] their orbits to get the highly inclined, elongated orbits we see today.”

Such changes are a smoking gun that an intruder must have plowed into the Kuiper belt, he says. Whatever disturbed the belt also removed 99.99 percent of its mass.
The obvious suspect is Neptune, the closest large body to the belt, says observer Mike Brown of Caltech. “That’s really the only thing that will scatter these objects all around.”

Now researchers are trying to figure out how and when Neptune barged into the belt and how quickly it did so, details that could help explain how and when the planets assumed their final positions in the solar system.

In one scenario, suggested earlier in the decade by Levison and his colleagues, Neptune and its three larger compatriots — Jupiter, Saturn and Uranus — were once packed together into a region only about half the diameter of Neptune’s current orbit (SN: 2/14/09, p. 26).

Gravitational interactions with the then-hefty belt gradually spread these planets out until the orbits of Jupiter and Saturn reached a special synchrony. That synchrony strengthened the mutual gravity of Jupiter and Saturn, which in turn hurled Neptune, and possibly Uranus, headlong into the Kuiper belt. Like a bowling ball, Neptune scattered most of the icy bodies toward the sun or out of the solar system entirely, and scrambled the orbits of those denizens that remained. With the belt emptied of much of its material, the gravitational tussle lessened and Neptune came to rest there.

Researchers now think that comparing the number of Kuiper belt objects in two particular orbits relative to that of Neptune could reveal when and how quickly Neptune’s big migration occurred — a natural speedometer.

Scientists had realized that as Neptune moved toward the belt early in the solar system’s history, some belt members fell under the planet’s gravitational spell and settled into special orbits: Each time that Neptune orbits the sun twice, these objects go around once. Some of the objects in these orbits, classified as in a 2:1 resonance with Neptune, appear to trail the planet, while others lead it.

The number of Kuiper belt objects trailing Neptune in this resonance would increase relative to the number leading it the faster that Neptune migrated, Ruth Murray-Clay of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and Eugene Chiang of the University of California, Berkeley reported in 2005. Counting these objects might ultimately reveal whether Neptune took roughly 1 million or 10 million years to migrate about 7 astronomical units from its birth site.

The timescale would help researchers “understand the properties of the proto-planetary disk around the sun at the time that Neptune formed,” Murray-Clay says. If the migration lasted only a million years, it could mean that the planetesimals in the disk were particularly abundant and had circular orbits. Which, in turn, could mean that planets coalesced relatively quickly.

Studies also show that giant extra-solar planets orbiting within roasting distance of their parent stars couldn’t have formed so close and therefore must have migrated inward. New clues about how Neptune migrated could provide hints about the movement of planets in systems far beyond Earth’s, Murray-Clay notes.

More answers, more mysteries

That goal isn’t just wishful thinking. A sky survey called Pan-STARRS, which will feature the world’s largest digital cameras — 1.4 billion pixels — attached to each of four small telescopes on Mount Haleakala in Maui, Hawaii, is poised to do the head count required to gauge Neptune’s migration speed, Murray-Clay says.

The survey, set to begin early this year, covers a vast portion of the sky each night. The initial 3.5-year study has the capability to find Kuiper belt objects as small as about 250 kilometers across, or about one-tenth the diameter of Pluto, says Matt Holman of Harvard-Smithsonian. From the northern sky, Pan-STARRS “will do a complete census of the Kuiper belt” and will have a field of view wide enough to record objects at very high inclinations to the plane in which the planets orbit the sun, he says. Earlier studies were limited to finding objects in or near that plane.

By providing a fuller picture of the belt and possibly finding as many as 10 times the number of Kuiper belt denizens now known, the survey is also expected to quantify the rarity of particular classes of Kuiper belt objects and the extent to which such objects’ orbits were altered by an early interaction with one or more of the outer planets.

And the survey will search for fainter, more remote bodies beyond the belt, such as Sedna, the most distant known object in the solar system (see “Solitary puzzle beyond the belt,” Page 20).

Surveys about to get underway in Chile will be the first comprehensive studies of the Kuiper belt from the southern hemisphere. And around 2016, the mammoth Large Synoptic Survey Telescope is scheduled to begin operation, providing an even more detailed study of the belt.

Other ongoing surveys are finding Kuiper belt objects. In sifting through 4.5 years of data collected by the Hubble Space Telescope’s Fine Guidance Sensors, researchers have found the first belt object smaller than a kilometer, observed as it passed in front of and occulted the light of a distant star. The newly discovered body has a diameter of 500 meters, Hilke Schlichting of the University of Toronto and Caltech and her colleagues report in the Dec. 17 Nature.

Another study, the Taiwanese-American Occultation Survey, has been using the same technique to look for small belt objects since 2005 and has found none.

The single finding in Schlichting’s study is a surprise because she and her collaborators calculate that small Kuiper belt objects should be about 35 times more numerous than the observations indicate. The deficit, Schlichting’s team concludes, suggests that over the lifetime of the solar system, small bodies in the belt have collided and ground down to dust. This process would produce a fainter version of the debris disks observed around a myriad of other stars believed to have planets and similar belts.

Millions of Quaoars

With new and continuing surveys, more clues to the early and still-evolving solar system are expected to emerge. And future findings may add to the list of odd belt characters already uncovered.

There’s Haumea, with its highly elongated shape, an average diameter of 1,500 kilometers and a family of ice cubes — satellites made of pure water-ice, the only such moons known in the belt. And Eris is bizarre for its highly inclined orbit. “We blame Neptune for all the inclined orbits in the belt,” Brown notes, but a 45-degree incline is too high to be generated by that planet’s gravity. “No one has been able to explain Eris’ incline; it’s kind of the dirty secret no one wants to talk about,” Brown says.

And then there’s Quaoar.

Discovered by Brown and his colleagues in 2002, Quaoar took center stage at a session of the planetary science meeting in Puerto Rico. Wesley Fraser of Caltech reported that he and Brown had used the Hubble Space Telescope to observe both Quaoar and its tiny moon, Weywoot. By measuring the motion of the orbiting moon, the researchers found that Quaoar is about 350 kilometers smaller than previously estimated, bringing its diameter to less than half that of Pluto. With a smaller diameter, Quaoar must be correspondingly denser; otherwise it wouldn’t wield a large enough gravitational tug to keep Weywoot in a bound orbit.

Quaoar must have a density akin to that of rock, despite  its residency in the icy belt, Fraser reported. That not only makes Quaoar a supreme oddball — perhaps the densest body in the Kuiper belt — but also puts it on par with rocky bodies that fill the asteroid belt, located between the orbits of Mars and Jupiter.

In an article now in press in Chemie der Erde, Erik Asphaug of the University of California, Santa Cruz offers an intriguing explanation for the new finding. His solution not only fits with existing evidence that the Kuiper belt was once a more crowded place, but also could explain other formation scenarios.

He envisions that Quaoar was originally covered by a mantle of ice that made it 300 to 500 kilometers bigger than it is today, and that it collided with another Kuiper belt body about twice its size — an object roughly the diameter of Pluto, possibly Pluto itself.

In this scenario, Quaoar is a bullet, striking a bigger body in the belt at a speed a few times higher than the escape velocity of that object. At that speed, Quaoar wouldn’t have stuck to the object but would have ricocheted off it. The bigger body would have emerged from the collision pretty much unscathed, but the encounter would have gravitationally and mechanically stripped Quaoar of most of its icy mantle, leaving only its denser, rockier core intact.

If Asphaug is right, the belt must have been rife with millions of Quaoars several billion years ago. That’s the era when Quaoar-sized objects were coalescing to make larger bodies like Pluto. A large number of Quaoars are required so that after most of these bodies were either ejected from the belt or accreted onto bigger bodies, there would still be enough left over to make a hit-and-run collision likely. A few other Quaoars might still lurk somewhere in the belt, awaiting discovery by future surveys, Asphaug suggests.

Anytime a small body has a much higher density than bigger bodies in the same region of space, this collision scenario may apply, Asphaug says. Indeed, hit-and-runs are by no means limited to the outer solar system.

Asphaug’s interest in such collisions was first piqued by puzzles closer to the sun. In the inner solar system, the dense, iron-rich planet Mercury stands out like a sore thumb. He speculates that a fluffier, bigger Mercury with a lot of mantle collided with Venus, the next-largest planet in the neighborhood, ridding Mercury of its lower-density, outer layers while leaving Venus relatively undisturbed. And in the asteroid belt, an exotic population of about 100 dense, iron-rich asteroids may be the remains of hit-and-run collisions with larger rocks.

In the standard picture of evolution in the solar system, Asphaug says, bodies of similar size were thought to collide and merge, and then a few chunks might be whacked off during a subsequent impact. In contrast, the hit-and-run model “gives a whole new pathway for planetary evolution,” Asphaug says. Understanding how such impacts occurred may indicate how crowded the early solar system was.

All this activity — formations, migrations, collisions — took place some
4 billion years ago, soon after the solar system’s birth. Today, Brown says, “we are left with this junk on the floor, the Kuiper belt objects, to try to reconstruct what might have happened.”

Solitary puzzle beyond the belt

Discovered by Mike Brown of Caltech and colleagues in 2004, an object called Sedna is the most distant body known in the solar system. Residing beyond the Kuiper belt, Sedna approaches as close to Earth as 76 times the Earth-sun distance (or 76 astronomical units) and travels as far away as 1,000 times that distance during its highly elongated, 10,500-year orbit (illustrated below).

Sedna’s very existence is a puzzle, Brown says. The body lies too far from the Kuiper belt to have been affected by any migration of Neptune but too close to the sun to have been pulled outward by a passing star. If Sedna were one of a group of objects in similarly remote orbits, it would indicate that the sun was born within a cluster of stars, long since dispersed, that tugged on these now-remote bodies and pulled them into their current orbits.

But a survey of the edge of the solar system has failed to turn up any other single object like Sedna. Meg Schwamb of Caltech, who collaborated with Brown, reported these findings in October at a planetary science meeting in Fajardo, Puerto Rico. The survey searched a sizable patch of sky — about 220 times the apparent area of the full moon — and was sensitive enough to detect large objects as far as 1,200 astronomical units from the sun. Sedna’s seemingly solitary status makes it “one of the strangest objects in the solar system,” Brown says.

Hit-and-run from Science News on Vimeo.

Quaoar is an oddball in the belt because of its rocklike density. In one formation scenario, Quaoar could have hit a bigger Pluto-sized body at high speeds and ricocheted off instead of sticking. This may have stripped away Quaoar’s outer, fluffy layers.

Video Credit: Craig Agnor, E. Asphaug