Outer Limits

Solar system at the fringe

Planetary scientist Mike Brown has had plenty of practice finding objects at the fringes of the solar system. In sky images spaced an hour apart, he and his colleagues have identified several of the solar system’s most distant denizens, revealed by their motion relative to the background of fixed stars. Early in 2004, soon after his team began using a new version of their discovery software, Brown was in his office at the California Institute of Technology in Pasadena reviewing images on his computer screen. He came across a sequence of pictures, taken by the Samuel Oschin telescope on Palomar Mountain near Escondido, Calif., showing a faint object moving so slowly that it must have been in the outer solar system, far beyond Pluto. “I think I fell out of my chair,” says Brown.

Among a slew of recently discovered objects at the far reaches of the solar system, an icy body dubbed Sedna is providing scientists with new clues about the origin and evolution of that distant region. M. Brown et al./ California Institute of Technology
REMOTE VIEW. The sky as seen from the proposed surface of the recently discovered Sedna, which strays farther from the sun than any other known solar system body. A. Schaller, NASA, STScI
NEPTUNES GALORE. In a newly proposed theory, the fledgling solar system contained five Neptunes (green circles), crammed into a region that begins 20 times as far from the sun as Earth’s orbit and extends to twice that distance. The interaction of these embryonic planets with Kuiper belt objects (yellow circles) helped to sculpt the outer solar system. Chiang et al.
PLUTOCRACY. Surface of Pluto seen with the Hubble Space Telescope. Pluto is no longer an oddball at the solar system’s fringe. Several newly discovered objects have about the same size, mass, and orbit as it does. A. Stern/SwRI, M. Buie/Lowell, NASA, ESA

The extremely slow motion of the object, now dubbed Sedna (SN: 3/20/04, p. 179: Planetoid on the Fringe: Solar system record breaker), indicates that the body lies so far out that “there is nothing in the solar system today that could have ever made it … and if you run time backwards, there’s nothing in the solar system today that can put [a body] in this orbit,” says Brown. Finding Sedna, he adds, “just blew our minds.”

Sedna is the strangest of a recently discovered batch of outer solar system residents, but other finds are also surprising in their size, spin speed, or location. The new discoveries, which include the first planet-size body found beyond Pluto, are forcing astronomers to retool their ideas about the evolution and origin of the outer solar system—just as NASA prepares to launch the first spacecraft targeted to explore Pluto and its outlying neighbors.

“We’re really not at the stage yet of making small refinements in our understanding [of the outer solar system],” says Matt Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “It’s still the case that the discovery of one or two new objects is changing our views.”

Solar system sprawl

The outer solar system used to be such a simple place. Beyond Neptune lay Pluto, the ninth planet, and that, it seemed, was that. Although theorists had suggested since the late 1940s that the solar system’s outer reaches contain an abundance of frozen, cometlike objects, it wasn’t until 1992 that astronomers found the first members of this proposed population.

Scientists now divide the outer solar system into two distinct regions, both of which serve as reservoirs for comets. The Kuiper belt extends outward from the orbit of Pluto to about 70 times the Earth-sun distance, which defines an astronomical unit (AU). The pancake-shaped belt contains a population of some 100,000 objects, with about 1,000 known members. It is the source of Halley and other comets that visit the inner solar system at least once every 200 years. According to theorists, a second, more remote reservoir surrounds the Kuiper belt. This proposed reservoir, the Oort cloud, would be the home of longer-period comets.

The recent far-out discoveries include an object that is not only unusually big—about three-quarters the diameter of Pluto—but also spins faster than any other large body in the solar system. That object, called 2003 EL61, resides in the Kuiper belt, where it now orbits the sun at a distance of 52 AU.

In July 2005, scientists announced another find: the first member of the Kuiper belt larger than Pluto (SN: 8/6/05, p. 83: Bigger than Pluto: Tenth planet or icy leftover?). Dubbed Xena, the object has a diameter 1.5 times as great as that of Pluto, making it the largest body found in the solar system since 1846, when Neptune and its moon Triton were discovered. Astronomers are informally calling Xena the tenth planet, pending a final ruling by the International Astronomical Union.

Xena is now located 97 AU from the sun but spends most of its time in the Kuiper belt. Its path tilts 44° relative to the plane in which most planets orbit the sun.

Researchers recently found yet another large object in the Kuiper belt. Dubbed Buffy, the body is nearly as big as Pluto, astronomers announced on Dec. 13, 2005. Buffy’s orbit has an even more extreme incline of 47°, Christian Veillet of the Canada-France-Hawaii Telescope on Mauna Kea and his colleagues report. That Kuiper belt object now is located 58 AU from the sun, or nearly twice as far from the sun as Neptune is.

Objects such as Xena and Buffy had eluded detection precisely because planetary scientists had concentrated their search for distant solar system bodies on objects orbiting in the same plane as the planets. Had a body as massive as Pluto resided in that plane, “we would have found it long ago,” Brown says.

While Buffy, Xena, and 2003 EL61 all belong to the Kuiper belt, remote Sedna, whose orbit extends from 76 to 126 AU, may be the first known member of the Oort cloud. No other known solar system object has such a remote orbit.

Individually, each of these newfound objects tells its own story about the solar system’s outlying regions. Collectively, the new discoveries—along with Pluto—speak volumes about planet formation throughout the solar system, says Brown. He and his colleagues, Chad Trujillo of the Gemini Observatory in Hilo, Hawaii, and David L. Rabinowitz of Yale University discovered or codiscovered three of the four newfound bodies.

With the spate of discoveries, Pluto is no longer an oddball. “Suddenly, we have all these [Plutolike] objects, and we can study how they appear at different temperatures and distances from the sun and at different sizes,” says Brown. “We’re getting a new window into planet formation.”

Full story

Among the newfound objects, astronomers have gathered the most details about the Kuiper belt object 2003 EL61. Thanks to its record-breaking spin—it rotates once every 4 hours—along with observations of its moons, “we’re learning more about this object and its internal structure than anything else out there,” notes Brown.

When researchers discover a Kuiper belt object, they usually can’t determine how big it is, only how bright. That’s because a small object that reflects a lot of sunlight looks just as bright as a large object that reflects only a tiny amount. In the case of 2003 EL61, however, “we got lucky,” says Brown.

Last summer, he and his colleagues discovered a moon orbiting the body. The orbit depends solely on the mass of 2003 EL61. By tracking the moon’s orbit for 6 months, the researchers determined that 2003 EL61 has one-third the mass of Pluto.

But determining mass doesn’t reveal the size of a body. It could either be small and dense, like rock, or large and porous, like ice. The rapid spin of 2003 EL61 provided the team with the answer.

Any extended object that spins becomes stretched like pizza dough tossed into the air, notes Brown. A high-density object elongates less than a low-density object does. Observations of 2003 EL61’s varying brightness show that its longest axis is only about the same length as Pluto’s diameter. Therefore, it must be as dense as solid rock.

Brown says, “Nothing else so large and so elongated or so quickly rotating is known anywhere in the solar system.”

That’s not all. Spectra recently taken by Brown and his colleagues with the Keck 1 Telescope atop Hawaii’s Mauna Kea reveal that the surface of 2003 EL61 harbors an abundance of frozen water. Taken together, the spectra and rotational data suggest that the object resembles a “rocky, squashed football that’s been sprayed with a frosting of water-ice,” says Brown.

Fleshing out the story line, researchers have also obtained spectra of the moon. The spectra indicate that the moon is a chunk of pure water ice, matching the surface composition of its parent body.

The similarity suggests that the moon came into being when a giant impactor struck 2003 EL61 soon after it coalesced from the same swirling disk of dust, gas, and ice that swaddled the infant sun and gave birth to the planets. Such a collision would have knocked icy material off the frosty surface of 2003 EL61 to form a cloud of vaporized debris around the main body. Over millions of years, debris from the cloud would have reassembled into an icy moon. The same collision would also have increased the spin speed of 2003 EL61.

“It’s a beautifully consistent story,” says Brown. “As we get more detail with bigger and better telescopes, we’ll learn even more about this object.”

In a Nov. 29, 2005 circular of the International Astronomical Union, he and his colleagues announced that they had found a second moon orbiting 2003 EL61. Brown says that he’s betting that spectra of the newly found moon will reveal that it’s also a chunk of pure water ice.

Kuiper enigmas

Collisions must have been common in the Kuiper belt’s early history. That’s the only way to account for the abundance of moons, says theorist Martin Duncan of Queen’s University in Toronto. Eleven percent of known Kuiper belt objects, including three of the four largest, have small orbiting partners.

These moons couldn’t have formed recently, Duncan notes. The density of objects in the belt today is too low for collisions to happen often, and the relative velocity of objects is too high for fragments of colliding objects to stick together and form a moon.

The orbits of Kuiper belt objects also indicate a violent past. Many of them travel in highly elliptical paths inclined at large angles to the plane of the solar system. The only way theorists can account for these wayward trajectories is by assuming that direct collisions combined with close gravitational encounters between objects.

Several researchers argue that the Kuiper belt didn’t form at its present location. In one scenario (SN: 5/28/05, p. 340: Roaming Giants: Did migrating planets shape the solar system?), proposed early this year by Hal Levison of the Southwest Research Institute of Boulder, Colo., and his colleagues, the new-formed planets huddled in a space only half as large as the region that they occupy today. Just beyond this crowd resided a slowly orbiting band of ice, gas, and dust. The forerunner of the Kuiper belt contained 100 to 1,000 times as many inhabitants as the belt does today. As particles from this band leaked out both toward and away from the sun, their gravity influenced the orbits of the young planets. Jupiter moved inward, while Saturn, Uranus, and Neptune moved outward.

The migration proceeded slowly and steadily until Saturn had drifted into an orbit exactly twice as wide as Jupiter’s. With the planets in this relationship, their mutual gravity caused Saturn’s circular orbit to suddenly elongate. This pushed the orbits of Uranus and Neptune so far outward that they barreled into the band of gas, dust, and ice.

Debris in the band dispersed. Some of it slammed into the inner solar system, while a small percentage—perhaps one-hundredth of the original population—was flung out further, becoming today’s sparsely populated but energetic Kuiper belt.

In an alternative model recently proposed by Eugene Chiang of the University of California, Berkeley and his colleagues, the newborn planets occupied about the same region of space as they do today. However, there were five Neptune-size planets instead of just one. That’s the maximum number of Neptunes that could have coalesced from the amount of ice, dust, and gas particles that then inhabited the solar system’s outskirts, notes Chiang.

In this model, gravitational interaction among the five Neptunes stirs up their orbits, transforming them from circular to highly elliptical. In several million years, the paths become so elongated that they intersect the orbits of Saturn and Jupiter. The gravity of those two giant planets then ejects four of the Neptunes, which pass through the Kuiper belt on their way out of the solar system. Their passage wreaks havoc on the belt, depleting it of material and throwing into disarray the circular orbits of belt residents.

It’s not yet clear how future observations could distinguish between Levison’s and Chiang’s scenarios. Both theories can account for nearly all the tilted, elliptical orbits of the Kuiper belt today.

However, Xena and Buffy stick out like sore thumbs. No theory, even one in which planets plow through the Kuiper belt, can explain such high tilts, notes Levison. He and other theorists are struggling to incorporate these new finds into their models.

Far out

Among the new finds, it’s Sedna that Brown places “scientifically head and shoulders above everything else out there, as important a discovery as Pluto was in 1930.”

Consider its orbit. The Kuiper belt ends at 70 AU, but Sedna gets no closer than 76 AU. “It’s not part of the Kuiper belt and never comes back into the belt,” says Brown.

To place Sedna where it is today, “there had to be something different about the solar system in the past,” he notes. Sedna’s presence suggests that that the sun, like many stars, was not born alone but in a dense cluster, says Duncan. Soon after this stellar nursery emerged, the sun and the other stars went their separate ways. By now, they would have moved so far apart that no hint of their common origin would remain.

Early on, however, during the first million-or-so years, the nearest sibling to the infant sun could have exerted a sizable tug on the outer solar system. That tug would have slowed and retained Sedna and perhaps other objects traveling rapidly outward at the fringes of the solar system, says Duncan. Without the pull of a neighboring star, these objects, flung outward by the gravity of the planets, would probably have sailed out of the solar system altogether.

That mechanism, some theorists argue, would have sculpted the entire Oort cloud. In this scenario, material originally residing between the orbits of Jupiter and Saturn was kicked outward by the gravity of these giant planets but remained within the outer reaches of the solar system, where the tug of passing stars and the mass of the Milky Way galaxy puffed the debris into a spherical cloud. Sedna may therefore belong to the inner part of the proposed Oort cloud, suggests Brown.

Moreover, by studying Sedna and hunting for other objects that may lurk beyond the Kuiper belt, astronomers may reconstruct the environment in which the sun flamed into existence.

Sedna provides “a fossil record of what happened at the birth of the solar system 4.5 billion years ago,” says Brown.

Levison says that the distant object “provides the first clue about what kind of star cluster we were formed in.”

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