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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 solar system, the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass. . In sky images spaced an hour apart, he and his colleagues have identified several of the solar systems--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 California Institute of Technology, at Pasadena, Calif.; originally for men, became coeducational in 1970; founded 1891 as Throop Polytechnic Institute; called Throop College of Technology, 1913–20.  in Pasadena reviewing images on his computer screen. He came across a sequence of pictures, taken by the Samuel Oschin telescope photographic plates]]. Since the focal plane is curved, these plates had to be preformed in a special jig before being loaded into the camera.

Construction on the Schmidt telescope began in 1939 and it was completed in 1948.

The camera has been converted to use a CCD imager.
 on Palomar Mountain Palomar Mountain (păl`ōmär), peak, 6,126 ft (1,867 m) high, S Calif., NE of San Diego, in Cleveland National Forest. It is the site of the

Palomar Observatory, operated by the California Institute of Technology.
 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.

The extremely slow motion of the object, now dubbed Sedna (SN: 3/20/04, p. 179), 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 re·tool  
v. re·tooled, re·tool·ing, re·tools
1. To fit out (a factory, for example) with a new set of machinery and tools for making a different product.

 their ideas about the evolution and origin of the outer solar system--just as NASA NASA: see National Aeronautics and Space Administration.
 in full National Aeronautics and Space Administration

Independent U.S.
 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 The Harvard-Smithsonian Center for Astrophysics (CfA) is located in Cambridge, Massachusetts. It consists of the Harvard College Observatory and the Smithsonian Astrophysical Observatory. The Center is located at 60 Garden Street.  in Cambridge, Mass. "It's still the ease 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 Kuiper belt: see comet; Kuiper, Gerard Peter.
Kuiper belt
 or Edgeworth-Kuiper belt

Disk-shaped belt of billions of small icy bodies orbiting the Sun beyond the orbit of Neptune, mostly at distances 30–50 times Earth's distance
 extends outward from the orbit of Pluto to about 70 times the Earth-sun distance, which defines an astronomical unit astronomical unit (AU), mean distance between the earth and sun; one AU is c.92,960,000 mi (149,604,970 km). The astronomical unit is the principal unit of measurement within the solar system, e.g., Mercury is just over 1-3 AU and Pluto is about 39 AU from the sun.  (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 according to
1. As stated or indicated by; on the authority of: according to historians.

2. In keeping with: according to instructions.

 theorists, a second, more remote reservoir surrounds the Kuiper belt. This proposed reservoir, the Oort cloud Oort cloud: see comet.
Oort cloud

Vast spherical cloud of small, icy bodies orbiting the Sun at distances ranging from about 0.3 light-year to one light-year or more that is probably the source of most long-period comets.
, 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). 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 “IAU” redirects here. For other uses, see IAU (disambiguation).

The International Astronomical Union (IAU) unites national astronomical societies from around the world.

Xena is now located 97 AU from the sun but spends most of its time in the Kuiper belt. Its path tilts 44[degrees] 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[degrees], Christian Veillet of the Canada-France-Hawaii Telescope The Canada-France-Hawaii Telescope is located near the mountain top of Mauna Kea in Hawaii at an altitude of 4,204 meters (13,793 feet). It is a Prime Focus/Cassegrain configuration with a usable aperture diameter of 3.58 meters.  on Mauna Kea Mauna Kea (mou`nə kā`ə), dormant volcano, 13,796 ft (4,205 m) high, in the south central part of the island of Hawaii. It is the loftiest peak in the Hawaiian Islands and the highest island mountain in the world, rising c.  and his colleagues report. That Kuiper belt object Noun 1. Kuiper belt object - any of many minor planets in the Kuiper belt outside the orbit of Neptune at the edge of the solar system

minor planet, planetoid - any of numerous small celestial bodies that move around the sun
 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, maybe 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 Chadwick A. "Chad" Trujillo (born November 22, 1973), is an astronomer and the co-discoverer of the dwarf planet Eris.

Trujillo works with computer software and has examined the orbits of the numerous trans-Neptunian objects (TNOs), which is the outer area of the solar
 of the Gemini Observatory For Other uses of Gemini, see .
The Gemini Observatory is an astronomical observatory consisting of two 8-metre telescopes at different sites. The Gemini telescopes were built and are operated by a consortium consisting of the United States, United Kingdom, Canada, Chile,
 in Hilo, Hawaii Hilo (pronounced IPA: /ˈhiːloʊ/) is a coastal city in the State of Hawaiʻ , and David L. Rabinowitz David Lincoln Rabinowitz (born 1960) is a researcher at Yale University. He has built CCD cameras and software for the detection of near Earth asteroids,[1] and his researched has helped reduced the assumed number of these objects by half, from 1,000-2,000 to  of Yale University Yale University, at New Haven, Conn.; coeducational. Chartered as a collegiate school for men in 1701 largely as a result of the efforts of James Pierpont, it opened at Killingworth (now Clinton) in 1702, moved (1707) to Saybrook (now Old Saybrook), and in 1716 was  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 e·lon·gate  
tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates
To make or grow longer.

adj. or elongated
1. Made longer; extended.

2. Having more length than width; slender.
 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 frosting

the slight graying of the haircoat around the face, particularly muzzle, in dogs with aging and as a regular feature of some breeds such as the Belgian shepherd dog.
 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 co·a·lesce  
intr.v. co·a·lesced, co·a·lesc·ing, co·a·lesc·es
1. To grow together; fuse.

2. To come together so as to form one whole; unite:
 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 va·por·ize  
tr. & intr.v. va·por·ized, va·por·iz·ing, va·por·iz·es
To convert or be converted into vapor.

 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 ENIGMANS 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 el·lip·tic   or el·lip·ti·cal
1. Of, relating to, or having the shape of an ellipse.

2. Containing or characterized by ellipsis.

 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 grav·i·ta·tion  
1. Physics
a. The natural phenomenon of attraction between physical objects with mass or energy.

b. The act or process of moving under the influence of this attraction.

 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), proposed early this year by Hal Levison of the Southwest Research Institute Southwest Research Institute (SwRI), headquartered in San Antonio, Texas, is one of the oldest and largest independent, nonprofit, applied research and development (R&D) organizations in the United States. Founded in 1947 by Thomas Slick, Jr.  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 1,0OO times as many inhabitants
:This article is about the video game. For Inhabitants of housing, see Residency
Inhabitants is an independently developed commercial puzzle game created by S+F Software. Details
The game is based loosely on the concepts from SameGame.
 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 e·lon·gate  
tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates
To make or grow longer.

adj. or elongated
1. Made longer; extended.

2. Having more length than width; slender.
. 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 The University of California, Berkeley is a public research university located in Berkeley, California, United States. Commonly referred to as UC Berkeley, Berkeley and Cal  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 sculpt  
v. sculpt·ed, sculpt·ing, sculpts
1. To sculpture (an object).

2. To shape, mold, or fashion especially with artistry or precision:
 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 Milky Way Galaxy

Large spiral galaxy (roughly 150,000 light-years in diameter) that contains Earth's solar system. It includes the multitude of stars whose light is seen as the Milky Way, the irregular luminous band that encircles the sky defining the plane of the galactic
 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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Author:Cowen, Ron
Publication:Science News
Article Type:Cover Story
Geographic Code:1USA
Date:Jan 14, 2006
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