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What Lies Beyond the Planets Everyone’S Heard of Frigid Pluto, Yet It’S Just the Tip of the Iceberg in the Solar System’S Still-Mysterious Kuiper Belt

What Lies Beyond the Planets Everyone’S Heard of Frigid Pluto, Yet It’S Just the Tip of the Iceberg in the Solar System’S Still-Mysterious Kuiper Belt

Inside the What lies beyond the Everyone’s heard of frigid , yet it’s just the tip of the iceberg in the ’s still-mysterious Kuiper Belt. by S. Alan Stern

stronomers break down our planetary system’s architecture into three distinct zones. Th e inner zone comprises the rocky planets and lies close to the ’s warmth. Th e giant, gaseous planets dominate the middle zone. And the outer zone — called the Kuiper Belt — contains Pluto, SEDNA’S ICY SURFACE glows dimly in the feeble light 100,000 “ice dwarf” worlds, and of the distant Sun. Sedna ranks among the largest objects found in the Kuiper Belt, although bigger several billion . ones likely lurk farther out. ADOLF SCHALLER/NASA/STSCI

© 2011 Kalmbach Publishing Co. This material may not be reproduced in any form 74 The Solarwithout System permission from| the2008 publisher. www..com www.Astronomy.com 75 orbiting between and . Astrono- mers soon realized this object, , has an unstable orbit. Th is strongly indicates it must have come from a more distant region of the solar system. And this, in turn, suggests many more such bodies likely orbit the Sun beyond Pluto the region of the giant planets. Later, in the 1980s, orbital simulations dem- onstrated that most short-period comets must PLUTO AND posed for the Hubble Space originate in a disk-like reservoir beyond Nep- ’s Faint Object Camera in 1994. This was tune. Th is fi nding harkened back to the concept Charon the first image that clearly separated the closely orbiting, distant worlds. NASA/ESA of a trans-Neptunian belt of primordial bodies that Kuiper had written about 4 decades earlier, understand how the frigid outer solar system spurring various searches for a trans-Neptunian is put together. “Kuiper Belt” beginning around 1988. Th e clincher came in mid-1992, when David PLUTO’S FAMILY now contains at least four An undiscovered continent Jewitt and Jane Luu discovered a distant object objects: Pluto itself, the relatively large Formally speaking, American astronomer Clyde orbiting on a near-circular, low-inclination orbit Charon, and the two small Nix and Hydra. Tombaugh discovered the Kuiper Belt in 1930. a billion kilometers beyond Neptune. Th is

HAL WEAVER (JHUAPL)/ALAN STERN (SWRI)/NASA/ESA Using a 13-inch camera at object, dubbed 1992 QB1, was just the fi rst of in Arizona, he found “ X,” an object what have now become more than 1,000 similar STATISTICS: Pluto vs. beyond Neptune that astronomers had been discoveries in the space beyond Neptune. trying to fi nd for the previous quarter century. Notably, however, only about 1 percent of Pluto Earth comparison Scientists quickly dubbed this new world the sky along the ecliptic has been searched so Mass 0.0276 trillion 0.2% of Earth’s both Planet 9 and Pluto, the latter aft er the far for Kuiper Belt objects (KBOs). When such trillion pounds Roman god of the underworld. Observations a survey is complete, astronomers expect it will Diameter 1,485 miles 18.7% of Earth’s showed it follows an unusually elliptical and Density 1.75 31.7% inclined orbital path. Pluto takes 248 years to grams/cc of Earth’s orbit the Sun, coming as close to our as 29.5 Weight of 9 pounds 5.9% 150-lb person of Earth’s astronomical units (AU; 1 AU is the average THE KUIPER BELT REMAINS Distance from Sun 3,670 million 39 times Earth’s distance between the Sun and Earth, approxi- miles distance mately 92.8 million miles [150 million kilome- largely unexplored. ... Even Orbital period 247.7 years 247.7 times ters]) and heading out as far as 49.5 AU. the most powerful tele- Earth’s When Tombaugh discovered Pluto, no one Length of day 153.3 hours 6 times Earth’s fully appreciated that he had revealed a third scopes on Earth ... render Mass: Total mass (1 trillion = 1012); Diameter: Equatorial diameter; zone of the solar system. It’s not a bad analogy Density: Average density in grams per cubic centimeter (water = 1); these worlds as little more Distance from Sun: Average distance; Orbital period: Sidereal period; to liken Tombaugh’s discovery of Pluto to Length of day: Average time between successive noons; Earth com- Columbus’ discovery of America. Astronomers than faint points of light. parison: Ratio of Pluto’s to Earth’s value. missed the conclusion that “Planet X” is the brightest member of a vast, undiscovered popu- For centuries, astronomers have scrutinized lation that constitutes an entirely new zone of reveal more than 100,000 KBOs with diameters the major bodies in the inner and middle zones. our solar system. Similarly, Columbus thought larger than 60 miles (100 km). Pluto’s context And, during the past 50 years, spacecraft have he had found India, but instead had stumbled is now clear. Th e Kuiper Belt occupies a far GIANT PLUTO (right of center) and Charon (right of visited every planet in these zones. Th ese probes upon a far more signifi cant, and then unrecog- greater expanse, contains a far greater mass, Pluto) stand watch over one of this world’s recently instead of abruptly. Nitrogen dominates the have returned spectacular images and answered nized, element of Earth’s geography. and embraces a far larger and more diverse suite discovered moons. The fourth moon appears as a atmospheric composition as it does the surface. many of the mysteries surrounding these For most of the 20th century, the more sci- of bodies than does the . bright dot well to Pluto’s left. NASA/ESA/G. BACON (STSCI) Because Pluto’s gravity is so weak (just 5 worlds. entists learned about Pluto, the more it didn’t or 6 percent as strong as Earth’s), the planet’s Yet, the Kuiper Belt remains largely unex- seem to fi t with our solar system’s eight inner The Pluto system refl ects 55 percent of incoming sunlight, indi- atmos phere escapes at a fairly high rate. Perhaps plored. No spacecraft has reached these far- worlds and myriad small bodies. Suggestions Over the course of almost 80 years, the march cating fresh ices cover the surface. several kilometers’ worth of surface ice has been fl ung objects. And even the most powerful about Pluto’s true context did appear, however. of technology has allowed astronomers to learn Spectroscopy shows that nitrogen ice dom- lost to space over the age of the solar system. on Earth and in space render these Dutch astronomer Gerard Kuiper (1905-1973) the basics of the Pluto system — despite its great inates Pluto’s surface composition, with small Such “escape erosion” is not found elsewhere worlds as little more than faint points of light. hypothesized that Pluto might be the fi rst distance, faintness, and small angular diameter. amounts of methane and carbon-monoxide among the known worlds of the solar system. Although this outer zone remains largely terra known example of a vast population of bodies, Pluto is a small planet with one large and ices also present. Some sort of darkening agent, Pluto’s atmosphere also has hazes, and observ- incognita, planetary scientists are starting to including comets and “planetoids,” that reside two small moons. It measures 1,485 miles with a still-undetermined composition, gives ers have seen its pressure change dramatically in the cold, trans-Neptunian wilderness 30 to 50 (2,390 km) in diameter. Th e Hubble Space Tele- the planet a reddish color. for still poorly understood reasons. PLUTO’S SURFACE shows bright and dark areas S. Alan Stern is the former associate administrator AU from the Sun. scope barely resolves Pluto. Still, these images Pluto also has an atmosphere, which astron- With Pluto’s great distance from Earth, it largely covered in nitrogen-rich ices. These Hubble for the Mission Directorate at NASA. He is the Th en, in 1977, American astronomer and other data show that it apparently has polar omers discovered by watching disappear should come as no surprise how long it took images, which reveal opposite hemispheres, also principal investigator for the New mission to Charles Kowal discovered a “miniature planet” caps and a variety of bright and dark provinces behind the icy dwarf. Such reveal observers to spot the ’s moons. U.S. hint at Pluto’s polar caps.

Pluto and the Kuiper Belt. a couple of hundred kilometers in diameter scattered about its globe. On average, Pluto an atmosphere when a star disappears gradually Naval Observatory astronomer James Christy S. ALAN STERN (SWRI)/ (LOWELL OBSERVATORY)/NASA/ESA

76 The Solar System | 2008 www.Astronomy.com 77 One theory to explain this quandary sug- gests the primordial Kuiper Belt had to be perhaps 50 times its present mass. Numerical simulations show the largest KBOs were well on their way to growing into large planets, perhaps something the size of , Earth, or even Neptune, when something suddenly inter- rupted their growth. A likely culprit: the growth of “nearby” Neptune. One eff ect of this “dynamical excitation” would have been that collisions became erosive, rather than accretional as they had been. Once this transition took place, the growth process DWARF-PLANET , seen here with its faint would have stopped. Erosion then would have moon Dysnomia (at the 8 o’clock position), currently ground much of the mass in the ancient Kuiper ranks as the solar system’s largest dwarf planet, Belt into dust. Th e Sun’s radiation pressure besting even Pluto. NASA/ESA/M. BROWN (CALTECH) subsequently would have blown this dust into interstellar space. Kuiper Belt’s total mass today at between 0.01 Astronomers have witnessed similar and 0.1 Earth-mass. processes taking place in what appear to be Most Kuiper Belt objects have relatively dark Kuiper-like belts around many stars in our surfaces, typically refl ecting just 3 to 15 percent , including Vega and . Th is of the light that falls on them. A small fraction scenario appears to explain the lack of a large have higher albedos, notably Pluto and Charon. planet in the Kuiper Belt as well as the dearth of In Pluto’s case, the high albedo of 55 percent mass found there. arises because its atmos phere regularly deposits Other astronomers think the mismatch fresh snow on its surface. Th e surface colors between the Kuiper Belt’s current mass and of KBOs range widely, from slightly bluish to the much larger amount required to form the KUIPER BELT OBJECT QUAOAR SPORTS A FINELY TEXTURED SURFACE in this artist’s impression. Quaoar extremely red. Astronomers also have unearthed bodies in it stems from the KBOs forming ranks among the largest of the estimated 100,000 KBOs. NASA/G. BACON (STSCI) evidence for water ice on some KBOs, like Charon, and even more-volatile substances, discovered the fi rst one, Charon, in 1978. In Charon. Astronomers quickly named the two like nitrogen, methane, and carbon-monoxide Roman mythology, Charon is the boatman who new moons Nix and Hydra. ices, just as on Pluto. PLUTO AND CHARON MAKE ferries souls of the dead across the river to In Roman mythology, Nix is the goddess of Most of the KBOs studied so far appear to the solar system’s only the underworld. Th e moon lies about 12,160 darkness and the , and mother of Charon. rotate on their axes in a few hours, although miles (19,570 km) from Pluto and makes a cir- Hydra is the nine-headed monster that guards some take days. In 2001, researchers found the true — their cular orbit in the planet’s equatorial plane. the gates of the underworld. Th e surface colors fi rst KBO moons. Today, astronomers recognize balance point lies between Charon’s diameter is almost precisely half of Charon, Nix, and Hydra are all a bland gray. that at least 20 percent of KBOs have moons. that of Pluto, a ratio no planet-satellite pair of Scientists do not yet have any information on Astronomers also have found that collisions them rather than inside the inner or giant planets comes close to. In the refl ectivities, surface compositions, or sur- play a key role in the Kuiper Belt, shaping the fact, its size (737 miles [1,186 km] across) and face appearances of Nix and Hydra. surfaces of KBOs and controlling the number the main body. mass (12 percent of Pluto’s) are so large relative From studies of Charon’s orbit around Pluto, of small bodies there. Th is realization led to the to Pluto that the pair makes the solar system’s astronomers can determine Pluto’s mass. And discovery that those short-period comets origi- THE SPACECRAFT looks back on a Kuiper Belt object shortly after its flyby. From a distance only true double planet — their gravitational knowing both Pluto’s mass and volume, scien- nating in the Kuiper Belt are fragments chipped elsewhere. Th ese researchers conclude the of 4.1 billion miles (6.7 billion km), the faint Sun glows inside the zodiacal dust cloud. NASA/ESA/M. BROWN (CALTECH) balance point lies in the space between them tists have learned that the dwarf planet’s density off larger KBOs only millions to hundreds of objects we now see orbiting in the 30 to 50 AU rather than inside the main body. measures close to 2 grams per cubic centimeter. millions of years ago. region formed closer to the Sun, in the middle each responsible for some aspects of the present- context to Pluto’s existence, which, prior to Charon refl ects less light than Pluto, return- Th is means Pluto must contain about 70-percent Some KBOs have small moons, some have zone of the solar system, where we know a day Kuiper Belt. 1992, appeared to be an oddity. Second, it pro- ing some 35 percent of the sunlight hitting it. rock inside, in addition to its surface ices. Th us, large moons, and others, like Pluto, have both. huge mass of rock and ice existed when the vided strong links between our solar system Unlike Pluto, water ice primarily covers Char- although scientists oft en refer to Pluto as an icy A moon allows researchers to calculate the giant planets formed. A clue to the past and the kinds of debris disks seen around main on’s surface, with trace amounts of ammonia world, it is primarily rocky. main body’s mass, and thus estimate its density. If the KBOs developed closer to the Sun, Overall, the Kuiper Belt is a highly varied col- sequence stars like Vega, Fomalhaut, and Beta compounds. Th ere’s no sign of nitrogen, meth- So, we’ve learned that while some KBOs consist then how did they get to their present orbits? lection of collisional shards and dwarf planets. Pictoris. Finally, it helped us realize that our ane, or carbon-monoxide ices. The third zone primarily of rock with icy outer shells like Pluto, According to some models of solar system Owing to its strongly heterogeneous population planetary system contains a “third zone” — an In 2005, astronomers used the Hubble Space It has now been 16 years since the discovery of others are practically all ice. formation, the migration of the giant planets and its as a collection of ancient bodies ancient, icy disk of miniature worlds dotting

Telescope to search for new Pluto moons in 1992 QB1, and the subsequent realization that How did the Kuiper Belt and KBOs form? transported the KBOs. Th is migration would left over from the formation of the outer planets, the space beyond the giant planets. support of the New Horizons Pluto-Kuiper Belt a vast assemblage of icy bodies rings our solar Computer simulations show KBOs could not have swept many smaller bodies from the the Kuiper Belt ranks among the most scientifi - Th e study of the Kuiper Belt remains in its mission then about to be launched. Th ey dis- system. Th e objects range in size from cometary have formed in today's belt. Why? Th ere is not middle zone out to the Kuiper Belt and even cally signifi cant parts of the solar system. infancy. Although we have learned much, we covered two small moons. Th ese bodies, each nuclei a mile or less in diameter to dwarf plan- nearly enough material in this region to accu- to the beyond. Th e Kuiper Belt’s discovery has revolution- have much to learn. If we can count on any- just 35 to 100 miles (60 to 160 km) across, orbit ets many hundreds to more than 1,550 miles mulate into objects hundreds and thousands of What actually happened? We do not know. ized scientists’ view of our home planetary thing, however, it’s that the Kuiper Belt will more than twice as far from Pluto as does (2,500 km) across. Astronomers estimate the kilometers across during the solar system’s life. In fact, both scenarios may have played a role, system in three important ways. First, it gave continue to surprise.

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