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Frigid Pluto Is Just the Tip of the Iceberg in the Solar System's Still

Frigid Pluto Is Just the Tip of the Iceberg in the Solar System's Still

Inside the

Frigid is just What lies the tip of the iceberg in the ’s still-mysterious Kuiper Belt. beyond by S. the

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

ADOLF SCHALLER/NASA/STScI

© 2014 Kalmbach Publishing Co. This material may not be reproduced in any 82 Explore The Solar System form without permission from the publisher.www..com www.Astronomy.com 83 the first known example of a vast population of bodies, including comets and “planetoids,” that reside in the cold trans-Neptunian wilderness 30 to 50 AU from the Sun. S/2012 P1 Pluto Then, in 1977, American S/2011 P1 Charles Kowal (1940–2011) discovered a “min- iature ” a few hundred kilometers in diameter orbiting between and Nep- tune. Scientists soon realized that this object, PLUTO AND posed for the Hubble Space ’s Faint Object Camera in 1994. This was , has an unstable orbit. This fact Charon the first image that clearly separated the closely strongly indicates that it must have come from orbiting, distant worlds. NASA/ESA a more distant region of the solar system. And this, in turn, was a clue suggesting that many incognita, planetary scientists are starting to more such bodies likely orbit the Sun beyond understand how the frigid outer solar system is the zone of the giant planets. put together, and they expect a major advance Later, in the 1980s, orbital simulations dem- PLUTO’S FAMILY now contains at least five : when NASA’s New probe arrives to onstrated that most short-period comets must larger Charon, smaller Nix and Hydra, and tiny S/2011 make an initial reconnaissance of planet Pluto originate in a disklike reservoir beyond . P1 and S/2012 P1. NASA/ESA/M. SHOWALTER (SETI INSTITUTE) and its moons in July 2015. After its flyby of the This finding harkened back to the concept of a Pluto system, is expected to con- trans-Neptunian belt of primordial bodies that tinue outward to explore one or two small Kui- Kuiper had written about four decades earlier, STATISTICS: Pluto vs. per Belt objects. spurring various searches for a trans-Neptunian “Kuiper Belt” beginning around 1988. Pluto Earth comparison An undiscovered continent 0.0276 trillion 0.2% of Earth’s Technically, American astronomer Clyde Tom- trillion pounds baugh (1906–1997) discovered the Kuiper Belt in Diameter 1,485 miles 18.7% of Earth’s 1930. Using a 13-inch telescope at Lowell Obser- THE KUIPER BELT REMAINS 1.75 grams/ 31.7% vatory in , he found “Planet X,” an object cc of Earth’s largely unexplored. Even beyond Neptune that had been try- Weight of 9 pounds 5.9% 150-lb person of Earth’s ing to spot for the previous quarter-century. the most powerful Distance from Sun 3,670 million 39 times Earth’s Scientists quickly dubbed this new world miles distance both Planet 9 and Pluto, the latter after the on Earth render 248.0 years 248.0 times Roman god of the underworld. Observations these worlds as little more Earth’s showed that it follows an unusually elliptical Length of 153.3 hours 6 times Earth’s and inclined orbital path. Pluto takes 248 years than faint points of light. Mass: Total mass (1 trillion = 1012); Diameter: Equatorial diameter; to orbit the Sun, coming as close to our as Density: Average density in grams per cubic centimeter (water = 1); Distance from Sun: Average distance; Orbital period: Sidereal period; 29.5 astronomical units (AU; 1 AU is the aver- GIANT PLUTO (right of center) and Charon (right of Length of day: Average time between successive noons; Earth com- age distance between the Sun and Earth, or The clincher came in mid-1992 when David with a still-undetermined composition, gives parison: Ratio of Pluto’s to Earth’s value. Pluto) stand watch over one of the distant world’s 0° 180° approximately 92.8 million miles [150 million Jewitt and Jane Luu discovered a distant object moons in this illustration. Another satellite appears the planet a reddish color.

kilometers]) and heading out as far as 49.5 AU. orbiting on a near-circular, low-inclination orbit as a bright dot well to Pluto’s left. NASA/ESA/G. BACON (STScI) Pluto also has an , which astrono- For centuries, astronomers have scrutinized When Tombaugh discovered Pluto, no one a billion kilometers beyond Neptune. This mers discovered by watching disappear the major bodies in the inner and middle zones. fully appreciated that he had revealed a third object, dubbed 1992 QB1, was just the first of The Pluto system behind the icy dwarf. Such “” reveal And, during the past 50 years, have zone of the solar system. It’s not a bad analogy to what have now become more than 1,000 similar Over the course of more than 80 years, the march an atmosphere when a star disappears gradually visited every major body in these areas except liken Tombaugh’s discovery of Pluto to Colum- discoveries in the space beyond Neptune. of technology has allowed astronomers to learn instead of abruptly. Nitrogen dominates the some of the larger . These probes have bus’ discovery of America. Astronomers missed Notably, however, astronomers have so far the basics of the Pluto system — despite its great world’s atmosphere as it does the surface. been highly successful, returning spectacular the conclusion that “Planet X” is the brightest searched only a small fraction of the sky along distance, faintness, and small angular diameter. Because Pluto’s is so weak (just about PLUTO’S SURFACE shows bright and dark areas, images and answering many of the ­mysteries member of a vast, undiscovered population that the for Kuiper Belt objects (KBOs). Pluto is a tiny planet with one large and 6 percent as strong as Earth’s), the planet’s atmo- largely nitrogen-rich ices, in these Hubble images surrounding these worlds. constitutes an entirely new region of our solar When such a survey is complete, astronomers four little moons. Owing to its distance, how- sphere escapes at a fairly high rate. Perhaps sev- that reveal opposite hemispheres. The right disk Yet the Kuiper Belt remains largely unex- system. Similarly, Columbus thought he had expect it will reveal more than 100,000 KBOs ever, the barely resolves eral kilometers’ worth of surface ice has been displays an odd bright spot rich in monoxide plored. No spacecraft has reached these far- found India, but instead had stumbled upon a with diameters larger than 60 miles (100km). Pluto. Still, these images and other data show lost to space over the age of the solar system. frost. NASA/ESA/M. BUIE (SOUTHWEST RESEARCH INSTITUTE) flung objects. And even the most powerful far more significant, and then unrecognized, Pluto, of course, is much larger — about 1,491 that it apparently has polar caps and a variety of Such “escape erosion” occurs nowhere else telescopes on Earth and in space render these element of Earth’s geography — the Americas. miles (2,400km) in diameter — but its context is bright and dark provinces scattered about its among the known worlds of the solar system. discovered the first one, Charon, in 1978. In worlds as little more than faint points of light. For most of the 20th century, the more sci- now clear: It’s no misfit. It was simply the first globe. On average, Pluto reflects 55 percent of Pluto’s atmosphere also has hazes, and observ- Roman mythology, Charon is the boatman who Although this outer zone remains largely terra entists learned about Pluto, the more it didn’t discovery of a population of small planets and incoming sunlight, indicating that fresh ices ers have seen its pressure change dramatically ferries the souls of the dead across the river seem to fit with our solar system’s eight large comets orbiting beyond Neptune. It is also now cover the surface. for still poorly understood reasons. to the underworld. The lies about 12,160 S. Alan Stern is the former associate administrator inner planets and myriad small bodies. Sugges- apparent that the Kuiper Belt occupies a far Spectroscopy shows that nitrogen ice domi- With Pluto’s great distance from Earth, it miles (19,570km) from Pluto and makes a cir- for the Mission Directorate at NASA. He is the tions about Pluto’s true context did appear, greater expanse, contains a far greater mass, and nates Pluto’s surface composition, with small should come as no surprise how long it took cular orbit in the planet’s equatorial plane. principal investigator for the New Horizons mission to however. Dutch astronomer Gerard Kuiper embraces a far larger and more diverse suite of amounts of methane and carbon monoxide observers to spot the ’s moons. U.S. Charon’s diameter is almost precisely half Pluto and the Kuiper Belt. (1905–1973) hypothesized that Pluto might be bodies than does the belt. ices also present. Some sort of darkening agent, Naval Observatory astronomer James Christy that of Pluto, a ratio no planet-satellite pair of

84 Explore The Solar System www.Astronomy.com 85 have formed in today’s belt because the region doesn’t have nearly enough material to accumu- late into objects hundreds and thousands of kilometers across during the solar system’s life. One theory to explain this quandary sug- gests that 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 the size of , Earth, or even Nep- tune — when something suddenly interrupted their growth. A likely culprit: the increasingly huge “nearby” world of Neptune. One effect of this “dynamical excitation” would have been that collisions became erosive DWARF PLANET , seen here with its faint moon rather than accretional as they had been. Once (at the 8 o’clock position), currently ranks this transition took place, the growth process as the solar system’s largest dwarf planet, besting would have stopped. Erosion then would have even Pluto. NASA/ESA/M. BROWN (CALTECH) ground much of the mass in the ancient Kuiper Belt into dust. The Sun’s radiation pressure sub- Most KBOs have relatively dark surfaces, sequently would have blown this dust into reflecting just 3 to 15 percent of the light that interstellar space. falls on them. A small fraction have higher Astronomers have witnessed similar pro- , notably Pluto and Charon. In Pluto’s cesses taking place in what appear to be Kuiper- case, the high of 55 percent arises like belts around many stars in our , because its atmosphere regularly deposits fresh snow on its surface. The surface colors of KBOs range widely, from slightly bluish to KUIPER BELT OBJECT QUAOAR sports a finely textured surface in this artist’s impression. Quaoar ranks extremely red. Astronomers also have un- PLUTO AND CHARON MAKE among the largest of the estimated 100,000 KBOs. NASA/G. BACON (STScI) earthed evidence of water ice on some KBOs, the solar system’s only like Charon, and even more-volatile substances the inner or giant planets comes close to. In In 2011, scientists looking for possible rings like nitrogen, methane, and carbon monoxide true — their fact, its size (756 miles [1,210km] across) and around Pluto found a fourth moon, provision- ices, just as on Pluto. mass (about 10 percent of Pluto’s) are so large ally named S/2011 P1. And in 2012, astronomers Most of the KBOs studied so far appear to balance point lies between relative to Pluto that the pair makes the solar found a fifth moon named S/2012 P1. No infor- rotate on their axes within a few hours, al- them rather than inside system’s only true double planet — their gravi- mation yet exists on the reflectivities, surface though some might take days. In 2001, re- tational balance point lies in the space between compositions, or surface appearances of Nix, searchers found the first KBO moons. Today, the main body. them rather than inside the main body. Hydra, S/2011 P1, or S/2012 P1. astronomers recognize that at least 20 percent Charon reflects less light than Pluto, return- From studies of Charon’s orbit around Pluto, of KBOs have moons. THE NEW HORIZONS SPACECRAFT looks back on a Kuiper Belt object in this illustration, with the Sun ing some 35 percent of the sunlight hitting it. astronomers have deduced the large world’s Some KBOs have small moons, some have including and Beta (β) Pictoris. This sce- glowing faintly from a distance of 4.1 billion miles (6.7 billion kilometers). NASA/ESA/M. BROWN (CALTECH) Unlike Pluto, water ice primarily covers Char- mass. And knowing both Pluto’s mass and vol- large moons, and others, like Pluto, have both. nario appears to explain the lack of a large on’s surface, with trace amounts of ammonia ume, scientists have learned that the dwarf plan- A moon allows researchers to calculate the planet in the Kuiper Belt as well as the dearth of compounds. There’s no sign of nitrogen, meth- et’s density measures close to 2 grams per cubic main body’s mass, and thus estimate its density. mass found there. zone out to the Kuiper Belt and even to the The Kuiper Belt’s discovery has revolutionized ane, or carbon monoxide ices. centimeter. This means Pluto must be about 70 So, we’ve learned that while some KBOs consist Other astronomers think the mismatch beyond. scientists’ view of our home planetary system in In 2005, astronomers used the Hubble Space percent rock. Thus, although scientists often primarily of rock with icy outer shells like Pluto, between the Kuiper Belt’s current mass and So, then, what actually happened? We do three important ways. First, it gave context to Telescope to search for new in refer to Pluto as an icy world, it is actually pri- others are practically all ice, and some are all the much larger amount required to form the not know. In fact, both scenarios may have Pluto’s existence, which, prior to 1992, appeared support of the New Horizons Pluto-Kuiper Belt marily rocky. Charon is a bit less dense, indicat- rock. The great diversity of object colors, densi- bodies in it stems from the KBOs forming else- played a role, each responsible for some to be an oddity. Second, it provided strong links mission that was about to be launched. They ing a near 50/50 mixture of water ice and rock. ties, rock fractions, and satellite systems in the where. These researchers conclude that the aspects of the present-day Kuiper Belt. Or between our solar system and the kinds of debris ended up discovering two small satellites. These Kuiper Belt is one of its hallmarks. objects we now see orbiting in the 30 to 50 AU perhaps something not even hypothesized yet disks seen around stars like Vega, Fomalhaut, bodies, each just 35 to 100 miles (60 to 160km) The third zone Astronomers also have learned that colli- region formed much closer to the Sun, in the might have occurred. and . Finally, it showed us that our across, orbit more than twice as far from Pluto It has now been 20 years since the discovery of sions play a key role in the Kuiper Belt, shaping middle zone of the solar system, where we planetary system contains a “third zone” — an as does Charon. 1992 QB1 and the subsequent realization that the surfaces of KBOs and controlling the num- know a huge mass of rock and ice existed when A clue to the past ancient icy disk of miniature worlds dotting the Astronomers quickly named the two new a vast assemblage of icy bodies rings our solar ber of small bodies there. This led to the discov- the giant planets formed. Overall, the Kuiper Belt is a highly varied collec- space beyond the giant planets. moons Nix and Hydra. In Roman mythology, system. The objects range in size from cometary ery that those short-period comets originating If the KBOs developed closer to the Sun, tion of collisional shards and dwarf planets. The study of the Kuiper Belt remains in its Nix is the goddess of darkness and the , nuclei a mile or less in diameter to dwarf plan- in the Kuiper Belt are fragments chipped off then how did they get to their present orbits? Owing to its strongly diverse population and its infancy. Although we know much, we have and mother of Charon. Hydra is the nine- ets many hundreds to more than 1,550 miles larger KBOs only millions to hundreds of mil- According to some models of solar system for- as a collection of ancient bodies left over much left to learn. If we can count on anything, headed monster that guards the gates of the (2,500km) across. Astronomers estimate the lions of years ago. mation, the migration of the giant planets trans- from the formation of the outer planets, the Kui- however, it’s that the Kuiper Belt will continue to underworld. The surface colors of Charon, Nix, Kuiper Belt’s total mass today to be between How did the Kuiper Belt and KBOs form? ported the KBOs. This migration would have per Belt ranks among the most scientifically sig- surprise us, both in telescopes and by the explo- and Hydra are all a bland gray. 0.01 and 0.1 times Earth’s mass. Computer simulations show KBOs could not swept many smaller bodies from the middle nificant parts of the solar system. ration it will receive from New Horizons.

86 Explore The Solar System www.Astronomy.com 87

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