Inside the Kuiper Belt Frigid Pluto is just What lies the tip of the iceberg in the solar system’s still-mysterious Kuiper Belt. beyond by S. Alan Stern the planets stronomers break down our planetary system’s architecture into three distinct zones. The inner zone comprises the rocky planets and lies close to the Sun’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 comets. 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.Astronomy.com www.Astronomy.com 83 the first known example of a vast population of bodies, including comets and “planetoids,” that Nix reside in the cold trans-Neptunian wilderness 30 to 50 AU from the Sun. S/2012 P1 Pluto Then, in 1977, American astronomer S/2011 P1 Charles Kowal (1940–2011) discovered a “min- iature planet” a few hundred kilometers in diameter orbiting between Saturn and Nep- Hydra tune. Scientists soon realized that this object, PLUTO AND CHARON posed for the Hubble Space Telescope’s Faint Object Camera in 1994. This was 2060 Chiron, 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 moons: when NASA’s New Horizons 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 Neptune. 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, New Horizons 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. Earth per Belt objects. spurring various searches for a trans-Neptunian “Kuiper Belt” beginning around 1988. Pluto Earth comparison An undiscovered continent Mass 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 Density 1.75 grams/ 31.7% vatory in Arizona, he found “Planet X,” an object cc of Earth’s largely unexplored. Even beyond Neptune that astronomers 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 telescopes on Earth render Orbital period 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 day 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 star 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 atmosphere, which astrono- For centuries, astronomers have scrutinized When Tombaugh discovered Pluto, no one a billion kilometers beyond Neptune. This mers discovered by watching stars 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 “occultations” reveal And, during the past 50 years, spacecraft 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 asteroids. 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 gravity 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 ecliptic 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 Hubble Space Telescope barely resolves eral kilometers’ worth of surface ice has been displays an odd bright spot rich in carbon 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 Styx 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 moon 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 Science 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 dwarf planet’s moons. U.S. Charon’s diameter is almost precisely half Pluto and the Kuiper Belt.