As a child, Mike Brown had all the trappings of an astronomer in the making, with space books, rocket drawings, and a poster of the planets on his bedroom wall. The poster depicted Pluto as “this crazy and very eccentric planet,” he says. “It was everyone’s favorite crazy planet.” Brown, now an astronomer at Caltech, still recalls the mnemonic he learned for the names of the planets: Martha visits every Monday and (the a was for asteroids) just stays until noon, period. “The ‘period,’ for Pluto, was always suspicious,” Brown says with a laugh. “It didn’t seem to fit. So maybe that was when I first got the idea that Pluto didn’t belong.”

Brown’s childhood insight now sounds like a premonition. In August 2006 the International Astronomical Unionofficially demoted Pluto, putting it into the new category of “dwarf planet,” a sun-orbiting object big enough to be forced into a spherical shape by gravity but not big enough to clear its own orbit. The IAU more recently deemed Pluto and two newfound bodies to be “plutoids,” bright dwarf planets that circulate mostly outside Neptune’s orbit. The decision was controversial and continues to generate protest, fueled by new findings from this icy realm.

These remote celestial bodies are the largest known members of the Kuiper belt, a band of rocky, icy objects that orbit the sun in a swath stretching from beyond Neptune to a distance of nearly 5 billion miles. Whatever you call them, one point is beyond debate: Pluto is no longer a lonely outpost in an otherwise empty frontier. A string of discoveries has revealed that it is merely the entry point to a vast and still mysterious wilderness that teems with an uncount-able number of unusual objects. They come in a variety of shapes, colors, and sizes, many with their own moons, some in peculiar orbits that have been pushed by Neptune or pulled by passing stars. Stranger objects are likely to be found, since astronomers are only on the edge of discovering this vast new terrain.

In the 1940s and 1950s, astronomers Kenneth Edgeworth and Gerard Kuiper independently predicted that a reservoir of icy rocks lay beyond the orbit of Neptune. Many of these objects became short-period comets, with orbits of 200 years or less, that blasted in toward the sun, crossing the paths of most planets. Excluding Pluto (discovered in 1930), the first official Kuiper belt object was not found until 1992, by astronomers Jane Luu and David Jewitt. Since then, in excess of 1,200 have been detected in the 2-billion-mile-wide Edgeworth-Kuiper belt (commonly truncated to Kuiper belt), including burly Eris, even larger than Pluto. Almost all of the biggest have been found by Brown and his colleagues. More than 100,000 objects at least 30 miles across may occupy the belt.

But our solar system doesn’t end there. Far beyond the Kuiper belt lies the mysterious Oort cloud, a spherical shell that stretches to the boundaries of interstellar space and blasts its own dark ice balls toward the sun. Trillions more bodies may lurk there. A few may be as big as Mercury or Mars. Imprinted in those far-flung worlds, scientists say, is the history of the solar system before planets came to be. Every Kuiper belt object and Oort cloud entity is a geologic fossil, preserved at low temperatures, largely unaltered by time, and made up of the material from which the solar system formed. Understanding their compositions—and why they are where they are today—will help scientists reconstruct the nascent moments of our planetary neighborhood and our sun’s younger days, when it was just one of a cluster of stars.

Name That Celestial Object Scientific nomenclature can barely keep up with the range of objects that astronomers are discovering in the solar system these days. Future discoveries will no doubt suggest new categories and further blur the boundaries of old ones. In the meantime, here’s a field guide to the known residents.

Planets are defined by the IAU as bodies orbiting the sun that have been made spherical by their own gravity and have cleared their orbit of other objects. Not every astronomer accepts this designation.

Dwarf Planets

are similar to planets but are too small to clear their own orbits. Most lie beyond the orbit of Neptune, making them plutoids, a subcategory named for the most famous member of their group.

Asteroids

are rocky, metallic, or carbonaceous bodies whose solar orbit takes them beyond Mars into the so-called asteroid belt. They typically lack sufficient gravity to retain an atmosphere. The latest count of asteroids in the inner solar system, inside Jupiter, is between 1.1 million and 1.9 million.

Comets

are icy bodies that follow elliptical orbits. Those originating in the Kuiper belt have orbital periods shorter than 200 years. Longer-period comets originate much farther out in the Oort cloud. Known comets number in the thousands, but trillions might be out there.

Classical Kuiper Belt Objects orbit the sun at 3.9 billion to 4.5 billion miles out. They are sometimes called cubewanos, after QB1, the first Kuiper belt object discovered (1992).

Resonant Kuiper Belt Objects orbit in synchrony with Neptune. Pluto is the prototype, orbiting twice around the sun for every three solar circuits made by Neptune. The majority of known RKBOs are thought to orbit with a similar 2:3 resonance and thus are called plutinos. About 100 other objects with resonant orbits have been detected.

Scattered Kuiper Belt Objects have very eccentric and tilted orbits that carry them from around 3.3 billion miles from the sun to almost 100 billion miles out. They may have been kicked into their far-flung orbits by Neptune’s gravity.

Deep red, large, and very far away, Sedna is an enigma. Since its discovery, the menagerie of big oddities in the outer solar system has expanded to include Eris and Makemake, a plutoid discovered in 2005. | Image courtesy of NASA

Looking Deeper and Farther As astronomers trip over one another hunting for planets outside our solar system, some of the most exciting finds may be closer to home.

Palomar Observatory Survey Since 1998, Mike Brown, Chad Trujillo of Gemini North Observatory, and astronomer David Rabinowitz of Yale University have used the 48-inch Samuel Oschin Telescope at Palomar Mountain in Southern California to scan the Kuiper belt and beyond for objects two-hundredths the brightness of Pluto. Every night a mosaic of 112 charge-coupled devices (CCDs) snaps digital images of a hand-size patch of sky. So far the search has turned up almost 100 new objects.

Deep Ecliptic Survey In this study, a mosaic of eight CCD detectors coupled to the 3.8-meter Mayall Telescope at Kitt Peak National Observatory near Tucson, Arizona, scanned for faint Kuiper belt objects as small as 30 miles in diameter. A second, identical camera was connected to the Blanco Telescope in La Serena, Chile. In all, the survey spotted 499 objects. The goal, says former survey leader Robert Millis, was to discover enough distant bodies to begin to understand the scale of the belt, the three-dimensional distribution of these objects in space, and their orbits. Currently the group is engaged in follow-up study and working to improve observational techniques.

Spacewatch For a portion of each month, the 0.9-meter and 1.8-meter telescopes at Steward Observatory on Kitt Peak scan the skies for comets, asteroids headed for Earth, and other small bodies in the solar system. Although Kuiper belt objects are not the main quarry of Spacewatch, the project’s astronomers did find 560-mile-wide Varuna, a dwarf planet candidate.

Taiwan-American Occultation Survey Astronomers are using four small robotic telescopes to simultaneously scan the same patch of sky. The telescopes look for the slight dimming of distant stars that occurs as Kuiper belt objects momentarily pass in front of them. The hope is that objects as small as a third of a mile wide will be detectable. The survey monitors 2,000 stars each night.

New Horizons NASA is gearing up for a historic first visit to the frigid world of Pluto and its moon, Charon. The designated explorer, a 1,000-pound, $550 million spacecraft called New Horizons, swung around Jupiter in February 2007 for a gravity boost and is scheduled to arrive in July 2015. It will snap the first close-up images of Pluto and Charon, map their surface features with visible-wavelength cameras, study their compositions in the near-infrared spectrum, and monitor Pluto’s thin atmosphere with ultraviolet spectrometers and radio waves.

The closest approach will bring the probe within 6,000 miles of Pluto’s surface. Among their many aspirations, NASA scientists hope New Horizons will clarify Pluto’s relationship to Triton, a moon of Neptune with a similar size, mass, and composition. Are they sister worlds or historically distinct entities? After 2015, the spacecraft will venture farther out into the Kuiper belt, where it will spend five years or more studying at least one of the smaller (30- to 60-mile-wide) objects there.

“This is our first foray into the deep, frozen, ancient outer solar system,” says planetary scientist Alan Stern. “This is what humans do, what great civilizations do: They explore and they make history.”

Understanding Your Solar System 2.0 It’s not about planets anymore. In the past decade, astronomers have spotted hundreds of objects in the Kuiper belt, one of them (Eris) even bigger than Pluto. Countless more await discovery. Even the distant Oort cloud is coming into scientific focus. Preserved in these nether realms are trillions of rocky, icebound insights into the early days of the sun and the formation of the solar system. In spatial scale and information potential, they are the true solar system—past, present, and future.

Inner Solar System: 0–5.2 astronomical units, or AU The inner solar system spans everything from the sun out to (but not including) Jupiter, 483 million miles away. The solar system as a whole formed roughly 4.5 billion years ago from a collapsing disk of dust and gas. Swarms of boulder-size objects called planetesimals slowly accreted to form the rocky planets—Mercury, Venus, Earth, and Mars—and their various and sundry satellites. The leftovers linger today in the asteroid belt. Farther out, well away from the sun’s warmth, gases like hydrogen and helium remained in place and accumulated to form the gaseous outer planets: Jupiter, Saturn, Uranus, and Neptune.

Asteroid Belt 2.3–3.3 AU (main belt) The majority of asteroids orbit about 210 million miles from the sun, between the orbits of Mars and Jupiter. A smaller group orbits at about 370 million miles; a third group occupies the same orbit (though not the same spot) as Jupiter. At least 100 asteroids are a mile or more in diameter, and a dozen or so are 200 miles wide. The largest, Ceres, is 600 miles wide and is now considered a dwarf planet.

One astronomical unit (AU) equals the distance from the sun to Earth, about 93 million miles.

Kuiper Belt: 30–50 AU The Kuiper belt is a vast reservoir of icy rocks that extends from about 3 billion miles from the sun, beyond Neptune’s orbit, to 5 billion miles out. Beyond that, the number of detected objects drops precipitously until one reaches the Oort cloud. There may be billions of Kuiper belt objects in all, including at least 35,000 that are more than 60 miles wide. Eris is the largest and farthest of the known objects; there is also some truly quirky stuff out there. 2003 EL61 looks like a deflated football hurtling end over end through space, dragging along two moons and a tail of icy debris. Pluto was once thought to be unique in having a moon (Charon), but dozens of Kuiper belt objects are now known to have such partners.

Most of the bodies in the Kuiper belt are believed to have formed roughly where they now reside, and many have stayed largely undisturbed in temperatures averaging –375 degrees Fahrenheit. They come in a variety of colors, from gray to dark red, but all are thought to be half ice and half rock. On occasion a gravitational bump from Neptune tosses a Kuiper belt object toward the sun. Then the heated ices sublime, producing a bright halo of glowing gas that trails behind the rock: the coma and the tail of a comet.

Oort Cloud 10,000–100,000 AU The entire solar system is enveloped by the Oort cloud, an enormous sphere of dormant comets extending more than 9 trillion miles into space. The cloud is thought to contain trillions of loose lumps of dust and volatile gases, near-perfect samples of the stuff from which the planets were made.

Dutch astronomer Jan Hendrik Oort first proposed the existence of the cloud in 1950 to explain the peculiar paths of comets that take more than 200 years to orbit the sun. This model suggests that the cloud’s contents started out closer by, near Jupiter and the other gas giants, until gravitational encounters with those planets knocked the primordial bits into the hinterlands. Barely 5 percent of that original material remains in the Oort cloud; the rest has been ejected entirely from the solar system.

Over time, the gravitational yanks of passing stars jostled the orbits of the outer Oort cloud objects, leaving no hint of their formerly neat, flat paths around the sun. (The orbits of objects in the loosely defined inner Oort cloud, located a few thousand astronomical units from the sun, do retain some memory of their origins, so this part of the cloud is thought to be flatter.) The outer, round cloud would have taken about a billion years to form, making it the youngest structure in the solar system.