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Beyond Pluto

We are only beginning to discover how vast and strange our solar system truly is.

By Kathy A Svitil and Don Foley
Nov 25, 2004 6:00 AMNov 12, 2019 5:02 AM


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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. On it, Pluto was depicted as “this crazy and very eccentric planet,” he says. “It was everyone’s favorite crazy planet.” Brown still recalls the mnemonic he learned for the names of the planets: Martha visits every Monday and—a 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.”

Nowadays Brown, a planetary astronomer at Caltech, has no doubt about Pluto’s place in the solar system: “Pluto is not a planet. There is no logical reason to call Pluto a planet.” Like a growing number of his colleagues, Brown believes Pluto is best understood as the largest known member of the Kuiper belt, a band of rocky, icy miniplanets that orbit the sun in a swath stretching from beyond Neptune to a distance of nearly 5 billion miles. “I don’t think it denigrates Pluto at all to say that it is not a planet. I think Pluto is a fascinating and interesting world, and being the largest Kuiper belt object is an honorable thing to be.”

No longer is Pluto 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 uncountable numbers 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. Astronomers are only on the edge of discovering this vast new world.

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 became short-period comets, with orbits of 200 years or less, that blasted in toward the sun, crossing the orbits 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 800 have been detected in the 2-billion-mile-wide Edgeworth-Kuiper belt (commonly truncated to Kuiper belt), including a few huge objects that are as much as three-quarters the size of Pluto (which is 1,430 miles wide). The two biggest have been found by Brown and his colleagues. At least 100,000 objects 30 miles wide may occupy the belt. Some of them, say researchers, are almost certainly the size of Pluto—if not larger.

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 4.5 billion years ago. 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.

Each of those objects has a tale to tell that is as lively as those that the nine—call it eight—planets have told so well. Let their stories begin.

A New Miniplanet Rises

Detected last November by Brown and his colleagues, using the 48-inch Samuel Oschin Telescope, Sedna is the largest, farthest, coldest, and arguably weirdest world yet discovered in the distant solar system. It may be as large as 1,100 miles in diameter, and it has a mysterious deep red surface—neither particularly dark, like a typical rocky object, nor bright and icy like Pluto and other Kuiper belt objects. Its orbit is even more peculiar. It passes no closer to the sun than 7 billion miles, nearly twice as far away as Pluto ever gets, and it sails beyond the outer edge of the Kuiper belt, possibly into the Oort cloud. Is it a stray Kuiper belt object, a comet from the inner Oort cloud, or something different?

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, some nearly the size of Pluto; countless more await discovery. Even the distant and far-reaching 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 solar system—past, present, and future.

Inner Solar System: 0–5.2 astronomical units, or AU

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

The inner solar system spans everything from the sun out to (but not including) Jupiter, 460 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 failed to vaporize and instead accumulated to form the gaseous outer planets: Jupiter, Saturn, Neptune, and Uranus.

Asteroid Belt:

The majority of asteroids occupy an 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 a hundred asteroids are a mile or more in diameter, and a dozen or so are 200 miles wide. The largest, Ceres, is 600 miles wide.

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 about 5 billion miles out. Beyond that, the number of detected objects drops precipitously until one reaches the even more distant Oort cloud. There may be billions of Kuiper belt objects in all, including at least 35,000 that are more than 60 miles wide. Pluto, at 1,430 miles wide, is the largest known member and the only one known to have an atmosphere. Its possession of a moon, Charon, was long thought to set Pluto apart from other denizens of the frozen region, but a dozen Kuiper belt objects are now known to have partners.

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°F. They come in a variety of colors, from gray to dark red, indicating their compositions, 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, which fade again as the object retreats past Neptune.

Oort Cloud: 10,000–100,000 AU

The entire solar system is enveloped by the Oort cloud, an enormous sphere of nascent 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 funny 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 was ejected entirely from the solar system.

Over time, the gravitational yanks of passing stars jostled the orbits of the objects, leaving no hint of their formerly neat, flat paths around the sun. (However, 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.


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


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 CCD detectors snap digital images of a hand-size patch of sky. So far the search has turned up 35 bright objects. Over the next two years, the team plans to scan the entire northern sky.


A mosaic of eight CCD detectors coupled to the 3.8-meter Mayall Telescope at Kitt Peak National Observatory near Tucson, Arizona, scan for faint Kuiper belt objects as small as 30 miles in diameter. A second, identical camera is connected to the Blanco Telescope in La Serena, Chile. To date, the survey has spotted 411 objects. The goal, says survey leader Robert Millis, is to discover enough objects to begin to understand the scale of the belt, the three-dimensional distribution of objects in space, and their orbits.

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