Introduction to Exploring the Suburban Solar System
One stormy night near the turn of the millennium, a friend and I were sitting beneath the massive Hale Telescope at Palomar Observatory near San Diego. The telescope was standing idle inside its closed dome because of bad weather, and so we passed the time discussing recent discoveries about the edge of the solar system. I stated my conviction that beyond Pluto—the most distant of the known planets since its discovery in 1930—more planets were waiting to be found. In fact, I was so certain that I was willing to bet on it. We set a deadline of Dec. 31, 2004, for the discovery of a 10th planet. Then we did something that might seem surprising. We chose our own definition for the word planet. In 1999—and even as of mid-2006—astronomers had not yet agreed on an official definition for this familiar object. We did not think we could calculate immediately how big any newly discovered object was. As a result, we decided our criterion should be the amount of sunlight the object reflected, a rough estimate of size.
The idea that there might be planets beyond Pluto had been percolating in astronomers' minds since 1992, when David Jewitt of the University of Hawaii at Honolulu and Jane Luu, then at the University of California at Berkeley, spotted a small, icy object circling the sun well beyond Pluto's orbit. The object they discovered was the first identified member of a vast population of such objects orbiting the sun in the outer reaches of the solar system. This area is called the Kuiper (pronounced KY pur) belt, for Dutch-born American astronomer Gerard P. Kuiper, who in 1951 theorized that the belt may be the source of short-period comets (comets that take fewer than 200 years to orbit the sun). Astronomers believe that objects in the Kuiper belt are “building blocks” left over from the formation of the planets 4.6 billion years ago. By the end of 2000, astronomers looking at small areas of the sky had discovered nearly 500 Kuiper belt objects (KBO's), with the largest being about one-third the size of Pluto. Pluto's diameter is about 2,300 kilometers (1,430 miles). Because we were looking at much larger segments, of the sky, it seemed just a matter of time—and hard work—until we would find more objects at least that size.
My confidence about the discovery of new planets was bolstered by the project that I was working on at the time. Chad Trujillo at the Gemini Observatory on Mauna Kea, Hawaii; David Rabinowitz at Yale University in New Haven, Connecticut; and I had just begun the broadest search ever made of the Kuiper belt to find new large bodies in the solar system. Our work involved taking pictures of the skies using the 1.2-meter (48-inch) Samuel Oschin Telescope at Palomar Observatory. Night after night, we searched a different section of the sky that—from Earth—covered an area about the size of a hand extended at arm's length. We were looking for faint objects moving at a certain speed across the background of stars so distant that they appear to be fixed (unmoving).
Every object orbiting the sun moves across the sky. Objects close to the sun—such as the terrestrial planets Mercury, Venus, and Mars and the small, rocky asteroids orbiting between Mars and Jupiter—move across the sky quickly because of their relatively short orbits. Between the orbits of Jupiter and Neptune, in the thinly populated realm of the solar system's giant planets, objects move at more moderate speeds. The objects we were seeking were the most distant—and, therefore, the slowest moving—of all. We took three pictures of each area three hours apart and then compared them (using sophisticated computer programs) with images of millions of fixed stars and galaxies. Occasionally, we found what we were looking for—a faint point of light that looked like any other star in the first picture but had changed position slightly in the second picture and moved again in the third. About once a week, our photographs captured a dim, slow-moving object that we could add to astronomers' collection of known KBO's.
Most of the KBO's we found were small—perhaps one-fourth the size of Pluto. However, on a few occasions, we found an object brighter than anything else in the Kuiper belt. Each time we found such an object, we thought that we had finally discovered something larger than Pluto. But after more detailed examinations using other telescopes, we realized that none of these objects was larger than two-thirds the size of Pluto. In late December 2004, we discovered our largest object yet, but even that body was only about three-fourths of Pluto's size. On Jan. 1, 2005, I sent a note to my friend conceding the bet. My prediction that new planets would be found within five years had not come true.
Our search did not end, however. Four days after losing the bet, I was examining an unremarkable part of the sky in the constellation Cetus, the sea monster. Suddenly, the computer picked out a bright object that looked like a star in the first picture but had shifted its position slightly in the second picture. In the third picture, it had shifted again. This object was one of the brightest that we had ever seen in motion. Moreover, it was moving more slowly than any other KBO we had ever seen, meaning that it was also the most distant. The fact that it was so bright despite being so far away meant that the object was big—bigger than any of the KBO's we had seen to date. A quick calculation on my computer verified what I had initially guessed. This object was not only bigger than Pluto but also the largest object discovered in the solar system since 1846. I had lost the bet four days earlier, but the discovery confirmed my belief that our solar system had more than nine planets. I temporarily nicknamed the object Xena, after the Greek warrior princess in the 1990's television series. But had we really found a new planet? Some astronomers thought not.
Defining Planet
Although the word planet has existed for thousands of years, its meaning has changed to reflect our understanding of the cosmos. The word comes from an ancient Greek word meaning wanderer or something that moves in the sky. The ancient Greeks counted seven planets: the five you can see with the unaided eye—Mercury, Venus, Mars, Jupiter, and Saturn—plus the moon and the sun. In 1543, the Polish astronomer Nicolaus Copernicus proposed that the sun, rather than Earth, lies at the center of the solar system, and that Earth and the other planets revolve around the sun. Once scientists had accepted these ideas, the definition of a planet changed to include Earth but exclude the sun as well as the moon, which orbits Earth. In 1610, the Italian astronomer and physicist Galileo observed objects orbiting Jupiter the way the moon orbits Earth. His discovery solidified the concept that planets travel around the sun while moons travel around planets. In 1781, the solar family of planets expanded to seven when the British astronomer William Herschel found Uranus, a large body similar to Jupiter and Saturn.
In 1801, the number of planets rose to eight with the discovery of Ceres, a large object that orbits between Mars and Jupiter. Most people have never heard of “planet Ceres,” though, because soon afterward, astronomers began to find other objects like it in that region of space. Rather than having discovered new planets, astronomers had discovered the asteroid belt, an area filled with many small, rocky bodies like Ceres. By the 1850's, Ceres and similar objects had been demoted from “planet" to “asteroid.” (Asteroids are also called minor planets.) With this move, the scientific meaning of the word planet had changed once again. The word no longer simply meant any object that orbits the sun; a small object that is one of many similar objects no longer qualified.
A Different Kind of Planet
Xena (scientifically known as 2003 UB313) is not the first celestial body to puzzle astronomers. When Pluto was discovered, its unusual characteristics led some astronomers to question whether it should be considered a planet. Pluto is, by far, the smallest of the nine planets, measuring only about half the diameter of Mercury, the second smallest. Pluto moves around the sun in an elliptical (oval) orbit, rather than the nearly circular orbit followed by the other planets. While the other planets orbit the sun in the ecliptic plane (an imaginary surface through Earth's orbit around the sun), Pluto is tilted by almost 20 degrees away from the rest. However, astronomers had no other way to classify it, and so Pluto was—and still often is—referred to as the ninth planet.
The discovery of the Kuiper belt in 1992 allowed astronomers to make better sense of Pluto. It was not a lonely oddball at the edge of the solar system but the then-largest known KBO. Just as discoveries 150 years earlier had forced astronomers to reconsider the status of Ceres and other asteroids, the discovery of the Kuiper belt forced astronomers to reconsider the status of Pluto. If Ceres is the largest member of a certain group of objects—and, therefore, not a planet—why should Pluto be treated any differently?
Some astronomers want Pluto to keep its planetary status because it—like all the other planets—is round. Astronomers are not merely attached to this particular shape. Objects in space are round only if they have enough mass (amount of matter) that their gravity overcomes the strength of the material they are made of. That is, if a boulder were dropped into space, it would maintain whatever irregular shape it originally had. However, if a large number of boulders were dropped into a particular region of space, their combined mass—and the gravitational force they produce—would crush the individual boulders into the shape of a single sphere. A rocky object must be approximately one-third the size of Pluto to have enough gravitational pull to become a sphere. Any space object that is massive enough to be round is massive enough to have severely modified whatever materials it was made of. According to this argument, Pluto qualifies as a planet.
One problem with defining a planet as a sphere is that many objects in the solar system are round, including the moons orbiting Earth, Jupiter, Saturn, and other large planets. Ceres, the asteroid formerly known as a planet, is round. The Kuiper belt has at least a dozen objects that are probably round. Should the word planet include all these objects?
Popular culture has certainly embraced Pluto as a planet. In 2000, when astrophysicist Neil deGrasse Tyson prepared an exhibit of planets at New York City's Hayden Planetarium (of which Tyson is the director), he excluded Pluto. The decision made the front page of The New York Times, and hundreds of letters and e-mails protesting the move poured into the museum. On the other hand, the public would probably not embrace the idea of instantly creating dozens of new planets. How are astronomers to proceed if members of the general public, who have a certain understanding of the word planet, are unwilling to accept the scientific definitions astronomers have proposed?
The discovery of 2003 UB313 brought this simmering debate to a boil. Should we call it a planet? No, if we use the definition that also excludes Pluto. Yes, if we use the spherical definition that also includes dozens of other objects. How can we reconcile these definitions and decide what to call the new object?
Some scientists argue that we should abandon any attempt to agree on a scientific definition of the word planet. They contend that people have used the word in its current context for more than 1,000 years, while modern science has existed for only a few hundred years. What if astronomers were to simply agree that a planet is any object that is at least the size of Pluto? If so, 2003 UB313 is a planet; and anything else that we find that is at least as large as Pluto would also be a planet. Admittedly, this definition is arbitrary and nonscientific, but it fits what most people think of when they use the word. Moreover, this definition is no less scientific than those of such words as continent, which is as strongly influenced by popular culture as it is by the discipline of geography. Australia, for example, is considered a continent, while the islands Iceland and Greenland are not. Europe and Asia comprise one land mass but are considered two continents.
Throughout 2006, the organization responsible for naming planets and other celestial objects—the International Astronomical Union (IAU)—struggled over the decision of whether to consider Xena/2003 UB313 a planet. Until the group decides, an official name for the object cannot be chosen. The IAU planned to announce an official definition of a planet in September 2006.
Discovering the Secrets of the Kuiper Belt
Are there any other planets in either the Kuiper belt or the extreme edge of the solar system beyond it? To know the answer requires an understanding of what we have learned about the Kuiper belt. Between 1930, when Pluto was discovered, and 1992, when Jewitt and Luu found the first KBO, many technological innovations transformed astronomy. Larger telescopes allowed astronomers to see fainter objects; more powerful computers automatically combed through images representing vast areas of the sky; and, most important, digital cameras replaced inefficient photographic plates. By the early 2000's, those cameras were large enough to capture wide expanses of the sky, allowing astronomers to find greater numbers of objects in the Kuiper belt. By 2006, astronomers had found at least 1,000 objects in orbit beyond Neptune. The objects range in size from 2003 UB313's diameter of about 2,400 kilometers (1,490 miles) to a few faint objects perhaps 50 kilometers (30 miles) across.
Much effort has gone into figuring out where KBO's are and what shape their orbits take around the sun. Such basic information is an important key to understanding the formation and history of the belt. An oceanographer might use clumps of driftwood washed up along a beach to chart ocean winds and currents. In a similar way, astronomers look for patterns of KBO locations and orbits to chart their interactions with planets and their movement during the 4.6 billion years of the solar system's existence.
So far, astronomers have found three such patterns in the Kuiper belt. The first pattern involves a population of objects with orbits similar to Pluto's. In a relationship called orbital resonance, Pluto orbits the sun twice for every three times that Neptune travels around the sun. Orbital resonance occurs when two orbiting bodies exert a gravitational pull on each other. In Pluto's case, orbital resonance prevents the planet from crashing into Neptune even though at one point, Pluto crosses the larger planet's orbit. Orbital resonance also has produced Pluto's tilted, elliptical orbit around the sun, which takes 248 years to complete. If Pluto were the only object with these characteristics, it might be an odd coincidence. However, astronomers now know that some 300 KBO's have tilted, elliptical, 248-year orbits in a 3:2 resonance with Neptune. Jewitt dubbed these objects Plutinos. A force other than coincidence must be at work.
The Power of Neptune
The exact nature of this force was first suggested in 1984 by the Uruguayan astronomer Julio Fernandez and the Taiwanese astronomer Wing-Huen Ip, then of the Max Planck Institute in Germany. In developing theories about the early formation of the solar system, Fernandez and Ip proposed that the planets from Jupiter to Pluto must have migrated to their present locations from orbits closer to the sun. They theorized that every time an object in the then-hypothetical Kuiper belt approached Neptune, Neptune's gravitational field would fling the object either inward or outward. (In the same way, space probes traveling to the outer solar system take advantage of Jupiter's gravitational pull, which slings them farther out into space, saving time and fuel.) Every time Neptune flung an object outward, Neptune itself compensated for the change by moving slightly inward. Every time Neptune flung an object inward, Neptune moved slightly outward. Over time, more objects were flung inward than outward because objects flung outward are still affected by the sun's gravitational pull. The pull of the sun generally drew them back into the inner solar system, where they interacted with Neptune again. However, objects pulled inward also encountered the other giant planets and were never seen again. They may have crashed into the giant planets and been destroyed, or the giants may have flung them back out of the solar system. According to Fernandez and Ip's theory, Neptune moved away from the sun at a snail's pace—about 0.02 kilometer (0.01 mile) per hour.
Fernandez and Ip's idea was not widely accepted until a number of Plutinos had been found. In 1993, the Indian-born astronomer Renu Malhotra at the University of Arizona in Tucson suggested that the Plutinos could have arrived at their present orbits only if Neptune's orbit once had been much closer to the sun. Over about 100 million years, the planet moved from an orbit of about 3 billion kilometers (1.9 billion miles) to an orbit of about 4.5 billion kilometers (2.8 billion miles) away from the sun. As Neptune's orbit expanded, more and more objects found themselves in resonance with Neptune. Once an object was in resonance, it was trapped by Neptune's gravitational attraction and was pushed outward. Neptune's gravitational attraction caused the object's orbit to gradually stretch and tilt. Pluto was only one of many objects swept up into such an orbit. When Neptune reached its current location, all the objects in resonance with it—including Pluto—were left with 248-year orbits.
The discovery of Plutinos proved that Neptune had migrated outward. The orbital patterns of a second group of KBO's called scattered KBO's (or scattered disk objects) support Fernandez and Ip's explanation of how Neptune had moved. Scattered KBO's have highly elliptical orbits that, at their closest point to the sun, almost intersect Neptune's orbit. These characteristics are precisely the orbits astronomers expected to find in objects that have been flung outward by Neptune, based on their computer models of how the early universe formed. The existence of scattered KBO's following highly elliptical orbits demonstrates that Neptune continues to move outward, though more slowly.
Scientists have discovered a third group of objects in the Kuiper belt, though they do not yet understand the role these objects have played in the history of the solar system. These objects are called classical KBO's, because their orbits most closely resemble those that computer models show would have existed in the earliest stages of the solar system's history. The orbits of classical KBO's are more circular than those of other KBO's, and classical KBO's generally travel in the same plane of the ecliptic as the rest of the planets. Unlike Plutinos and scattered KBO's, which have been pushed and flung around by Neptune, most classical KBO's appear to have made no significant movements in the past 4.6 billion years.
Both scattered and classical KBO's seem to come to an abrupt end at a distance of about 7 billion kilometers (4.5 billion miles) beyond Neptune. As astronomers find more objects within the Kuiper belt, they hope to better map out the patterns of orbits and perhaps finally learn what forces brought them to the locations objects occupy today.
The search for planet-sized objects in the Kuiper belt that my colleagues and I are conducting most likely has uncovered all the large classical KBO's and, perhaps, all the large Plutinos. New planets, however, may exist among the scattered KBO's. When scattered KBO's are close to the sun, they appear bright and so are readily visible to astronomers on Earth. However, on the outer parts of their long, elliptical orbits, KBO's are dim and hard to spot. The newly found scattered KBO, 2003 UB313, is currently as far from the sun and Earth—and thus as faint—as it ever gets. By 2010, our survey of the entire Kuiper belt will likely be completed. At that time, if any other new planets exist, my colleagues and I will have to assume that they are in the Oort cloud, the most distant region of the solar system, not in the Kuiper belt.
The Outermost Edge of the Solar System
Although a number of astronomers theorized about the existence of a vast sphere of comets at the edge of the solar system, the Dutch astronomer Jan Oort developed the theory that became most widely accepted. In 1950, Oort proposed that long-period comets—those that take at least 200 years to revolve around the sun—originally had been part of the inner solar system. At some point, Jupiter and the other giant planets had flung them out to their current location. Oort realized, however, that even though Jupiter was massive enough to fling objects to the edge of the solar system, another force must have stopped the comets from continuing on into interstellar space. That force, astronomers now believe, comes from nearby stars that exert just the right gravitational tug on fleeing comets to force them to remain within the solar system.
All the comets in the Oort cloud are far too distant and faint to be detected with existing telescopes. Therefore, knowledge of Oort cloud objects comes to us indirectly. Occasionally, a passing star's gravitational tug will send an Oort cloud object back into the inner solar system, where it can be seen as a comet on a highly elliptical orbit.
In early 2004, Trujillo, Rabinowitz, and I discovered one such object during our search of the Kuiper belt. The object—now named Sedna, after the Inuit goddess of the sea—follows a distant elliptical orbit that, unlike those of scattered KBO's, never comes close to Neptune or to any other planet. In fact, Sedna's orbit is unlike anything astronomers have ever seen. Sedna's orbit looks more like that of an object from the Oort cloud (that is, like that of a long-period comet) than that of a KBO, except that it is about a hundred times closer to the sun than it should be.
The simplest explanation for this puzzle is that parts of the Oort cloud may be closer than we thought because the stars that helped create the Oort cloud are hundreds of times closer to the solar system. Astronomers have made fairly accurate measurements for the distances to these stars, and so we know they are not really closer. However, they may have been much closer in the past. Many stars form within vast clusters of stars. Over time, the stars gradually drift away from one another. If our sun formed within a cluster, its companion stars would have been much closer. As a result, the Oort cloud would have been closer to the center of our solar system. After the cluster stars drifted away, this early Oort cloud would have been left, frozen in place as a kind of fossil record of the earliest history of our section of the Milky Way Galaxy.
Although this idea of how the Oort cloud may have formed is scientifically compelling, it cannot be proved simply by the existence of Sedna. Like Pluto before we knew about the Kuiper belt, Sedna is a singular object that for now can be described as an oddball on the outer fringe of the solar system. In the future, however, as astronomers discover more and more objects in the region beyond the Kuiper belt, we should be able to understand Sedna's place in the solar system and how it formed.
Based on the number of comets seen so far, astronomers believe that the inner Oort cloud contains an even larger number of icy bodies than the millions thought to inhabit the Kuiper belt. Because there are so many more comets, the largest ones in the Oort cloud will probably turn out to be several times as large as the largest objects in the Kuiper belt.
Today, we are able to see inside distant galaxies. However, much as we can see the moon on a clear night but not the tiny mosquito buzzing somewhere above our head, our current technology is probably not powerful enough to find new worlds in the Oort cloud. To discover new worlds, we will need to depend on future technological advances—and a new generation of astronomers willing to bet on the possibility.