Overview

Map of Pluto based on Charon eclipses

"Trans-Neptunian object" is a generic term describing all of the assorted objects out beyond Neptune. The first objects known to man that were related to this outer belt of flotsam and jetsam were the comets, although no one actually realized this at the time. The first object to be definitely discovered and known to orbit in this region was Pluto. Since nothing else had been discovered out there, astronomers assumed that Pluto was another planet, out beyond Neptune. Decades later, they began to realize that the comets were inner solar system interlopers, dislodged from some vast cloud of icy objects out even beyond Pluto. Then they begin to discover other bodies similar to Pluto, some with identical orbital characteristics. As the number of objects continued to grow, astronomers began to question whether or not Pluto was truly a planet. Finally, with the discovery of Eris—a trans-Neptunian object larger than Pluto—the issue was forced, and in 2006 Pluto was officially demoted from the ranks of the planets.

Trans-Neptunian objects are generally divided into three groups: the Kuiper Belt, the scattered disk, and the Oort cloud.

Kuiper Belt

The Kuiper Belt is similar to the asteroid belt in that it is composed of a large number of relatively small bodies, of which Pluto was the first to be discovered. However, it resides on the outskirts of the solar system, stretching from around the orbit of Neptune out to nearly twice as far. And whereas asteroids are composed of mostly rock and metal, Kuiper Belt objects (KBOs) are composed largely of ices, such as frozen water, ammonia and methane.

Today, the number of known KBOs has grown to over a thousand, and estimates put the total possible number of objects at over 70,000. The total mass of the Kuiper Belt is anywhere from 20 to 200 times as massive as the asteroid belt.

Individual members of the Kuiper Belt are correspondingly larger than their counterparts in the asteroid belt. The largest member, Pluto, is nearly two and a half times the diameter of Ceres, the largest asteroid. Even Pluto's largest moon (it has 3), Charon, is larger than Ceres.

Scattered Disk

The scattered disk is much less well defined than the Kuiper Belt. It consists of objects—mostly out beyond the Belt—with eccentric orbits that take them close enough to Neptune for that planet to affect their motions. Although fewer scattered disk objects have been discovered (around 100) than KBOs, astronomers believe that ultimately there are at least as many.

The largest scattered disk object is Eris. Eris was discovered in January of 2005, and was quickly realized to be larger than Pluto. At that time, Pluto was still considered a planet, in spite of rising evidence that it was merely another member of the Kuiper Belt. The discovery of an object larger than Pluto forced the International Astronomical Union to make a decision, and in the end they (properly!) demoted Pluto and thus formally recognized the true nature of the trans-Neptunian objects.

Eris was originally dubbed "Xena" by its discoverers, before the IAU got around to giving it a real name. Eris has one small moon, named Dysnomia. Unlike some other objects such as Pluto or Sedna, Eris is not red, and is highly reflective. Indeed, is the second most reflective body known in the solar system, right after Saturn's moon Enceladus.

Sedna is another body often considered a scattered disk object, yet that designation is in question, since it never comes anywhere near the orbit of Neptune. Right now it is slightly closer to the Sun than Eris, but its highly elliptical orbit will eventually take it several times further out. Another designation, detached object, is sometimes used to describe Sedna and similar objects.

Certain classes of short term comets are believed to originally come from the scattered disk.

Oort Cloud

At this point in time, the Oort cloud is still essentially theoretical. It is postulated to lie at a distance of nearly a light year from the Sun, as a vast sphere of icy objects on the very edge of the Sun's gravitational influence. Another hypothetical cloud, the Hills cloud, is believed to lie further inward toward the Sun, and have the form of a disk in the plane of the ecliptic. Most likely, the two clouds would not have a distinct boundary, but would merge into one another.

Long term comets and certain classes of short term comets are believed to originate in the Oort/Hills cloud, as gravitational influences of nearby stars and possible passing objects disturbs the orbits of individual members. It is also postulated that there still might be more planets out beyond the Kuiper Belt and scattered disk which could also serve to disrupt the orbits of Oort cloud (and scattered disk) members. Estimates range from a planet or planets the side of Earth all the way up to the size of Jupiter. Such planets would be very difficult to detect, since their motion across the sky would be extremely slow, and because of the very low light levels that far from the Sun.

Exploration of the trans-Neptunian objects

At this time, no spacecraft have yet actually explored any of the trans-Neptunian objects. Four probes, Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 have traveled at least partway through this belt. Voyager 1 is the furthest man-made object from the Sun, and has actually passed Eris and Sedna.

In January of 2006, New Horizons was launched, with the goal of reaching Pluto in July of 2015. A year later it took some of the highest resolution pictures ever of Jupiter, using a camera that had been designed for much lower light levels. If the Pluto flyby is successful, mission planners are hoping to then send it past at least one more trans-Neptunian object.