Quick answer: It is still out there and continues to orbit the Sun as it has for eons.
Pluto is a distant and frozen world orbiting the Sun once every 249 years at an average distance of 3.7 billion miles away. To the best of our knowledge, it continues to be a frigid ball of rock and ice orbiting the Sun in the same way it has done for eons. Since Pluto’s discovery in 1930, our understanding of the solar system has progressed significantly and many more outer solar system bodies have been discovered. Despite this increase in knowledge, much remains to be learned about the icy world we call Pluto.
Pluto’s largest moon Charon was discovered in 1978. The precise sizes of Pluto and Charon only became known in the late 1980’s. In 2005, two additional and much smaller moons of Pluto were imaged with the Hubble Space Telescope. These newly discovered moons received the names Nix and Hydra (respectively in order of their distance away from Pluto).
Pluto shares characteristics of other solar system objects orbiting in a broad flat ring of rock-and-ice bodies just beyond the orbit of Neptune known as the Kuiper Belt. Named after astronomer Gerard Kuiper, the Kuiper Belt is home to the left over icy debris from an early time in the formation of the solar system. Although Kuiper proposed the existence of this belt in the 1950s, it wasn’t until the 1990s that Kuiper Belt objects (KBOs) were discovered. At the time of this writing, there are 1100 known KBOs. For the vast majority of these, only the positions and orbits are known.
Because of their great distance away from the Sun, KBOs are very cold and retain the simple molecules of the solar system in the form of ices on their outer layers. Depending on the amount and type of ice on the surfaces, the objects can appear more or less bright in telescopes. Pluto and the KBOs are so distant that the images appear as fuzzy dots or blobs even with the most powerful of telescopes. The best photographs of Pluto taken by the Hubble Space Telescope now show that the surface has light and dark markings which change seasonally. Scientists think this is caused by ice which sublimates into a gas, leaving one location only to refreeze elsewhere on the surface.
Some astronomers believe that Pluto is the king of the KBOs, that is, it is among the largest, brightest, and nearest of them. Based on its density, Pluto is estimated to be composed of 70% rock and 30% water ice. For comparison, water is much less than 1% of the total composition of Earth.
Another trait that Pluto has in common with other KBOs is a highly tilted and oblong orbit compared to the major planets. Size is yet another differentiator. Although the largest KBOs are considered to have a significant amount of rock in their composition, they are generally much smaller in size compared to the rocky inner planets of the solar system, Mercury, Venus, Earth, and Mars. The smallest of these four is Mercury. Pluto’s diameter is slightly less than half that of Mercury.
In the past decade, planet-like objects similar in size to Pluto have been discovered in the Kuiper Belt. The largest of these, later named Eris, was discovered by the team of Michael Brown, Chad Trujillo, and David Rabinowitz in 2005. Eris, with its single moon Dysnomia, lies further away from the Sun on average compared to Pluto. Measurements of the diameter of Eris show that it is slightly smaller than Pluto but not by much. A handful of other objects have been found in a similar size range but no KBOs larger than Pluto have been discovered so far.
After its discovery in 2005, the community of astronomers and space scientists had great difficulty with the classification of Eris. Astronomers became caught in a name game that turned out to be a long, controversial, and mostly empty debate about the definition of the word “planet”. Does Eris become a tenth planet with potentially more planets joining the ranks or is it in a different class? If Eris is in a different class then does this have implications for the classification of Pluto?
In 2006, the International Astronomical Union (IAU), an international society of astronomers, created a new classification called “dwarf planet”. According to the new convention, the word “planet” is reserved for the 8 largest Sun-orbiting bodies of the solar system. Pluto and Eris were assigned into this new dwarf planet classification by virtue of a vote by society members at an IAU meeting. Pluto was thereby downgraded and Eris never made it to full planet status.
The controversial vote received intense media attention and caused confusion in the general public. For now, many researchers have adopted the IAU nomenclature. Not surprisingly, the debate still rages on among astronomers and non-astronomers alike. Perhaps it illustrates that we still have a long way to go in our understanding of the solar system. Regardless of the naming convention, most would agree that Pluto and its cousins in the Kuiper Belt are important members of our solar system and greater study is needed.
Although no spacecraft has ever visited Pluto, one is on the way right now. The NASA New Horizons mission spacecraft has already passed the half-way point and is on schedule to arrive at Pluto in 2015. The purpose of the mission is to make measurements of the chemical composition of the surface and the extremely thin nitrogen atmosphere of Pluto. NASA scientists also plan to take high quality photographs of the surface of Pluto and Charon to better understand the nature of the ice and the surface topography. They will also look for other moons beyond the three that are currently known to orbit the remote icy world. After visiting the Pluto system, New Horizons will move on to other targets in the Kuiper Belt.