The solar system is our home, our own comfortable little neighborhood of the galaxy, and as mankind has grown as a species and become more technologically capable, we have begun to venture forth into this neighborhood and to familiarize ourselves with it. The results are far more wondrous and varied than we could have imagined when all we knew of the cosmos was what we could see with our eyes only.
The planets that have been known of since antiquity are those that are easily visible with the naked eye – Mercury, Venus, Mars, Jupiter, and Saturn. Even before we knew of these things as astrophysical objects, they were seen as bright points of light that seemed to wander independently of the background of the stars. In the 1700s and 1800s the solar system became populated with more than just these familiar few, and as these discoveries were made it became clear that it was a more diverse place than we had suspected. As Uranus (1781) and Neptune (1846) were discovered, it was clear that these objects were of the same nature, albeit a little farther and a little dimmer. But the objects Ceres (1801) and Pluto (1930) were not so clearly determined.
Pluto and Ceres share common ground in that they both have introduced us to regions of our solar system populated by hundreds of thousands of objects: the asteroid belt in the case of Ceres, and the Kuiper belt in the case of Pluto. With these discoveries came the need to further consider the nature of these objects and the proper ways to categorize them. Ceres and Pluto trailblazed the identification of asteroids and dwarf planets, but for all the discussion they have introduced, we’ve never really gotten a good look at either one. Two recent missions have set out to correct this deficit and make us a little bit more familiar with some of the Earth’s diminutive cousins.
On January 1st, 1801, Giuseppe Piazzi discovered a new object orbiting at about 2.8 AU from the sun. Although it was difficult at first to ascertain the exact size of this object, dubbed Ceres, it was clear that it was not quite in the same category as the known planets. Therefore, the term ‘asteroid’ (‘star-like’) was coined. Nevertheless, some textbooks listed Ceres as a planet among the others, and continued to do so for a number of decades.
Soon after Ceres became known astronomers found other objects to populate the region of space between Mars and Jupiter, a region that has since become known as the asteroid belt. Ceres is often referred to as an asteroid, although the new designation ‘dwarf planet’ also applies.
On March 6th, 2015, the Dawn spacecraft (http://dawn.jpl.nasa.gov/) arrived at Ceres. As it approached the asteroid, it was able to achieve stunning photos of this object, revealing a pock-marked face reminiscent of the lunar surface. Dawn continues to explore the surface and atmosphere of Ceres, revealing to us information about the formation and evolution of the solar system.
The discovery of Pluto, which took place in 1930, is much more recent, and has proved to be a bit more controversial. Many people are familiar with the discussion that took place a few years ago that resulted in the re-categorization of Pluto as a dwarf planet. Some took this hard, and consider this more of a demotion than a categorization. For the entirety of the twentieth century after its discovery, Pluto was considered the ninth planet. It was only as other objects began to be discovered in the same region of space, now referred to as the Kuiper belt, that the call for reclassification came.
The deciding moment came in 2005 when the object Eris was discovered and found to be roughly the same size as Pluto, though orbiting at a greater distance. Rather than considering Eris the tenth planet, and opening the way for some as yet unknown number of similar objects to become planets, the International Astronomical Union (IAU) decided to meet and formally consider the definition of ‘planet’. They settled on the following three properties:
- It must be in orbit around the sun.
- It must be massive enough for its gravity to pull it into an approximately spherical shape.
- It must have “cleared its neighborhood” of similarly sized objects and become the gravitationally dominant body.
Objects that meet the first of these two requirements, such as Pluto and Ceres, but fail the third are categorized as dwarf planets.
On July 14, 2015, the New Horizons spacecraft (http://www.nasa.gov/mission_pages/newhorizons/main/) is expected to arrive at Pluto, achieving a fly-by distance of about 10,000 km. Due to the distance between the Earth and Pluto, it will have taken New Horizons over 9 years to reach its destination. When it arrives, it will be able to take images superior to those possible from Earth with even the most powerful telescopes.
For all the discussion of categorization, in the end, names are just names. A dwarf planet, by any other name, would orbit just as sweet, to butcher Shakespeare. Categorizations are a useful tool to help us understand Nature and its workings, but there will likely always be natural phenomena that defy these classifications, no matter carefully we define them. The important matter is not what we call something, but what new insights into the universe it has to offer.