With the advent of powerful new telescopes on the ground and in space, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable to and larger than Pluto. These discoveries have rightfully called into question whether or not they should be considered as new ‘planets.'

The International Astronomical Union (IAU), have concluded two years of work defining the difference between “planets”, “dwarf planets” and “small solar system bodies” such as comets and asteroids and other objects. The astronomers gathered August 14-25, 2006 at the IAU General Assembly in Prague, where they announced voting results on the definition of a planet. http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf

The new definition of a planet has 3 parts: A Planet is a celestial body that is in orbit around the Sun. It has sufficient mass that it’s self-gravity forms the body into a (nearly) spherical shape. It has cleared the neighborhood around its orbit. This means that it has enough mass to “sweep” out debris, such as small asteroids, from its orbital path.

According to this definition, the Solar System contains 8 classical planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In addition, the IAU voted to create a new class of objects called Dwarf Planets.

Dwarf Planets are defined as: The first objects to be classified as Dwarf Planets have been Pluto, Eris, Ceres, Haumea, and Makemake. Dwarf Planets are defined as: Artist concept of Eris. Image credit: NASA/JPL-Caltech A celestial body in orbit around the Sun. Has sufficient mass for its self-gravity to form it into a (nearly) spherical shape. Has not cleared the neighborhood around its orbit. Is not a satellite. Ceres – From Dawn Spacecraft Image credit: NASA Pluto – From New Horizons Spacecraft. Image credit: NASA

Pluto and Charon by New Horizons during approach on July 13 and July 14, 2015. The relative reflectivity, size, separation, and orientations of Pluto and Charon are approximated. Image credit: NASA Because Pluto’s moon, Charon, is about half the diameter of Pluto, there was much discussion of classifying the system as a “double planet”. The IAU did not re-classify Charon, so for now it remains a satellite of Pluto. Charon may later be classified as a Dwarf Planet.

Currently a dozen candidate “Dwarf Planets" are listed on IAU's "watchlist" which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

Credit: Lynette Cook and W. M. Keck Observatory Small Solar System Bodies is the IAU classification name of all other objects in the Solar System except satellites. This includes most comets, asteroids, and Kuiper Belt objects. Asteroid image: http://www.keckobservatory.org/view_album.php?album_id=4 Credit: NASA Credit: Lynette Cook and W. M. Keck Observatory

Dwarf Planets and Small Solar System Bodies typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). They also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity). All of these distinguishing characteristics are scientifically interesting in that they suggest a different origin from the 8 classical planets.

Most Dwarf Planets, including Pluto, reside in the Kuiper Belt, in the outer region of our Solar System. These objects, which orbit beyond Neptune, are known as Trans-Neptunian Objects (TNOs) Pluto has much more in common with other recently discovered TNOs than with the planets, which led to its designation as a “Dwarf Planet”. It is expected that many more Trans-Neptunian Objects will be discovered in the coming years.

These 3 newly defined classification systems are not without controversy in the astronomy community! The International Astronomical Union (IAU) is an organization of over 8,500 professional astronomers from all over the world. The IAU is recognized as the official authority in astronomy, and is responsible for naming all planets, asteroids, and other celestial bodies and phenomena. Despite the large number of members, only about 5% of members were involved in voting on the 2006 resolution which narrowed the definition of a planet. Many astronomers have pointed out ambiguities in the definition. For instance, several planets, including Earth, have not fully “cleared their neighborhood” of debris. Many astronomers are also clearly disappointed that Pluto was not classified as a double planet. What Do You Think??