Is Pluto still a planet? Using the current definition of planet, set by the International Astronomical Union, the answer is no. But Pluto itself has not changed since it was discovered by Clyde Tombaugh in 1930. It is still shaped like a planet. It still orbits the sun at a distance of 3764.58 million miles. So why was Pluto reclassified as Dwarf in 2006?
Why was Pluto Downgraded (Reclassified) as a Dwarf Planet?
One of the reasons is that much more is now known about the distant icy rock Pluto. The combination of powerful telescopes and manned/unmanned space missions has resulted in a rapid expansion of knowledge. Already the baby of the solar system, it was discovered that Pluto was even smaller than first thought in 1978, when astronomer James Christy spotted that part of Pluto's supposed mass was actually a satellite, which went on to be named Charon.
Since the 1990s, more objects have been found in the Kuiper belt, the region of space that is home to Pluto. Some (such as Eris) are comparable in size to Pluto, which posed a problem. Were they planets too? Or did that mean Pluto itself (already significantly different from the other major planets due to factors such as its tilted orbit, and its large moon Charon) should no longer be regarded as the ninth planet?
Initially, terminology and opinions about the newly discovered objects were mixed. A variety of descriptive words such as planetoids and minor planets came into usage. Inevitably, this led to a situation developing where the International Astronomical Union (the IAU), who are responsible for approving the names of celestial bodies, had to delay approvals due to uncertainty about what sort of object was being named.
The IAU rules about assigning different names to specific categories of object are strict, but incredibly there had never been a formal definition for a 'planet'. Eventually, in 2006 IAU members voted in new guidelines, intended to help establish what characteristics define different celestial objects. These are not laws, but are generally adhered to by the greater scientific community. The discovery of new Kuiper belt objects was a clear deciding factor in their creation.
Planet Classification Criteria “Clearing the Neighbourhood”
The IAU’s criteria for defining a planet is as follows:
- The object needs to be in orbit around the sun.
- The object needs to have achieved hydrostatic equilibrium by having enough gravity to pull itself into a spherical shape.
- The object needs to have cleared the neighbourhood around its orbit.
Pluto fits the first two planetary criteria, but not the third. Although, that third classification is somewhat arbitrary, causing an ongoing debate amongst scientists, as well as confusion amongst the general public.
The term “cleared the neighbourhood” means that the planet has to be dominant in its solar orbit, sweeping away smaller objects, excluding its own satellites (moons) if any, into a different gravitational field, or causing them to acrete (fuse) with the dominant object. Pluto shares its orbit with other objects of comparable size, and so it is not deemed to have cleared the neighbourhood.
Dwarf Planet Classification and Plutoids
The IAU uses the term dwarf planet as a descriptive category, generally for planets smaller than Mercury. The guidelines for a dwarf planet follow:
- The object is in orbit around the sun.
- The object has achieved hydrostatic equilibrium by having enough gravity to pull itself into a spherical shape.
- The object has not cleared the neighbourhood around its orbit.
Pluto fits in this category, along with fellow Kuiper belt objects Eris, Makemake and Haumea. Ceres, the largest object in the asteroid belt between the inner and outer planets, makes it a group of five currently recognised dwarf planets. It is likely that many more dwarf planets will soon be recognised officially, such as the large Kuiper belt objects Sedna and Quaoar.
The fact that some scientists (such as Marc Buie who has studied Pluto for three decades) argue that all these objects should still be called planets, and that the IAU openly admits its decisions are not law, means that officially Pluto can still be called by whatever terminology one prefers, but the wider scientific community has accepted the term dwarf planet is here to stay.
However, to add further confusion, there is now another term. In 2008 the IAU added the word plutoid to the lexicon. This is now used to describe all the dwarf planets located beyond Neptune. Depending on one’s viewpoint, this is either a fitting nod to Pluto’s long reign as the ninth planet, or a completely un-necessary further complication. Either way, it’s probably not the last word on the subject of Pluto’s planetary status.
References
For information and fact checking, this article is indebted to the following sources:
Astro Biology Magazine, 30/12/04, “Planetoids Beyond Pluto”, astrobio.net, accessed 26/05/10
Buie, Marc, 03/2005, “Definition of a Planet”, Southwest Research Institute, boulder.swri.edu, accessed 26/05/10
Cain, Fraser, 10/04/08, “Why Pluto is Not a Planet”, universe today.com, accessed 26/05/10
Hamilton, Rosanna L, 1998, "Charon", Views of the Solar System, solarviews.com, accessed 26/05/10
International Astronomical Union, iau.org, accessed 26/05/10
Nasa, nasa.gov, accessed 26/05/10
Michelle Strozykowski - Michelle Strozykowski lives in a small brewing town smack bang in the middle of England. She loves films, especially arty European ones ...
