Should Pluto be reinstated as a Planet?
The recent discovery of its fifth moon has reignited the debate as to whether Pluto, which was made a ‘dwarf’ planet in 2006, should be reconsidered as a planet.
Earlier this month the Hubble Space Telescope discovered a fifth moon orbiting the dwarf planet Pluto. The first moon Charon was discovered in 1978. A further two moons, Nix and Hydra, were discovered in 2005, a year before Pluto’s demotion, while the fourth, S 2011 P1 or simply P4, was glimpsed only last year.
Charon is noteworthy for being similar in size to Pluto itself. This is unusual as the majority of moons are substantially smaller than their planets. Charon and Pluto are an example of a binary system as their size and relative closeness mean their centre of mass is not located within the interior of either object. Some astronomers believe that Pluto and Charon should be classed as a double planet system.
The news of this most recent discovery, temporarily named S/2012 (1034340) 1, or P5, has caused some people, including scientists, to question Pluto’s current planetary status. The total of five moons surrounding Pluto is more than any of the inner planets combined - Earth has only one moon, Mars has two, and both Mercury and Venus have none. However, Pluto still cannot compete with the gas giants of our solar system – currently Jupiter has 66 known satellites, while Neptune has the least of the outer planets, with only 13 discovered.
The question is should the presence, or number, of moons decide whether an object is a planet or not? According to the International Astronomical Union (IAU) - a group of professional astronomers active in the research and education of astronomy - it should not. The decision at their 2006 General Assembly was to introduce a new class of objects termed ‘dwarf planets’. These were objects that did not meet the criteria agreed upon in their discussion of what defines a planet. The discovery of Eris in 2005, a dwarf planet larger than Pluto, contributed to this new term, as well as the demotion of Pluto. In order to be defined as a planet, the Union deemed that a body has to:
Orbit around the sun; and
Have sufficient mass to maintain a spherical shape; and
Have a strong enough gravitational pull to clear smaller objects from its orbit.
Whilst Pluto meets the first two criteria it fails to fulfill the last one. Nonetheless, some believe P5’s discovery should make the IAU rethink its decision and that moons should play a part in classification. Needless to say, others disagree.
Although our knowledge of space and our classification of its objects continues to evolve and change as more is revealed, it seems that despite its unusually large number of moons, and its unique relationship with Charon, Pluto will have to stay a dwarf planet for the foreseeable future. However, Pluto can take some solace in being one of the most intriguing objects in outer space.
Planet or not, there is nothing else quite like it in our solar system!