Until the year 2006, every school science book harped upon there being nine planets and thereafter the count got revised to eight! After 76 years since its discovery in 1930, during the International Astronomical Union meeting held in Paris, the scientist fraternity astounded the world by declaring Pluto, the ninth and the smallest planet of the solar system no longer considered as a planet, generating mass confusion about its identity!
Pluto, named after the Roman God of death, is a cold, frigid world with an average surface temperature of about -235 degrees Celsius. It takes 5.5 hours for the sunlight to reach this celestial body and it completes one revolution around the sun in 248 Earth years; one day on Pluto is equivalent to 6.5 Earth days. A person on Pluto would weigh 1/15 of what they weigh on Earth. Its largest moon Charon is nearly half its size. However, they are much similar in mass which causes Pluto to wobble as it travels through space, an un-planet-like behaviour. This along with the discovery of similar-sized but more massive Eris and many other comparable celestial objects near Pluto prepared the grounds for it to be booted out from the elite group of planets and into a brand-new category of celestial objects called ‘dwarf planets’.
The word “planet” has been redefined over the years since it was first coined centuries ago by ancient Greeks. Any object that appeared to wander through the skies was termed a planet. In popular perception, totally based on fantasy, a planet is conceptualised as a place one can visit and walk around on. Another way of instinctively defining a planet is centred on one or more of its physical features like its appreciable size, the existence of atmosphere, or perhaps the presence of satellites.
The magnificent images of Pluto that are sent back from the New Horizons probe showcase a vast array of towering exotic ice mountains, serpentine valleys and sprawling plains, dunes and volcanoes which makes Pluto look nearly Earth-like. But if these features were the deciding criteria, our moon would also qualify to be a planet, while the gas giants like Jupiter and Saturn would fail to make the cut.
Likewise, the tiny-sized Mercury with no moons and a faint atmosphere is a planet while Ganymede, the largest satellite within the solar system or Titan, the largest moon of Saturn with a thick atmosphere of Pluto with five moons isn’t. Thus popular acuities are not scientific practices of defining what a planet is.
The definition of a planet isn’t arbitrary. A new, subtle and nuanced approach to what constitutes a planet was formulated in 2006.
A heavenly body is designated as a planet if it meets three essential criteria – the object should revolve around the sun, it should be spherical in shape and the area around its orbit should not have any equivalent or bigger cosmic object. In other words, owing to its gravity the planet should clear asteroids or any other celestial objects out of its way.
While Pluto fulfils the first two benchmarks, it falters on the third and was subsequently demoted to the position of a dwarf planet which currently is a group of five entities, Pluto being the largest.
Elimination of Pluto from the exclusive planet club has proved to be one of the most divisive issues in modern science. But removing the controversial third condition as a prerequisite would reclassify not only Pluto, but also our moon, many moons of Jupiter and Saturn, Ceres and some more asteroids as planets, taking the total tally of planets in our solar system to more than 100. So it seems that the status of Pluto as a dwarf planet is unlikely to change in times to come.
Pluto may have lost its tag as a planet, but it continues to be a noteworthy and mysterious place with unparalleled complexity, the king of the Kuiper belt, with potency to help us gain insights into the ultimate frontier of our solar system and thus help apprehend the secrets of its birth.
(The author is PGT- Physics at Shiv Nadar School, Noida.)
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George is Digismak’s reported cum editor with 13 years of experience in Journalism