The solar system has eight planets. But it wasn’t always the case. In 1930, when Pluto was first discovered by American astronomer Clyde Tombaugh, it was considered the ninth planet of the solar system. In fact, it continued to be known as a planet till 2006 when the International Astronomical Union (IAU) redefined the definition of a planet and reclassified Pluto as a "dwarf planet". While astronomers found an agreement over Pluto, it didn’t last long. In recent years, questions have been raised on classification of Pluto and interestingly, it has been suggested that it could even be a comet or an asteroid.
As per the updated definition given by IAU, Pluto continues to be excluded from the list of planets. The organization said that for a celestial object to be classified as a planet, it must satisfy three criteria:
1. It must orbit the Sun.
2. It must be spherical in shape, meaning that it has enough mass to be pulled into a roughly round shape by its own gravity.
3. It must have cleared its orbit of other debris.
While Pluto meets the first two criteria, it does not meet the third as it shares its orbit with many other small, icy objects in the Kuiper Belt. However some astronomers do differ from this classification.
Dr. Mark Littmann , the astronomy and journalism professor at the University of Tennessee-Knoxville contends that Pluto does have merits to be considered a planet. He said that Pluto is too large to be an asteroid since “(It) has about three times more mass than all the asteroids in the solar system put together”. He also adds that Pluto is in the wrong place to be an asteroid since the majority of the asteroids in the solar system are placed between Jupiter and Mars in the asteroid belt.
On the other hand, famous American astrophysicist Neil deGrasse Tyson believes that Pluto could be a comet. He said in an interview, “Pluto’s orbit is significantly tipped out of the plane of orbits of the other planets. Do you know what else has tipped orbits? Comets. More than half of Pluto’s volume is ice. If you brought Pluto to where Earth is now, heat from the Sun would evaporate the ice and it would grow a tail”.
Littman finds a midway compromise to preserve the identity of Pluto. He explained, “Pluto and its moon Charon have potential value to science, not as representatives of a class of asteroids or comets, but as large, nearly pristine planetesimals — the objects from which the gas…planets and their moons are made”.
For now, IAU standard continues to hold on and Pluto continues to be classified as a dwarf planet. However, it would be interesting to see if in the future, it gets reclassified as a comet or an asteroid.
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