Pluto, an object which was once known as a planet

Many people ask me why Astronomers classified Pluto from planet to dwarf planet category? Last few years the voices demanding for the planet status to Pluto has weakens. But it is very interesting question people still wondered that what was the necessity of classifying Pluto. Debate for the status of Pluto as a planet was not recent ones. Astronomers has taken doubt time to time on Pluto as new observations came since its discovery. We will see that how Pluto became a dwarf planet. But, before that let me tell you a short story of Pluto's discovery. It's a fascinating story. The search for Pluto began before Clyde Tombaugh, the discoverer, was even born. It began with the Percival Lowell who during his lime time was best known for his work for a Mars planet.

Percival Lowell

Lowell believed that Mars had intelligent life. He thought there were canals on Mars that the Martians had built. He suffered a lot of ridicule from other astronomers for believing such fantasies. Lowell in the 1890 had actually founded an entire observatory to study planet Mars which still exist to this day in Arizona, United states. 

In order to get even with other astronomers who were ridiculing him for his work on Mars, he wanted to discover a new planet in the solar system. He called this hypothetical planet, a Planet X. He thought that if he could predict the location of another planet, and then find that planet, he would attain the prestige that had eluded him in all his work on Mars. 

He began his calculations, using observations of the motion of planet Neptune. He was hoping to see deviations in that planet's motion around the sun that would signal the gravitational effect of more distant planet on the orbit of planet Neptune. Unfortunately, Lowell died in 1916, before Planet X was ever found. Lowell observatory had to stop their search for planet X because of many reasons. 

Clyde Tombaugh

But then in 1929, Lowell observatory hired a young man from Kansas (U.S.), Clyde Tombaugh. Clyde Tombaugh was only 22 at that time. He was very skilled observer from the beginning of  his young age only. He implemented a search for the Planet X. Tombaugh started taking photographs of sky in night time from telescope and in day timings he used to check those photographs. He was checking any movement of object in these series of photographic plates taken from same sky region with regular interval of days. It was laborious work to check these photographic plates. During his search he found several objects like asteroids and comets. 

on February 18, 1930 he saw an object moved which was not seen before. It took many days for the confirmation then they came to conclusion that it is a planet. The discovery of this Planet X was inspired by Percival Lowell and Clyde Tombaugh's hard work which he put into searching millions of stars to find this tiny, faint object in distance space.

Telescope which was used to search Pluto, Source;sciencephotolibrary

The Lowell observatory team were allowed to name it, because the had discovered it. They put a public request for suggesting interesting name for this newly find object. They settled on PLUTO, a name suggested by an eleven year old school girl from Oxford, England (it wasn't named after the Disney's famous character, but the roman god of the underworld). It was perfect name for this distance world after the Roman god of underworld and its two initial letters PL were initials for Percival Lowell.

The solar system now counted nine planets in it.

Since its discovery astronomers could see very little of this distance world. Very less was known about Pluto for long time. Astronomers weren't sure about Pluto's mass and size until the discovery of its largest moon, Charon in 1978. By knowing its mass which is 0.0021 Earths, they could more accurately gauge its size. The most accurate measurement currently gives the size of the Pluto is 2400 km which is smaller than earths moon. Pluto's moon also showed that it is not rotating around Pluto, but these two object rotate around the common center of mass which is located outside of the Pluto. Astronomers thought its not what planet shows with their moons. They suggested its more like the binary planet system.

Pluto and Charon rotating around common center of mass

Pluto is tiny, but it was considered lager than anything else beyond the orbit of Neptune. Over the last few decades, powerful new ground and space based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the solar system, Pluto and its moons are now known to be just a larger example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune to outer region of solar system up to 55 astronomical units. (1 Astronomical Unit is a distance between Earth and Sun).

Astronomers estimate that there are at least 70,000 icy objects with the same composition of Pluto. Astronomers again asks what Pluto should be called a planet or not a planet?

Astronomer's nervousness, Pluto's data was not indicating it as a planet

Astronomers had been turning up with larger and larger objects in the Kuiper Belt. Object Makemake, discovered by Caltech astronomers team was only little smaller than the Pluto. There are several other Kuiper Belt objects in that same classification. Caltech team discovered another object further than the orbit of Pluto. It was same size of the Pluto even some believed it was bigger than Pluto. officially named 2003 UB313, the object later designated as Eris. Since, its discovery, astronomers have determined that Eris size is approximately 2,600 km. It also has approximately 25%  more massive than Pluto. 

With Eris being larger, made of the same ice/rock mixture and more massive than Pluto, the concept that we have nine planets in the solar system began to fall apart. What is Eris? a planet or Kuiper Belt object. What is Pluto then? Astronomers decided they must take decision about the planet at the 26th general assembly of International Astronomical Union. It was held from August 14 to August 25, 2006 in Prague, Czech Republic.

26th General Assembly of IAU in 2006, voting for Pluto

Astronomers from the various association were given the opportunity to vote on the definition of planets. One version of the definition would have actually boosted the number of planets to twelve. Pluto was still a planet and so were Eris and even Ceres, which had been thought of as the largest asteroid. A different proposal kept the total at nine, defining the planets as just the familiar ones we know without any scientific rationale, and a third would drop the number of planet down to eight and Pluto would be out of the planet club. In the end, astronomer voted for the controversial decision of categorize Pluto (and Eris) to newly created classification of "Dwarf Planet". They put some rule for an object to known as planet.

For an object to be a planet, it needs to meet these three requirements defined by IAU

It need to be in orbit around the Sun - Yes, so may be Pluto is planet

It need to have enough gravity to pull itself into spherical shape -Pluto is still planet

In needs to have "clear neighborhood" of its orbit - Uh ohw... according to this its status is in trouble

What does "clear neighborhood" means? As planet form, they become the dominant gravitational body in their orbit in the solar system. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

Any object that doesn't meet this third criterion is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.

Even though Pluto is a dwarf planet, and no longer officially a planet, it will still be fascinating target for study. It will help us to understand the outer region of our solar system. NASA sent their New Horizon spacecraft to just look at planet Pluto. New Horizon passed by Pluto in July 2015 and transmitted first ever pictures of this dwarf planet and its moon Charon. From these pictures astronomers now know much about Pluto. We will see about this mission some other time.

New Horizon flyby Pluto in 2015 

Many people still asks what is the necessity of classifying Pluto or any other object in this category? Yes their is necessity for such categorization. Because, science is the systematic study of the universe. We tried to understand and define things/events/phenomenon around us. For the ages there were not a single definition available for the objects called planets. The only way to define them was they were wanderers across the earth's sky. In the beginning years of astronomy these objects were few, because of limitations of human eyes. When telescopes enhances our vision we began to start see many of them. Then the need pf classification came up with our new observations. It was very necessary to define these new objects for our better understanding of solar system. Now, with new definition or criteria we can study and understand our solar system as well as extra-solar system objects too.

references and images for this article are taken from universe today, space.com, gify and IAU websites.