And Then There Were Eight

Pluto has the dubious distinction of being the only one that got thrown out of the Planets Club. So why did Pluto get downgraded? It wasn’t a simple case of the telescopes back in those days not being sophisticated enough (although that did play its role too).

Rather, people were looking for a planet. Why were they looking for one? Well, because Uranus’ orbit appeared to be disturbed in the 1840’s. They suspected another planet beyond Uranus was the culprit. And thus they found Neptune. On further analysis, it looked as if Neptune couldn’t have done it alone. And hence the hunt for another planet started. Percival Lowell (of the family referred to in the ditty about Boston being the home of the bean and the cod, where the Lowells spoke only to Cabots, while the Cabots spoke only to God) assigned his observatory to finding Planet X, as it was called then. Clyde Tombaugh found Planet X after Lowell’s death. And that’s why Pluto got to be called a planet.

Then the questions started coming as better telescopes brought more data in. Did it make sense to call Pluto a planet when it was so tiny (a fifth the mass of the earth's Moon)? Wasn’t its orbit different from all the other planets (way more eccentric, and its orbital plane way too titled)? But these questions obviously weren’t enough to changes Pluto’s status.

Then, in 1989, Voyager 2 sent back data that revised the mass of Neptune. That revised mass meant that there was no disturbance of Uranus that needed to be accounted for (beyond that caused by Neptune). Ergo, the “need” for a 9th planet no longer existed. This put Pluto on shakier ground.

Then astronomers began to find more and more objects of similar sizes in a band (now called the Kuiper Belt) around Pluto. Given that all the other planets had “cleaned up” objects in their orbital path, people began to wonder why Pluto didn’t have to do the same to be called a planet? On the other hand, if cleaning up the debris wasn’t a necessary condition, shouldn’t all the other Kuiper Belt objects be called planets too? Then came the knockout punch: in 2005, they found Eris, a Kuiper Belt object larger and heavier than Pluto.

At this point, astronomers had 3 choices: (1) Increase the number of planets to include Eris and Ceres, yet another large Kuiper Belt object, or (2) Keep the planet count at 9 (retain the status quo because what’s done is done), or (3) Drop Pluto from the planets list (and thus have a valid reason to exclude all the other Kuiper Belt objects as well).

In the end, astronomers voted to demote Pluto (and Eris) down to the newly created classification of “dwarf planet”.

Comments

  1. KumarOctober 31, 2011 at 10:59 AM

    Here is a case where physics (astrophysics, if you wants me to be precise) is showing signs of the behavior of the biological sciences. In Physics, the emphasis is more on precise cause-effect relationships. In biology, categorizing and labeling is more important because the science needs that approach for better understanding, since some or many individual characteristics become variable. (Unlike in physics/chemistry where every sodium atom is identical to all other sodium atoms etc., in biology no two trees of the same kind need to be identical to enable precise methods like in physics. Like like differences in many areas permeate biology so much so that labeling through character becomes almost science itself there.)

    At last, in defining "What is a planet?" physics is going by "Select characteristics" approach. As children, we were taught that "Objects that go round the sun continuously and having elliptic orbits are planets." Now they say, "Yes. But with that if characteristic X is present then will 'classify' them as "planets". On the other hand, added to the general basic idea of the planet, if characteristic Y is found then we will classify them as "dwarf planets".

    The layman cannot fathom in what way the knowledge of the solar system becomes better understood through this grouping into two categories. This kind of thing is meant for the astro-physics savants and possibly those laymen who would like these subjects as their hobby.

    Though I have always maintained some interest for physics even decades after my graduation in physics, on this matter, somehow I have no more interest than I might have in some obscure point in economics - a domain in which my knowledge is negligible!

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