High School High School Space and Planetary Sciences

Pluto: The Outcast of the Family

A primer on Pluto.

(Originally published on June 2nd, 2014 on The Wannabe Scientist)

When Pluto was casted out of the ‘planets’ family in 2006, our generation collectively shook our heads in denial. We’ve all learnt about The Solar System and its nine planets throughout kindergarten and grade school before the revolutionary change that challenged the very definition of a planet. To really understand what happened and why the change happened, we have to take a step back and take a look at the history of Pluto.

Named after the Roman god of the Underworld, Pluto has always been sort of a mystery from the beginning of its discovery. Since early pieces of evidence for Pluto’s existence emerged in the early 1900s, astronomers have tried collectively for decades to search for ‘Planet X’(1), which was finally confirmed through various photographs in early 1930. Up until 2006, Pluto has remained (happily) a part of the big family, even though its mass is very small and its orbit is unusually chaotic relative to the other planets in the Solar System.

Just how weird is Pluto? Well, to start off, its volume is about 0.6% that of the Earth, its mass less than 0.24% of the Earth, and its surface area is a good 10% smaller than that of South America. To put it in perspective, Pluto’s diameter is about two thirds that of the Moon’s. Its orbit is elliptical with an extremely high eccentricity, so it actually comes closer to the Sun than Neptune at times. (But much, much farther from the Sun than all the other planets at other times. The Sun is nothing but a larger speck of light in the Plutonian sky.) Still, these reasons were still not strong enough to kick Pluto out of the family yet.

When Eris, (now you know what to blame) a new trans-Neptunian object (2), was discovered in 2005, Pluto’s status as a planet was challenged. Eris turns out to be 27% (which is quite a lot!) more massive than Pluto. At first, it was said that Eris was considered to be the tenth planet. However, since there are potentially hundreds more undiscovered trans-Neptunian objects orbiting the Sun that are similar to Pluto and Eris, the IAU (International Astronomical Union) must give a precise definition as to what possibly may classify as a planet.

Thus, there are now three main classifications for a planet:

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
  3. It must have cleared its neighborhood(3) around its orbit.

Pluto failed to fulfill the third condition. It is now dubbed as a dwarf planet, a planet that fails to fulfill the third criteria for it to be a planet. I guess Pluto can enjoy being the poster child of ‘dwarf planets’ now…

(1) Percival Lowell started an extensive project to search for the ninth planet in The Solar System in 1894, which he dubbed as ‘Planet X’

(2) Any minor planet that orbits the Sun in the Solar System at a larger average distance than Neptune

(3) Which means the planet has to be the dominant gravitational force compared to the objects orbiting the planet.

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