My Very Educated Mother Just Showed Us Nine (Minus One) Planets

Whatever Happened To Pluto?

In grade 3, when I was learning about the solar system for the (almost) first time, my friend taught our class a mnemonic device to help us remember the order of the planets:

My (Mercury)

Very (Venus)

Educated (Earth)

Mother (Mars)

Just (Jupiter)

Showed (Saturn)

Us (Uranus)

Nine (Neptune)

Planets (Pluto)

pluto
Pluto: My (once) favourite planet

Well…not quite anymore huh? Since learning this nifty trick some 17 years ago, the scientific community has provided a few adjustments to this order. The biggest one, of course, was the removal of distinguished planetary status from our beloved outlier, Pluto. Before its unceremonious demotion, Pluto was the planet which lay furthest from our Sun (which is the star around which all the planets in our solar system, including earth, orbit around).

So what happened to Pluto? Did it disappear? Did it change, somehow shrink into obscurity? Was it replaced by a more deserving celestial body? Why isn’t Pluto a planet anymore?

The Answer? Reclassification

charon
Pluto (right) in comparison to its moon Charon

Well this whole story started in the late 1970s, when scientists were able for the first time, accurately determine the size of one of Pluto’s moons, Charon. This discovery led them to re-evaluate the size of Pluto itself. Originally, Pluto was believed to be roughly the same size or a bit larger than Mercury. However, upon this re-evaluation, astronomers were able to realize that Pluto was actually must smaller in mass than Mercury.

Later on, in the early 2000s, scientists began to record the presence of other extraterrestrial bodies which were in fact larger than Pluto (even our own moon turned out to be bigger than our tiny ninth planet), which led researchers to realize that someone needed to develop an official definition of a planet. They came up with these 3 major criteria for classification as a planet.

For a celestial body to be considered a planet, it must;

 

 

1) be in orbit around the sun.

2) have sufficient mass to assume hydrostatic equilibrium (i.e. must have enough gravity to overcome other forces to maintain a spherical shape)

3) have cleared the neighbourhood around its orbit.  (i.e. must have cleared its orbit of all other objects of a similar size)

size comparison
Size comparison of the smaller planets

While Pluto is able to maintain the first two conditions (i.e. it’s spherical in shape and does indeed orbit around the sun), it’s the third requirement that disqualifies poor Pluto from planetary acclaim. Because Pluto is so small, its orbit actually sits INSIDE the orbit of Neptune (the next closest planet to the sun), and it also shares its orbit with a number of other celestial bodies.

For this reason, in 2006, Pluto was officially reclassified as a “dwarf planet”. So yes, Pluto still exists, it’s still out there orbiting the Sun, and it still sits in pretty much the same place as it always did. However, it can no longer be grouped with the other major planets in our solar system, and we no longer count Pluto as one of our official nine planets.

images (15)

*****

This October, we’ll be taking an “Extra Terrestrial Trip”. We’ll dive briefly into the world of astronomy and explore some more about life beyond the borders of the earth.  Eventually, we’ll even take a look at the existence of life outside of Earth. What REALLY is out there? Are you excited? Because I definitely am! Stay tuned in the upcoming weeks for more.

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