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Hello Pluto

Tomorrow, July 13th we get our first good look at Pluto. The internet is buzzing with excitement of what we will see.


Google has already put up their picture for the flyby. (Source: Google.com)

It has taken New Horizons almost nine and a half years and 3 billion miles from launch to reach Pluto. It’s a long journey but it’s almost there. You can find out everything about the mission from the Press Kit.

There’s an exact count down to when New Horizons will make it’s closest approach


Live Countdown at http://pluto.jhuapl.edu/

We also finally know how big Pluto really is. 1,473 miles (2,370 kilometers) in diameter, larger than previous estimates, and Pluto is larger than all other known solar system objects beyond the orbit of Neptune.  (Source)


Compared to Earth, Pluto is quite small (Source: NASA)

If you went to elementary school prior to 1992 you probably remember something like “My Very Educated Mother Just Served Us Nine Pizzas” or some other similar mnemonic to remember the names of the nine planets,  (Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto). Though due to Pluto’s reclassification we only got served Nine. Nine of what kids today will never know. However, a 2015 New York Times article suggested some mnemonics including, “My Very Educated Mother Cannot Just Serve Us Nine Pizzas—Hundreds May Eat!” to remind us of the other classified dwarf planets as well. (Source) (Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Haumea Makemake Eris)

So what happened to Pluto, why did it get demoted?

The credit of booting poor Pluto goes to the International Astronomical Union under Resolution 5A and 5B. In 2006 a formal vote was taken on what makes a planet a planet. Their determination is as follows:


Resolution 5A is the principal definition for the IAU usage of “planet” and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a “Planet” in the Solar System

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.


The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A “planet” [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

IAU Resolution: Pluto

The IAU further resolves:

Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

So let’s see how Pluto matches up to 5A.

1a: Orbits the sun, check.

1b: Nearly round, check

1c: Has cleared the neighborhood, ah there is the rub.

According to 1c, a planet must have cleared the neighborhood around it’s orbit. So let’s take Earth. Earth has an orbit of 365.256363004 days, so it crosses it’s path once an Earth year clearing away anything in it’s path into beautiful displays of meteor showers for us here on the surface (we won’t mention the dinosaur incident). Pluto on the other hand makes a trip around the sun once every 90, 465 Earth days, or 247.68 Earth Years. Which means since Pluto was first identified on February 18th, 1930 it has only gone 1/3 of the way around the Sun.

If you have little kids I want to imagine you’re a full fledged Planet. You have to pick up everything they leave behind, or you’re not a Parent (Planet), you’re just another kid. So you go through life picking up what is left behind around you from your kids and others. Say you pick up things from your given path once a day. The next day there is more stuff and you have to clean it all up again. This is what Earth does as it goes around once a year clearing the way. (Though, if it truly cleared the way, we wouldn’t have these regular shows at all.)  Now imagine you only walk that same path that you have to clear once every 247 days. Do you think the path from the previous 246 days is going to stay clear for you with kids still around? This is what Pluto’s task is to be classified as a full fledged planet. Keep the walk way clean for  247 Years.

Neptune, Pluto’s next closest big planet brother has an orbit of 164.8 years. Still quite a lot, but Neptune is also 20 times the size of Pluto which helps when gravity helps pull in the neighborhood debris.

Pluto is an underdog, but Pluto is not alone. It has Ceres, Haumea, Makemake, and Eris to keep it company in the dwarf category.

This story also covers poor Pluto’s plight. In light of it all it is still Pluto’s day to shine. Pluto does not fear the planet dwarfing humans, but instead greets us with a loving heart and welcomes us to his Neighborhood.


Source: NASA Facebook

For more news and updates check out the links on the Pluto Toolkit.


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