The Missing 9th Planet

NASA / JPL-Caltech, Public domain / Wikimedia Commons

Earlier this week, I happened upon this article from the BBC that talks about a missing 9th planet. I paused when I saw the title - "If Planet Nine exists, why has no one seen it?"  

Because when I was growing up, I was taught that there were nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. 

I know that about 15 years ago, Pluto suddenly got the boot and was demoted to a dwarf planet. So did the article mean ANOTHER planet?

Dwarf Planets

The International Astronomical Union (IAU) is the official organization that promotes and safeguards the science of astronomy through international cooperation. They are also responsible for assigning official names and designations to celestial bodies. So that means they get to set the parameters or rules of what defines a planet... or any other celestial body.  

They determined that there are four things that classify a dwarf planet:

1. it orbits the sun, 
2. it has enough mass to assume a nearly round shape, 
3. it is not a moon, and 
4. it has not cleared the neighborhood around its orbit.

Pay close attention to the last rule because that is what has kicked Pluto out of its planet status and into that of a dwarf planet. By this definition, five bodies have been labeled dwarf planets: Ceres, Pluto, Eris, Makemake, and Haumea.

Lexicon, CC BY-SA 3.0 / via Wikimedia Commons

The thing is, Pluto orbits in the Kuiper Belt which is an area beyond Neptune containing many comets, asteroids, and other small bodies made largely of ice. It's like an asteroid belt, but much, much bigger. And in fact, three of the dwarf planets that have been identified are found in the Kuiper Belt: Pluto, Haumea, and Makemake.

But there's still a lot of debate about Pluto's status because when the New Horizons spacecraft passed by Pluto in 2015, it sent back photos that revealed Pluto has remarkable geographic features, including towering mountains and unique "ice-ridged terrain." So Pluto's status remains a hotly debated topic.

Sedna, Bob3Studios, CC BY-SA 4.0 / via Wikimedia Commons

Another 9th Planet?

Recently, there's been a lot of speculation that there is another 9th planet that has yet to be discovered. This hypothesis is fueled by the unusual orbits of the minor planet Sedna and 18 other objects, all of which appear to be pulled in the same direction. All 19 are tilted on their axis also in the exact same direction. The chances of this happening randomly is pretty unlikely. So scientists continue to hypothesize that there is another planet (or a black hole!) that is affecting these celestial bodies.

Why haven't we seen it?

I couldn't help but wonder, as old as astronomy is, why on earth wouldn't we have discovered this other planet years (or even centuries) ago? It all boils down to distance. 

Sedna and the other objects in question are located in the Oort Cloud in the farthest reaches of our solar system. They are so far away, only one machine is capable of seeing so far into space: the Subaru Telescope found atop a dormant volcano in Hawaii. And the astronomers who are searching for the elusive 9th Planet only get access to it three nights each year, if they're lucky. 

 

Robert Linsdell from St. Andrews, Canada, CC BY 2.0 / via Wikimedia Commons

Extend the Learning!

Here are some additional resources to learn more about dwarf planets, Pluto, and more.

• Ceres and Pluto: Dwarf Planets as a New Way of Thinking about an Old Solar System (Lesson Plan / 5th - 8th) :: NASA
• Space Shorts: What Is A Dwarf Planet? (Video / K-12) :: JPL
• Sizing Up Pluto (Activity / 4 - 12) :: JPL
• Pixel Puzzler: A 'Pi in the Sky' Math Challenge (Activity / 4 - 6) :: JPL
• Is Pluto a Planet? (Lesson / 6 - 8) :: ScienceNetLinks
• Solar System Objects (comets, asteroids, dwarf planets, meteors) Foldable (interactive notebook / 5 - 8th) :: Sandy's Science