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Space & Astronomy

NASA proposes redefining the word 'planet'

By T.K. Randall
February 20, 2017 · Comment icon 20 comments

How should we define a 'planet' ? Image Credit: NASA
The move, if successful, could mean that our own solar system actually has more than 100 planets in it.
In order for a body to be classed as a planet under the current definition of the word, it must be in orbit around its parent star, it must be massive enough for its own gravity to make it round and it must have "cleared its neighborhood" of smaller objects within its own orbit.

Now though, scientists at NASA have come up with a new definition of the word 'planet' that focuses, not on its orbital attributes, but on its physical properties.

If this were to become formally established it would mean that our own solar system would have over 100 planets including Pluto ( which was recently demoted ) and even our own moon.

"We propose the following geophysical definition of a planet for use by educators, scientists, students, and the public," the scientists wrote.
"A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."

In other words, to be considered a planet a body simply needs to be round and smaller than a star.

This new definition makes a lot more sense, the researchers argue, as the existing definition fails to take in to account planets in orbit around other stars or those that don't have a parent star at all.

Secondly, the 'zone-clearing' requirement is flawed because the size of the zone must be defined and in many circumstances clearing this area is impossible regardless of the size and properties of the world in question. An Earth-sized object within the Kuiper Belt for instance would never be able to clear its immediate vicinity of debris even though it is otherwise clearly a planet.

The full text of the proposal can be found - here.

Source: Science Alert | Comments (20)

Recent comments on this story
Comment icon #11 Posted by EBE Hybrid 6 years ago
When I was young I was taught the mnemonic Mother Very Thoughtfully Made A Jam Sandwich Under No Protest (Mercury Venus Terra/Earth Mars Asteroid Belt Jupiter Saturn Uranus Neptune Protest). It was a great way to learn the order of the planets and also where the asteroid belt lays. I motion that planets orbit stars and moons orbit planets, re-promote Pluto to planet status, then the mnemonic fits again and as a bonus we have 9 planets, which means we can call the next discovered planet as Planet X (X being romaneral for 10) which sounds cool!
Comment icon #12 Posted by Waspie_Dwarf 6 years ago
There is no sane definition of a planet which can include Pluto but exclude the rest of the dwarf planets.You can have 8 planets in the solar system or you can have dozens (potentially hundreds). Nine is not an option*. *Edited to add: Unless of course the hypothetical Planet 9 is proven to exist,in which case 9 will be an option, but only if you don't include Pluto. What ever definition of a planet is finally settled on you are going to have to learn a new mnemonic.
Comment icon #13 Posted by EBE Hybrid 6 years ago
Oh buggar, my brain is so full of old stuff I'm finding it tricky to fit any new mnemonics in. However I really should learn a new mnemonic for resistor colour coding (as in electronic), that was near the mark in the 80's and to speak it aloud would probably be classed a hate crime now!
Comment icon #14 Posted by Four Winds 6 years ago
I'm glad I will never have to name them all on a test. The change seems to make sense though.
Comment icon #15 Posted by taniwha 6 years ago
It makes no sense! †Everyone knows a moon is a satellite orbiting a planet.† It so happens there exist round moons too. There is nothing wrong with calling them what they are. † If a planet was orbiting it's moon then the moon would be the planet and vice versa. †But they don't. †There is an obvious distinction, so let it be. That is not to say in other solar systems twin planets, even triplets, might exist, perhaps even sharing the same atmospheres. †
Comment icon #16 Posted by Brok 6 years ago
† No. Just no. We shouldn't change definitions just to comply with people's bias and nostalgia. It's counterproductive and the opposite of what science stands for. Pluto isn't a planet any more than other dwarf planets, so it shouldn't be considered one by any sensible definition.
Comment icon #17 Posted by Derek Willis 6 years ago
Some astronomers - though not by any official definition - define the Earth-Moon system as being a "double planet". †
Comment icon #18 Posted by MWoo7 6 years ago
Comment icon #19 Posted by highdesert50 6 years ago
I find the new definition an interesting compromise that reflects the history of planetary discovery within our solar system while providing opportunity for broad application beyond our solar system as new discoveries are made.
Comment icon #20 Posted by Merc14 6 years ago
We'll have to see what the IAU says about it.

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