jump to navigation

What’s a planet?

According to the International Astronomical Union

  1. A “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, 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
  3. All other objects except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

According to me

  1. A planet is any object that has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium and which isn’t massive enough to fuse deuterium in its core.

Yes, that’s it. Simple, huh? And yes, I know, it’s a good jump from the current understanding of the word. I used to think that it would be better to try to compromise a bit and not jump all that much from the “9 planets” thing that has been with us for our whole lives (unless you’re over 76), but the nonsense that came out from the IAU made me change my mind. I’ll be explaining the reasoning behind this in the blog itself.

Comments»

1. Zazaban - October 6, 2006

Totally. I have to agree, the defenition of the IAU doesn’t makem uch sense. here’s my defenition.

(i) is in orbit around a star or stellar remnants;
(ii) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and
(iii) has a mass below the limiting mass for thermonuclear fusion of deuterium.

Mix of the stuff on Wikipedia.

2. Robert Shepard - August 24, 2009

Yep! I’m with you on this one 100%, Jorge! Let’s keep the definition really simple, and come up with as many sub-categories of “planet” as we need to describe what’s being discovered in our galaxy (not just our solar system).

What follows are merely my thoughts.

Size categories (measuring the diameter):

dwarf – below 4000 km (a bit smaller than Mercury)
terrestrial – between, say, 4000 and 16000 km (like Earth)
large – between, say, 16000 km and 48000 km (a bit smaller than Uranus and Neptune)
giant – above 48000 km

Dynamic categories:

dominant planet – orbits a star, dominates its orbital zone
belt planet – orbits a star, found within an asteroid belt or the like
moon planet – orbits a larger planet
binary planet – two planets in a mutual orbit whose barycenter is above the surface of either planet (Pluto/Charon, for one example)
resonant planets – large planets which jointly dominate their orbital zones, but are in some sort of orbital resonance
solo planets – found in interstellar space

Compositional categories:

ice planets
rock planets
gas planets
(fill in the blank) planets

Then, we just combine the categories. Like “icy dwarf planets” or “rocky terrestrial planets”.

If that means we have dozens of “planets” in our solar system, including the large moons, so be it. I’m fine with that.

3. Jorge Candeias - August 24, 2009

Yes, something like that will have to be the solution for this, in the end. I’m not a fan of establishing arbitrary size limits, although I do know that some level or arbitraryness is inevitable. Still, I cringe a bit with your size categories. With the rest I mostly agree.

Bob Shepard - August 24, 2009

Oh, no matter. I’ll leave it to the experts to come up with the sizes. Maybe it’ll be some sort of logarithmic scale, based on mass rather than diameter. Whatever makes sense.

4. Swayam Sampurna mishra - January 5, 2010

hi
i m a girl of 9 years old and i see sow many planets thank u to saw me .

regards

swaym
sampoorna
mishra

5. Devin Serpa - February 5, 2011

I like your definition of a planet, simple. But an even more succinct definition, and the historical, “a wandering star”. Basically everything found under your definition plus that which your definition and the IAU leaves out. Check out http://www.whatsaplanet.com let me know if you like, what can I improve on, and what we can build on.

6. p - October 26, 2012

its good and bad

7. Rico - March 5, 2014

I completely agree as well! I wrote an extensive article showing the folly of the IAU definition. You can read it here: http://www.alienrobotzombies.com/2011/04/theyre-all-planets-part-1.html

8. mossy2100 - February 26, 2017

This is great – I totally agree! I wrote a blog post about this back in 2007 in response to the same IAU nonsensical definition that inspired you. https://shaunmoss.blog/2012/10/11/definition-of-a-planet/


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: