Phoenix Astronomical Society

Go Back   Phoenix Astronomical Society > Public Forums > Public Discussion

» Upcoming Events
» Newsletter Deadline
» May 2020
2627282930 1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31 123456
Thread Tools Display Modes
Old 10-23-2009, 11:20 AM
Terri Finch's Avatar
Terri Finch Terri Finch is offline
Event Manager
Join Date: May 2008
Location: Phoenix AZ
Posts: 2,745
Default PLANETS & Dwarf Planets

A question came in to PAS in which a wonderful teacher in the grade school levels is seeking information:
Her question is:

Alison writes:
The reason I'm emailing is my students have been asking me a lot lately what the difference between a dwarf planet and a regular planet is. I tried looking this answer up online, but to be honest it was rather confusing. Is there an easy way to explain this to 5th graders and me ?


Here are the answers she got from our Telescope Team:

Tim writes:
Hi Alison,

I don't know if you've found the simple IAU final definition for the three body types, but let's start there:

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

Basically, it boils dow to whether the body has cleared its orbit of other bodies. In the case of dwarf planets, that has not occurred meaning that the dwarf planet would potentially collide with another body in its orbit (versus colliding with something from out of its orbit such as an asteroid).

Does that help narrow things down?


Bob writes:

The difference between a planet and dwarf planet can be confusing. Even experts within the astronomy community differ ? a significant number still consider Pluto to be a planet. (I do, but my thinking doesn?t count)

For 5th graders, it may be helpful to discuss the physical characteristics, and then the key "effect" of the characteristics.

Physical characteristics:

1. Planets and dwarf planets both orbit the sun.
2. Each body (planet & dwarf planet) has enough mass (size) and gravity (pull) to form itself into a ball-shaped sphere.
3. The dwarf planets ?tend? to be smaller than the known planets.
4. The orbits of the dwarf planets are beyond the orbit of Neptune (the outermost planet now).
4. Neither planets nor dwarf planets fit the description of asteroids or comets. Asteroids are rocky objects with roundish or irregular (most usually) shapes and are relatively small: +/- 100 km in diameter. Comets are simply lumps of ice and dust from outer regions ? very, very much farther from the
Sun than where Pluto is located. While some comets do orbit the Sun, they lose mass when their orbit brings them closest to the Sun. Planets and dwarf planets do not lose mass because of their orbits.

The Effect Characteristic

1. While dwarf planets have enough mass and gravity to form themselves into ball-shaped spheres, they do not have enough gravity to ?digest? matter in the area of their orbits. Astronomers call this ?clearing their orbit.? Planets can, and do, clear their orbits ? dwarf planets cannot. This is the primary differentiation between planets and dwarf planets.

The dwarf planets are named: Pluto (ugh!), Ceres, Eris, Makemake, and Haumea. There may be newly identified ones that I am not aware of.

Perhaps you will receive input from my fellow club members that hopefully is consistent, perhaps easier to relate than mine, and does not create added confusion.

Hope this is helpful.


Rod writes:

Hello Alison:
My Name is Rod Sutter, I am the current President of Phoenix Astronomical Society. Let me try to explain this so that hopefully,
5th graders could understand it. Difference between Regular Planets and Dwarf Planets is that a regular planet must have lots of gravity so
that it can take on a round shape. Dwarf planets don't have enough gravity to do this. Also a regular planet has to be large enough so that
when it runs into objects in it's orbit, it can either take a good hit and absorb the object or fling it out of it way. A Dwarf planet cannot clear the
objects out of its way so it is constantly running into these things and bouncing things off of it's self and other objects.

Hope this helps. If not let me know and i'll try a different approach.

Thank You for contacting P.A.S.


Leah writes:

Alison, here is a good article that describes it:

the article is written by Dr. Tony Phillips, who is the author of the very nice website

after your students read the article, they might want to participate in the petition!


Alison writes back to Tim:

> Thank you! I found the first definition you emailed me but I was confused at this part: has cleared the neighbourhood around its orbit.
> So in a nut shell a dwarf planet could potentially collide with another planet? That is the part I was stuck on. I am not quite sure how to explain this part to kids.

Tim writes:

Or other spatial debris - it could be asteroids, another planet (but that makes them both Dwarf Planets, doesn't it :-) ), or bits from their own formation that haven't been either consumed or ejected from the orbit.

This is what puts Pluto into that camp - there is still debris and other planets (Neptune possibly - see that are still within its orbit.

I explained it to an elderly neighbor like this:

If you are driving on the loop 101 freeway (pretend it makes a complete loop for this example) in the middle lane and there are no other cars in the middle lane on the entire freeway, you could consider yourself a planet orbiting Phoenix (the Sun). However, if there are any cars in the same middle lane anywhere around the loop, you are a dwarf planet since you are not in your own clear lane and could possibly meet up with one of those other vehicles as you continue in your orbit.

So why isn't Neptune in this same camp? Because Neptune would consume Pluto if they actually collided.


Mike writes:

Hello Alison,

I know why this is a tricky question. First some background, as our telescopes get better we are finding many icy bodies beyond Pluto and at least one is bigger than Pluto. There could be millions of these icebergs beyond Pluto, do all get to be named planets? Politics is also involved. At a recent European astronomy conference they defined a dwarf planet as an icy body and a planet as a massive spherical object that cleared it's orbit of debris. This is not a clear definition. Pluto and Charon are both spherical and both have enough mass that some considered them a double planet. Pluto and Charon both are clearing their solar orbit by impact and orbital capture such as the moons Nix and Hydra. If icy bodies could be considered planets what about asteroids? How would you define an asteroid, comet, moon, dwarf planet, planet, gas giant, or brown dwarf star?

In our solar system is Jupiter and some other tiny debris such as planets. Jupiter outweighs all the other planetary material combined and sets stable orbital distances. The asteroid belt's average orbit is halfway to Jupiter's orbit. Mars circles one quarter of the way to Jupiter while Earth's orbit is only 1/6th. Venus is 1/8th of the way but Mercury is 1/16th. Saturn's orbit is twice as far as Jupiter and Uranus is four times the distance. Neptune is six times as far but Pluto's orbit averages eight. An icy world larger than Pluto has been found at sixteen times Jupiter's distance and another large frozen body was found at thirty two times as far as Jupiter's orbit. A string of any length can be folded and marked to get the ratios of these planetary distances.

Hope this helps,


And that's what we have so far.

If anyone wishes to add to the definition, please do, below.
Any future requests to know about this subject can be
referred to this Forum Thread for the answer.

Thank you all for your shared ideas and knowledge.
It is always better to have a TEAM of heads working
on a question or issue, than only 1 head.
The saying is right: 2 heads are better than 1.
In this case, we had several heads helping and
I love all their answers. Thanks again TEAM!!!
Terri Finch, PAS Events Manager since 2008 We are All Star Stuff !!!
Private Music Instructor: Music Lessons in your home
Reply With Quote

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -7. The time now is 04:36 AM.
Powered by vBulletin
Copyright 2000-2010 Jelsoft Enterprises Limited.