The planets all go around the sun, of course, but so do plenty of things that aren't planets (asteroids and comets, for example).
The planets are all more or less round: they have enough mass to draw themselves into a rough sphere. But there are large asteroids and dwarf planets that do that.
A true planet is large (massive) enough that it's gravity has cleared all the space around it.
That means we know of 8 planets and 11 dwarf planets. Oops, they found another dwarf planet yesterday! So that makes 12 dwarves, so far!
So, more or less round? Aren't the planets ball shaped? Actually, most of them are a little squashed from spinning. The faster the spin, the more they bulge at the equator. It's really noticeable with Saturn!
Here's an easy way to see why this happens: Cut out two long thin strips of construction paper and punch holes on both ends.
Thread the holes onto a pencil so that you form a ball with the pencil just inserted a little way into the ball (we stapled the part where the strips cross to give some stability).
When you twirl the pencil, the ball flattens from the centrifugal force, just like the planets!
But even the true planets aren't very similar. Some are small and rocky, and some are huge and made of gas. Ever wonder why? And why are the close ones rocky, and the gas ones far away?
We think it has to do with the way the solar system was formed. The sun formed from a cloud of dust and gas: as bits bumped into each other and stuck, the gravitational pull increased, sucking more and more of the cloud into a central location and starting things spinning.
You can see this if you take a black trash bag and lay it flat. Mix together talcum powder to represent the lighter gases and a bit of salt to be the heavier rocky elements. Drop the mixture onto a corner of the trash bag while you (or a bunch of kids!) blow. You'll find powder clear across the bag, but most of the salt will stay close to where you dropped it.
And, of course, you can see it in the solar system with the near rocky planets, the distant gas planets, and the far icy and rocky Kuiper belt dwarf planets.
First up, we looked at Mercury: so small, so rocky, so hot (800 facing the sun), so cold (-300 facing space), and so fast (zipping around in it's 88 day year)! To look at why the close planets have such short years, I took a long stick with a knob at one end, and a knitting needle with a ball at the end. I stood them side by side, ball side up, and then let them fall to the ground. The larger stick moved faster, but the shorter needle took less time: it was covering less distance.
Then we looked at Venus: 900 degrees - hotter than Mercury - how can that be? Earth and Venus started out not only the same size, but also containing the same proportions of elements. The two were nearly identical! But Venus was too close to the sun. That extra heat developed a thick carbon dioxide atmosphere that held in heat and created clouds of sulfuric acid. There's a reason we send rovers to Mars, not Venus!
I had set a pair of identical ceramic tiles in a 300 degree oven half an hour before the class. At the beginning of the class, I took them out and covered one with a wool blanket to simulate Venus's insulating atmosphere. Now, 20 minutes later, I removed the blanket and took the temperature of each tile with a scanning thermometer. The covered tile was 140, the uncovered 100.
Next up, we have Earth, the Goldilocks planet! Not too hot, not too cold, juuuust right for liquid water.
I took the kids downstairs, turned out the lights, and showed them the seasons on our globe using a flashlight. When our part of the Earth is tilted towards the sun, we have summer. When it's tilted away, we have winter.
I have found that most kids think that the Earth is closer to the sun in summer. Here in the Northern hemisphere, in the summer the Earth is actually at its furthest point away from the sun (and in our winter, the Earth is closest to the sun). The opposite is true for the Southern hemisphere, which is why their seasons, all else equal, are slightly more extreme. The 23 degree tilt towards or away from the sun is really what makes the difference.
The last of the inner planets, Mars is only half the size of Earth. Too cold, too small to hold a good atmosphere, most of the surface water has evaporated. It's a dead planet in the largest sense: it's core has cooled and solidified.
It no longer has volcanoes, moving tectonic plates, or, critically, a magnetic field. Earth has a magnetic field because our core is liquid and moving as the planet spins. That field shields us from all sorts of solar and cosmic radiation which would otherwise wipe out life on the surface of the planet.
My "Mars" rock isn't from Mars, it's common iron stone that we found on a fossil expedition, but it's similar to the iron rich rocks which give "The Red Planet" it's color (and weirdly enough eroded to look alien!
One thing they always agreed on: remember the asteroid belt in Star Wars?
That is absolutely NOT what an asteroid belt looks like! If there were that many rocks, you'd have a planet there. In real life, standing on an asteroid in the densest part of the belt, you would not be able to see any other asteroids.
The total mass of all the asteroids is estimated to be about 4% of the mass of our moon.
Stay tuned, tomorrow I'll post the rest of the solar system!