Classifying objects in our Solar System

What does the word category mean? A group of things or objects that have something in common with each other. What are some ways that you sorted your cards yesterday? Were your sorts scientific?

There are many diverse objects in the Solar System. What makes an object a planet? Discuss Look at your cards. How many in the set are stars? One! Remember the Solar System is centered around the Sun. This is the only STAR in our Solar System.

First thing, find the Sun and set it aside. It is it’s own category. Next: Planet Not a planet Don’t know/Not sure Put the cards for each category in that spot. Remember, you need a REASON for your sort. Talk at your table and make a decision. Some groups can share their sorts.

I knew it was a planet. It looks round/spherical. It looks about the right size. It looks like it has an atmosphere. It orbits the sun. Size Spherical shape. Did the cards tell us this??? Is it difficult to determine what is a planet and what is not? Um.. Yes...

Find #23 and #26. How are these similar and different? Similar shape, but different sizes. Did anyone think these were planets? These are NOT planets even though they orbit the Sun.

#23 is Ida, an asteroid. It orbits the Sun in the asteroid belt between Mars and Jupiter. Asteroids are small, rocky objects that orbit the Sun. The Asteroid Belt is a collection of many multi-sized chunks that orbit the Sun.

#26 is the nucleus of Comet Borrelly. Like asteroids, comets orbit the sun. Their orbit is elipitical. The nucleus of a comet is often referred to as a “dirty snowball” because it is a mixture of ice, gas and rocky dust. When comets get closer to the sun, the ice will vaporize, a coma or glowing cloud will form around the nucleus. When comets get closer to the sun, they will have tails that trail behind them. Most comets are not close enough to the sun to have tails. Most comets are thought to have originated in either the Kuiper Belt or the Ort Cloud.

A full comet with a coma and a tail? #32 is Comet Hale-Bopp. This is one of the brightest comets of the 20 th century. A comet that has broken into 2 pieces? Do you remember comet Shoemaker-Levy? The fragments were observed crashing into Jupiter in Find #9 This is the nucleus of Comet Halley, one of the most famous. Orbit takes 76 years. Find #21 and #29 These are both comet nuclei. Start a new sub category of “Not a Planet” for the comets.

#24 and #33 Both are asteroids that are not part of the asteroid belt. Both of their orbits carry them close to Earth’s orbit. There have been space probe missions to both of these asteroids. Start an asteroid pile under the not a planet category.

#31 Dactyl Orbits the asteroid Ida. (Crazy, right?) First moon ever discovered of an asteroid. Discuss at your table. Is it a moon or an asteroid? Remember, even though they orbit the Sun, asteroids are not planets. Place #31 where you think it should go. Sort the remaining comet and asteroid cards. Comets: 9, 21, 26, 29, 30, 32 Asteroids: 12, , 31, 33

Find #8 and #11 These are both similar in size and appearance but one is a planet and one is a moon. What would you need to know to determine which one was a planet and which one was a moon? We would need to know what they orbit. #8 is Mercury and #11 is Callisto, a moon of Jupiter. Place these in the correct piles. Moons also orbit the Sun as well. They orbit with their planet.

#10, Ganymede #12, Io #13 Europa #11, Callisto All of these are moons of Jupiter. Place them in the not a planet category along with all the other moons.

Find #4 and #14. Both have cloudy atmospheres. One is Venus and one is Titan the largest moon of Saturn. #4 is Venus. Titan is much smaller. Place these in the correct piles. So is an atmosphere an criteria for being a planet? Nope! Some moons have an atmosphere and some planets have hardly any like Mercury.

After we have discussed and deleted some stuff off the list, here is what is left: Large, spherical and orbits the Sun. What cards would you pick that fits this criteria? #8, Mercury #4, Venus #1 Earth #6, Mars #5, Jupiter #7, Saturn #16, Uranus #18, Neptune

Moons orbit planets. #3 The Moon, orbits Earth #10 Ganymede, orbits Jupiter #11 Callisto, orbits Jupiter #12 Io, orbits Jupiter #13 Europa, orbits Jupiter #14 Titan, orbits Saturn #15 Tethys, orbits Saturn #19 Triton, orbits Neptune #20 Ariel, orbits Uranus #22 Phobos, orbits Mars #25 orbits Saturn

Pluto has long been considered a planet. However, in 2006, the International Astronomical Union voted on a resolution to set new criteria for an object in the solar system to be considered a planet. Orbits the Sun It is large enough that gravity pulls it into the shape of a sphere. It has cleared the neighborhood around its orbit. Pluto did not meet the third and was demoted and is now called a dwarf planet.

Find #17, #27, #28 and #35. Discuss at your tables all the ways that these objects are similar to each other. Some astronomers think these objects should be classified as planets. Do you think any of them should be? Which objects? #17 is Ceres, an asteroid. Astronomers do not consider it a planet because it is too small. #27 is Pluto and #28 is Charon, a moon od Pluto. Charon is not a planet because it is smaller than Pluto and orbits Pluto. #28 is Eris. It is similar to Pluto but slightly larger than Pluto. Eris is the largest known object in a collection of objects beyond Neptune called the Kuiper Belt. Like the asteroid belt, the Kuiper Belt is composed of multi-sized objects. Pluto is considered to be a Kuiper Belt object or KBO, like Eris.

Discuss at your table. Is Pluto a planet or a KBO?

Astronomers categorize objects using certain characteristics. The objects we looked at today can be categorized in many different ways depending on what characteristics you choose to look at. Astronomers use a certain criteria to classify objects in our solar system. Shape Appearance What they orbit Size Distance from the Sun Remember, not all objects can be easily categorized!