Pluto & The Dwarfs

There’s nothing in the definition of a planet, or a dwarf planet about having a moon or having dozens of moons. In fact, anything goes when it comes to moons.

Currently, there are 169 known moons orbiting the eight planets in our solar system and 4 orbiting the three dwarf planets. Dozens more moons orbit other small solar system bodies. The moon-count continuously increases as telescopes and spacecraft discover new ones that have been difficult to see.

The giant gas planets have the most moons. Jupiter currently has 63 and Saturn is close behind at 60. Uranus is next with 27 moons, followed by Neptune with 13. The inner planets have fewer moons: Mars has two tiny moons while our planet Earth has one large moon. Mercury and Venus have no moons at all.

Among the dwarf planets, Ceres has no moons (although other objects in the asteroid belt do), Eris has only one known moon and Pluto has three moons; two tiny ones and one almost as large as Pluto!

What does it take to be a moon? A moon is a natural satellite that orbits another body larger than itself. Moons come in all different sizes. The largest moon is one of Jupiter’s moons Ganymede with a diameter of 5,276 kilometers (3,280 miles)! That’s larger than the planet Mercury and the largest dwarf planet Eris. The Cassini spacecraft discovered tiny moons about 3.2 kilometers (2 miles) in diameter in orbit around Saturn. There may very well be moons less than a mile in diameter hidden in the rings of the other giant planets.

How did moons come to be? The larger moons most likely formed along with the parent planet, out of coagulating matter – as they got bigger, they attracted smaller fragments close by them, sweeping their orbit path clean. The smaller moons are probably captured asteroids or comets which wandered too close and got caught by the planet’s gravitational field.

In the case of Pluto’s large moon, scientists think that a massive body slammed into Pluto early in its history, sending massive amounts of debris into orbit around the planet. Over time, the rocky debris accreted and formed Charon. It’s likely that Earth’s moon formed this way as well.

Something to ponder….are there moons of moons? None are known but by definition, a moon could have a smaller moon orbiting around it. The question is the long-term stability of the orbits. In most cases, the gravity of other nearby objects (most probably the primary planet) would perturb the orbit of the moon’s moon until it broke away or impacted the primary. Ouch!

If I mentioned either dwarf planet Eris or the Kuiper Belt in any of my science classes last year, I was likely to draw several blank stares. All my students knew Pluto had been renamed a dwarf planet, and that other newly discovered objects also were classified as dwarf planets. But only a handful of my students knew the most distant object to join or solar system family was Eris. One good way for them to get to know Eris was to engineer a visit!

We read and discussed this dwarf planet: where it is, its freezing temperatures (-405 degrees Fahrenheit!) the frozen methane on its surface, and its orbit. And how about that amazing New Horizons spacecraft? It’s traveling out toward Eris on a journey that will take up to 15 years to complete.

I decided to have my students form teams and take on the role of engineers redesigning the New Horizons
spacecraft—their version had to hold four astronauts. I wanted the teams to explore the engineering design process used by engineers, including those at NASA:

• Identify the problem
• Research the problem
• Develop possible solutions
• Select a solution
• Construct a model
• Evaluate the model
• Share the model
• Redesign as needed

Teams brainstormed and researched current information about what astronauts were experiencing by living on the International Space Station. They came up with lists of necessary items to be included in the spacecraft. From there they began to create prototypes of possible spacecraft designs, incorporating the items needed for the long journey. Individual members came up with specific ideas sharing them with other team members.

Teams decided on the final design for their prototype. Then each team created a labeled, scale model drawing of the spacecraft. They created several views of the craft as well as cut-a-way drawings showing the interior.

We held an Engineer’s Symposium where all design teams shared their drawings. Questions were encouraged. After the symposium several teams went back and redesigned their spacecrafts based on input from other teams.

The last part of the project was having all design teams create 3-D models of their drawings. We used 1 cm = 1 m as the design ratio. Teams worked for about a week using nothing but paper, glue, pencils, and scissors.

After the models were built, we held another Engineer’s Symposium. With great fanfare, each design team showed their creation and explained the equipment they included in the models. There were some great presentations. Most teams came up with very creative ideas and explanations for their designs.

At the conclusion of the symposium, we hung the models in the classroom. For the rest of the year they were a source of interest and curiosity to anyone coming into the room.

For all my science students, 4th graders through 8th graders, learning about our solar system is one of the most exciting topics they study.

Last year, many of my students were surprised to find two new additions to our solar system. One of these new additions, dwarf planet Ceres, was of particular interest to many students. They thought the Asteroid Belt was filled with small rocks and boulders. Now, there’s a dwarf planet in there. Plus, it’s weird looking.

When Ceres was discovered in 1801, it was classified as a planet. However, it wasn’t long before more and more objects were detected in the same general area between Mars and Jupiter. Ceres was the biggest object noted, but it was still only 590 miles in diameter. With all these boulders being discovered, scientists decided they all couldn’t be classified as planets. It was decided that the larger objects would be called asteroids and the area they were found in would be named the Asteroid Belt. Ceres was renamed an asteroid and stayed that way up until 2006 when it was officially designated a dwarf planet. Ceres has the distinction of being the only object in our solar system to have been named a planet, an asteroid, and a dwarf planet.

Ceres has not had an easy existence. There are millions of boulders in the Asteroid Belt, all tumbling this way and that. Ceres has been bombarded. As a result, its surface is heavily cratered.

Discussing the craters with my older elementary and middle school science students leads easily into an exploration of teams creating their own Ceres craters. They’ll identify the factors affecting the appearance of impact craters, and the material ejected from the impact site, as well as the size and velocity of the impacter.

Teams use cardboard boxes about 40 cm square and about 7-8 cm high. Each cardboard box is filled about three-quarters full of all-purpose flour. The flour is leveled, and then the top is dusted lightly with dry tempera paint. I use a color that contrasts with the flour for the most striking results. Spreading newspapers under the boxes makes clean up much easier.

As impacters, teams can use marbles and ball bearings of about the same size, as well as golf balls. Teams drop these from a series of heights onto a prepared "Ceres surface." Using impacters of different mass dropped from the same height allows students to study the relationship of the mass of the impacter to crater size. Dropping impacters from different heights allows students to study the relationship of velocity of the impacter to crater size.

The teams agree beforehand on the method they will use to "smooth" and resurface the flour between impacts. The flour should not be packed down. Shaking or tilting the boxes back and forth produces a smooth surface. They reapply a fresh dusting of dry tempera paint after each experiment. Be sure to remind everyone that better experimental control is achieved with consistent handling of the materials. For instance, cratering results may vary if the material is packed down for some trials and not for others.

Allow some practice time for dropping marbles and resurfacing the materials in the boxes before students actually begin recording data.

Be sure to have all teams record each experiment. You may want them to graph height and size of impact craters as well as type of impacters used. Be sure to have them draw a detailed picture of one of their craters. After all experimenting is done, ask teams to share their results.

New questions may come out of these debriefing discussions, which in turn will lead to new experiments. Allow students the opportunity to continue to explore cratering, if possible.

By the time students finish this activity, they know what Ceres has gone through as a dwarf planet residing in the Asteroid Belt. About this time you may want to ask them if they think craters on the Moon were formed the same way. Show them photos of Moon craters and compare the pictures to what their cratering looks like.

All of the (now eight!) planets in our solar system have been visited by space probes.  Pluto was still a planet when NASA decided it was about time to pay it a visit and send a spacecraft on a long journey to the uncharted, outer edge of our solar system.

On January 19, 2006, NASA launched the New Horizons mission to Pluto and the Kuiper Belt (the region beyond Neptune swarming with thousands of small, icy objects).  Seven months later, Pluto was reclassified as a dwarf planet.

The New Horizons spacecraft will travel through our solar system for nine years, covering almost 5 billion kilometers (3 billion miles) before it finally reaches Pluto on July 14, 2015.  If all goes well, New Horizons will then venture out further into the Kuiper Belt and take a look at a few more icy dwarfs from 2018 to 2022. That’s a long way away.

New Horizons carries instruments on board which will photograph the surface of these icy bodies, determine what they are made of, how cold they are and what their atmosphere is like.

So what do we hope to learn from New Horizons?

Pluto has always been a little different. For one thing, except Uranus, the other planets have poles that point roughly up and out of their orbit planes. Both Pluto and Uranus rotate on their sides….did something tip them over?

Also, we know that Pluto has three moons. Two of Pluto’s moons, Nix and Hydra are tiny but one of Pluto’s moons, Charon, is more than half the size of Pluto! Charon is the largest moon compared to its planet of any moon in the solar system. Not only is Charon big, relative to Pluto, its very close – 20 times closer to Pluto than our moon is to us. Why is Charon so close to Pluto and how did it get there?

Pluto is a cold ball of rock and ice covered with a thin, frosty atmosphere which slowly escapes into space. Is Pluto similar to a comet?

And, what’s it really like in the cold, distant Kuiper Belt? Just how many dwarf planets are there?

Scientists hope that by studying these distant bodies we will gain an understanding of how the solar system formed 4.5 billion years ago. Sit back, relax and stayed tuned. Dwarf planet history is about to be made….starting in about eight years!

So why isn’t Pluto a planet anymore?

In August 2006 the International Astronomical Union had a meeting in Prague, Czechoslovakia to come up with the first real definition of a planet. They decided that to be called a planet, an object must pass three tests: it must orbit the Sun, have enough mass and self-gravity to pull itself into a round shape and be big enough to sweep its orbit free of other objects in its path.

Pluto failed the third test and hence was downgraded to a “dwarf planet”. But Pluto wasn’t alone in its new classification and was joined by Ceres, the largest body in the asteroid belt lying between Mars and Jupiter and Eris, a distant icy body located beyond Pluto in the Kuiper Belt. All three of these objects were deemed dwarf planets because they passed the first two tests but failed the third.

Dwarf planets are not planets because they are not big enough to have cleared the neighborhood around their orbit!

How small then is a dwarf planet? We always knew Pluto was the tiniest of all the planets. In fact, a fun way to visualize Pluto’s relative size to the other (now eight!) planets is what I like to call a solar system supper. In order of distance from the sun, Mercury is a green pea (about 1/3 inch), Venus is an unshelled walnut (less than 1 inch), Earth is a pearl onion (slightly less than 1 inch), Mars is a cherry tomato (almost one-half inch), Jupiter is a large head of lettuce (roughly 10 inches), Saturn is a head of cabbage (roughly 6 inches), Uranus is a grapefruit (slightly bigger than 3.5 inches) and Neptune is a large orange (slightly less than 3.5 inches).

Now let’s add the dwarf planets, Pluto, Ceres and Eris into this food fest. In our solar system supper model, Pluto, with a diameter of 2,300 kilometers (1,430 miles), is a peppercorn (less than 1/5 inch wide). Ceres however is less than half the size of Pluto with a diameter of 960 kilometers (590 miles). Can you think of what to add to our solar system supper that is smaller than half a peppercorn? Maybe a poppy seed or large grain of salt? The most distant dwarf planet, Eris, is actually larger that Pluto with a diameter of 2,400 kilometers (1,500 miles). Maybe we just find a slightly larger peppercorn than we did for Pluto to represent Eris.

So far we’ve identified three dwarf planets and probably more to come. We know that they’ll all orbit around the Sun and be round, but I wonder how small they will be? I’m guessing they’ll be somewhere between a grain of salt and a peppercorn in our solar system supper model!

Dwarf planets in our solar system? Pretty exciting!

While I was excited by the changes, I wondered how my students would react. Teaching science to students from 4th to 8th grade gave me a chance to see a range of reactions.

Throughout the classes, there was much discussion about Pluto being “demoted.” There’s only one thing demotion means in school, and it’s not good.

The reclassifying of Pluto was controversial for many of my younger students. Out of all the planets in our solar system, most of my 4th and 5th graders had a special attraction to Pluto.

Many of them thought it was really named after Mickey Mouse’s dog, and even if is wasn’t, Pluto was a name they could identify with. They didn’t know that in 1930, an 11-year old girl actually chose the name, inspired by the initials of Percival Lowell, the man who predicted the discovery of our ninth planet.

For my middle-school science students, the decision that planets had to meet specific criteria (orbit around the Sun, be large enough that it takes on a nearly round shape, and be able to clear its orbit of other objects) made sense.

Most of them understood that without criteria for calling a planet a planet, our solar system was going to change dramatically, perhaps 50 new planets being named.

I think quite a few of my students were excited that the solar system was alive with change—a reminder that new discoveries were being made, during their lifetime.

The question I was asked most often by all the science classes was what Pluto should be called now. Dwarf planet Pluto? Dwarf Pluto? Nothing clicked.

Several proposed that since Pluto had been called a planet for so long, it should just be allowed to keep the title—sort of “grandfathered” in.

What was the reaction in your classroom?