1. Chapter 11: Part 1 Pluto and the Dwarf Planets
Even though Pluto is no longer a planet, it is still a very special object in our Solar System and deserves its own focus. In Part 1 of Chapter 11, we will discuss Pluto, the former ninth planet of the Solar System. We will discuss the controversy behind Pluto and why the International Astronomical Union decided to remove its planetary status. We will also discuss what type of object it is now classified as and the other objects that have been discovered that are in the same class as Pluto and show you the exciting new images released by the New Horizons Mission that arrived at Pluto in July 2015.

2. Pluto: Ruler of the Underworld
Pluto was appropriately named after the God of the Underworld or God of the Dead. The reason why Pluto is named this is because the Underworld is a dark place…since Pluto is so small and so faraway from the Sun and thus very dim, this is a very fitting name for it.
Because it was so faint, Pluto was not discovered until Like Neptune, Pluto was hypothesized to exist mathematically before it was detected visually by the telescope. The discovery of Pluto was the result of a long search for an additional planet that was believed to be slightly distorting the orbit of Uranus. Finally , in 1930, Clyde Tombaugh hired at age 23 to search for a new planet, found the dot of light that is Pluto.
The process of finding this hypothetical planet was very tedious and difficult; it was like looking for a needle in a haystack! Clyde Tombaugh had to compare a year’s data of photo graphs taken of an area of space where that planet was thought to be and detect which of the small “dots” changed positions from one image to another image taken on a different night.
It took him a year of tedious study of the photographic plates he obtained at Lowell Observatory in Arizona. From its slow motion with respect to the stars from night to night, he identified Pluto as a new planet. On February 18, 1930 Tombaugh noticed that one dot among many had moved between January 23rd, and January 29th, The two side by side images indicate the position of Pluto (indicated with the red circle) on those two nights.
Although it was very tedious work, it was worth it as this discovery made Tombaugh a celebrity over night. After all, the discovery of new planets in our Solar System is rare, the last time a planet was discovered (Neptune) was almost a century before this!
Pluto was discovered by Clyde Tombaugh in 1930 by comparing one image of
the sky taken one night with an image taken 6 nights later.

3. The discovery of Charon
In 1978, there was a surprising new discovery about Pluto…that it had a moon. This was discovered by James Christy by comparing two images of Pluto taken on different nights. The images on this slide show that Pluto seemed to be asymmetric in shape and that its shape changed with time. Well, it is not possible for a planet’s shape to change this quickly with time so the only explanation was that there was an object orbiting around it and that the image didn’t have enough resolution to reveal that there was a separate object. That was how the moon of Pluto was foundThe moon of Pluto was named Charon. This name goes along with the “death” them as Charon was named for the boatman that who rowed passengers across the River Styx to bring them to the Underworld.

4. Charon and Pluto
Pluto’s moon Charon is rather large; it is about ½ the size of Pluto. Together they are about the same size as the continental U.S.

5. HST discovers new moons!
2012
In 2012, Pluto gives us another surprise. Observations from the Hubble Space Telescope confirmed that Pluto has more than one moon…it has 4 additional moons! These additional moons are labeled in the image Hydra, Nix, P4, and P5! P4 and P5 are now given the names Styx and Kerberos. Including Charon, there is a total of 5 moons that orbit around Pluto!
The moons' orbits are in the same plane as the orbit of the much larger satellite Charon (discovered in 1978). This likely means the moons were not captured, but instead were born, along with Charon, in what is commonly theorized to have been a titanic collision between two Pluto-sized objects over 4 billion years ago.
Astronomers believe that the formation of the Pluto system is similar to that of our Earth and Moon. In both cases a comparable-sized body slammed into the parent planet. Simulations show that debris from the collision would go into an orbit around the planet and coalesce to form one or more satellites. Investigating how Pluto ended up with five moons while the Earth has only one should yield valuable insights into the processes by which satellite systems form around planets.
Although Pluto is no longer a planet it still is a very fascinating object as it continually surprises us all the time!

6. New Horizons visits Pluto July 2015
On January 19th, 2006 NASA luanched the New Horizons spacecraft to fly by the outer system, take photographs of the outer planets an their moons, but in particular take close up photographs of Pluto and its moons. More than nine years later, on Jul 14, 2015, New Horizons arrived at is closest approach to Pluto and released the most detail photos of Pluto and its moons. For the first time, we were able to see what these mysterious worlds looked like with clarity. Remember that before then the best image of these objects were taken by the Hubble Space Telescope that showed just blurry spheres because they were taken from so far away. This was an exciting day for the astronomical community. Perhaps you already heard about this on the news?

7. Pluto revealed!
Here is a beautiful image of Pluto taken by New Horizons. It shows craters on Pluto, but most interestingly a large heart shaped feature. This heart shaped region is named the Tombaugh region, after Pluto’s discoverer of course. The preliminary studies of this region show it to be consisting of mainly nitrogen ice flow, the flow is similar to the flow of glaciers on the Earth.
Some people actually see this region as the shape of the Walt Disney dog, “Pluto” that was named after the planet!
From preliminary analysis, Pluto seems to have a thin atmosphere, but because it has such low gravity, the atmosphere is slowly escaping from the planet. This atmosphere may cause erosion that helps to smooth out Pluto’s surface, which appears to have fewer craters than we thought it would have.

8. Pluto and Charon
Here is a close up picture of Pluto and its largest moon Charon. The data is still be interpreted, but Charon seems to have far fewer craters than predicted suggesting that its icy exterior is constantly resurfaced possibly because it continuously captures Pluto’s atmosphere, which consists mainly of nitrogen and methane.
Again, think about how exciting it is to be able to see these distant worlds for the very first time!

9. What is Pluto?
Strange object; located far out from the Sun with gas giants but small size and very elliptical and highly inclined orbit
Pluto is a mixture of ices and rocks
composition similar to satellites of giant planets
Could be captured Kuiper Belt Object (e.g. comet)?
When Pluto was discovered, it was announced as the ninth planet because astronomers really didn’t know what else to call it. Its properties were closer to a planet than any other then known object in the Solar System such as a moon, asteroid, or comet. It was spherical and orbited around the Sun like the other planets. But even when it was designated the ninth planet, some astronomers didn’t agree with this designation because, although it has similar properties to the other planets, Pluto also had its own unique and different properties.
For example:
It is located faraway from the Sun but unlike the other outer giant massive planets like Jupiter, Saturn, Neptune, and Uranus, Pluto was small and low mass (it has 1/500 the mass of Earth). In addition, its orbit is the most elliptical of all the planets. And whereas the orbits of the other planets lie pretty much in the same plane as the Earth’s orbit, Pluto’s orbit is very inclined (tilted) relative to that plane (see the following slide).
In addition, Pluto was thought to be made of rock and ices which are similar compositions to the moons of the giant planets. Because of these strange characteristics, some scientists thought that it should be classified as a different type of object even when it was discovered. But what?
The discovery of a set of Kuiper Belt objects (comets) with orbits not very different from that of Pluto gives a possible home for categorizing Pluto. Some scientists believed that Pluto was probably just the largest or one of the largest and among the closest of the Kuiper belt objects. However, even though Pluto had similar properties to the comets, it was also very different, as at the time, no comet came close to matching Pluto’s mass, size, or had a spherical shape.
Because there was no other object exactly like Pluto discovered at that time, astronomers were okay with keeping the designation of Pluto as a planet. However, if Pluto had been discovered today, it would have never been called a planet because of the new discoveries made possible by today’s technology.

10. Pluto’s strange orbit
As can be seen by this diagram, Pluto’s orbit is the most eccentric (elliptical) and is inclined by the greatest angle with respect to the Earth’s orbital plane, “ecliptic” near which the other planets revolve.
Pluto’s elliptical orbit is so eccentric that part lies inside the orbit of Neptune. Pluto was closest to the Sun in 1989 and moved farther away from the Sun than Neptune in It is barely visible through a medium sized telescope under dark-sky conditions.
This was one of the strange properties that Pluto possessed which made astronomers question whether it was like the other 8 planets.

11. Quaoar, Sedna, Earth, Moon, and Pluto size comparisons
Pluto’s planetary designation held for over 70 years, because during that time, the technology was not good enough to detect other objects like it. But starting in 2002, astronomers started to find other objects that closely resembled Pluto. They resided past the orbit of Neptune, were spherical in shape, and were similar in size and mass to Pluto. In addition, they were thought to be made of rock and ice like Pluto. In 2003, they discovered Quaoar. They thought that it possessed properties very much like Pluto but were not ready to call it a planet or take away Pluto’s planetary status so they classified Quaoar as the largest Kupier Belt Object (objects that exist past Neptune, which at the time before the discovery of Quaoar were the comets). The image shows that Quaoar is almost half the size of Pluto, but pretty much has the same properties as Pluto.
In 2003, they discovered yet another Pluto-like object which they named Sedna. They again classified Sedna as a Kuiper Belt Object, but the controversy behind Pluto’s planetary designation was heightening. Many astronomers were not comfortable calling these objects Kuiper Belt Objects and were also seriously rethinking Pluto’s planetary status since in their minds, Pluto, Sedna, and Quaoar have the same properties.

12. 10th Planet???
The controversy over Pluto really took off in 2003, when astronomers found yet another Pluto-like object. Originally it was called UB313, now named Eris. This object forced astronomers to bring the discussion of Pluto to the table because whereas Sedna and Quaoar’s size were smaller than Pluto and, if need be astronomers can argue that Pluto is a planet because of its size (Pluto is larger than both of these objects, thus there would not be a contradiction), Eris was thought to be larger in size than Pluto (which would be a contradiction to that argument). Now, astronomers had to decide once and for all if Pluto should be kept a planet, and if so, what should they call these other new objects and should they come up with a more precise definition of a planet?
If they develop a new definition of a planet what should it be based on?
If it is based on shape alone, let’s say all planets had to be spherical, then Pluto would remain a planet and we would have to categorize Quaoar, Sedna, and Eris as planets.
If they decided to use size in the definition, say they cut off the minimum size to be the size of Pluto, then Pluto would be a planet, and so would Eris, but anything smaller such as Sedna and Quaoar should not be planets.
No matter what definition they decided on, one thing was certain, astronomers needed to find a consistent category for Pluto and these objects because in science there cannot be any exceptions or contradictions.

13. 8 Planets ! IAU Definition A "planet" is a celestial body that:
(a) is in orbit around a star,
(b) has a nearly round shape
(c) and has cleared the neighborhood around its orbit.
From 2003 on, astronomers began to detect more and more of these Pluto-like objects so they had to develop a formal definition of a planet. In August 2006, they organized a meeting of the International Astronomical Union (IAU) to be held in Prague, Czechoslovakia astronomers met to vote on the new definition of a planet. The definition that passed is the on one this slide.
As of 2006, a planet must fulfill three characteristics:
It has to:
Orbits around a star
2) Has a nearly round shape (the reason why they included the term “nearly round” is because planets are not technically round since they are wider at the equator than at the poles)
3) Has cleared the neighborhood around its orbit
You can see that astronomers did not include size in the definition because the size cut off would be arbitrary…that is how small should an object be to be considered a planet? Because of its arbitrary nature, using size in the definition would not be very scientific.
They did include the shape…(that planets must be nearly round) to rule out the asteroids and the comets from being called planets.
If astronomers decided just to keep the first two as the definition of a planet like it was in the past, then Pluto, Sedna, Eris, and Quaoar as well as the other round objects they recently found would all be planets. If all round objects orbiting a star are considered planets then we would currently have more than 40 planets in our solar system. This number would increase with time because they are continuing to discover more and more of these objects everyday. Astronomers hypothesize that there could be as many as 200 of these Pluto-like objects out there, its just that we haven’t been able to detect them yet since they are so faraway and small.
Most astronomers at the meeting agreed that these objects should not be designated as planets and added a third component to the definition that would rule out these objects.
The third component of the definition states that a planet has to clear the neighborhood around its orbit. This is the component that eliminates Pluto, Sedna, Quaoar, and Eris, as well as the other round objects as planets. I explain this component more in the next slide and why Pluto and other Pluto-like objects do not meet this criteria.
8 Planets !

14. Planets sweep up all the material along their orbit
The third component of the definition has to do with how planets formed and how they established their orbits.
Remember this slide? This was the slide that I used to discuss the current accepted theory that says that planets formed from the collection of the dust and gas.
As a reminder:
In the disk of gas and dust, the dusty material began to clump and smaller clumps joined together to make larger ones and eventually planetesimals, bodies that range from millimeter size up to a few hundred kilometers across were formed. Planetesimals are just big chunks of floating rock or ice. Gravity pulled many planetesimals together and when it is massive enough, gravity pulls equally on all sides of the object and shapes it into a sphere, at this point, when it assumes a spherical shape, we call it a protoplanet. The protoplanets, as they orbit around the protosun begin to sweep up and collect all the leftover rocky debris in their path, growing more massive in the process. After all of the material is cleaned up, the protoplanets then cooled to make the planets we have today.
But some of the leftover debris from the formation of planets that did not finish the process (that is they didn’t completely sweep up (collect) the material in their orbit because they got knocked out of the plane of the disk) are still orbiting the Sun in the form of asteroids, comets, meteroids, and dwarf planets. The non-spherical debris, the leftover planetesimals, are now called asteroids and comets. Whereas the ones that collected enough mass to have a round shape but didn’t complete the process, the protoplanets, are now called the dwarf planets.
It makes sense that since there were leftover planetesimals in the process of planet formation that there should be leftover protoplanets. These dwarf planets have collected more material than the asteroids and comets but not enough material to be a planet.

15. Pluto’s strange orbit
How you can determine that the dwarf planets did not clear the neighborhood around their orbits is that their orbits do not lie in the same plane as the orbit of the planets and what is more, their orbits all cross the orbit of another object.
This slide again shows you the orbit of Pluto. Notice again that Pluto’s orbit is highly inclined from the ecliptic plane and also crosses Neptune’s orbit.

16. Dwarf Planet Orbits are Strange
This slide shows you the orbits of the planets in orange as compared to the orbits of the dwarf planets. You can see that the orbits of Pluto, Sedna, Eris all cross the orbit of other objects and are much more elliptical. This shows that these objects got knocked out of their paths as they were trying to collect the material and grow in size; thus, they did NOT clear the neighborhood around their orbits like the other 8 planets in our solar system.

17. Dwarf Planet Definition
A “dwarf planet is” a celestial body that:
(a) is in orbit around the Sun,
(b) has a nearly round shape
(c) has not cleared the neighborhood
around its orbit and is not a satellite
Pluto, Eris, Sedna, as well as the other Pluto-like objects are now given a new category. They are all now known as dwarf planets which is defined as any object that orbits the Sun, has a nearly round shape, and is not a star, planet or a moon. There are 5 known objects that are considered dwarf planets, but astronomers hypothesis that there could be hundreds of them in our Solar System, but have not yet been discovered.
Interestingly, one object that used to be designated as an asteroid (Ceres) because it was located in the asteroid belt is now considered a dwarf planet under this new definition because it has a spherical shape.
Not everyone agrees with this new definition, though. In fact, the people at New Mexico State University staged a protest against Pluto being demoted from a planet to a dwarf planet. I’ve posted a little video on the website for you to see the protest. The reason why there was the protest at New Mexico State was because the people there were a little biased…Clyde Tombaugh used to be a professor at New Mexico State so he was their claim to fame.
Also, my little nephew didn’t like the new definition…when he heard that Pluto was no longer a planet, he cried 
In any case, if you are curious what I think about this…I agree with the new definition. In fact even before the vote, I didn’t believe that Pluto was like the other planets and thought that it must be a slightly different kind of object. Thus, when this new definition came out I was on board with it and happy because it makes more sense to me.
Although a lot of the general public don’t fully understand why Pluto is no longer a planet and some are even upset by it, and there has to be a lot of changes to library books and text books and solar system mobiles…I believe that in order to remain true to the scientific process, some ideas, even though they have been accepted for years, have to be changed if they have been proven inaccurate with the latest discoveries and new information…that is how progress is made…and that is how science is done.
5+ Dwarf Planets !