SCIENCE in ACTIONACTION:::: Pluto, Dwarf Planets, and the Iau

SCIENCE IN ACTIONACTION:::: Pluto, dwarf planets, and the iau

Science is always changing. Scientific theories explain what is known about nature—the world around you. However, sometimes new discoveries are made and these theories and explanations must be changed if they don’t explain the new facts.

The Discovery of Pluto At the beginning of the twentieth century, only eight planets were known. Many astronomers thought there had to be another planet orbiting the Sun beyond the orbit of Neptune. This planet, nicknamed Planet X, would be similar in size to Neptune. In 1930, a new object was discovered orbiting the Sun. This object was in the region of the sky where calculations predicted Planet X would be. The new object was named Pluto and at first was thought to be Planet X. Soon, however, data showed that Pluto was much too small to be Planet X. Astronomers realized that Pluto was not the planet they were looking for. It was a coincidence that Pluto happened to be in the same region of space where Planet X was predicted to be.

Is it a new planet? Although Pluto was not Planet X, astronomers decided to label Pluto the ninth planet in the solar system. One reason was that Pluto was estimated to be about the same size as the other planets—when it was first discovered, Pluto was thought to be about the same size as Earth. However, as time went on, some data showed that Pluto might be smaller than Mars but bigger than Mercury. Other astronomers estimated that Pluto had more mass than the planet Venus.

Charon helps astronomers decide. In 1979, a moon was discovered orbiting Pluto. This moon was named Charon (KER un), and its motion Nix enabled astronomers to make more accurate estimates of Pluto’s size and mass. According to the Hydra new data, Pluto had a diameter of about 2,300 km. This meant that Pluto was less than half the size Charon of Mercury, which has a diameter of 4,900 km. Pluto’s mass was now estimated to be only about 0.2 percent of Earth’s mass. In fact, Pluto is Pluto smaller and has less mass than Earth’s moon. In 2005, two smaller moons, named Nix (NIKS) and Hydra (HI druh), were discovered orbiting Pluto, as shown in Figure 1.1.1. Figure 1: This photo of Pluto and its three satellites was taken by the Hubble Space Telescope . [Credit: NASA]

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The Kuiper belt Since 1992, many new objects have been discovered orbiting the Sun beyond the orbit of Neptune. These objects are called trans-Neptunian objects (TNOs) and are part of a region of the solar system called the Kuiper (KI puhr) belt. The Kuiper belt, shown in Figure 2, is a group of small objects made of rock and ice that orbit the Sun in a zone that extends from about the orbit of Neptune to about 50 AU from the Sun. Since 1992, over 800 objects have been discovered in the Kuiper belt.

Figure 2 The Kuiper belt extends from about 30 AU to 50 AU from the Sun. Pluto orbits the Sun within the Kuiper belt.

Figure 2 also shows that Pluto’s orbit is different than the orbit of the other eight planets. Pluto’s orbit is much more elongated and tilted compared to the orbits of the other planets. Instead, Pluto’s orbit is much like the orbits of TNOs. Also, like many TNOs, Pluto’s motion around the Sun is strongly affected by the gravitational pull of Neptune. This led many astronomers to wonder if Pluto should be considered a planet or a trans-Neptunian object.

How should Eris be classified? In 2003, a new TNO named Eris (EE riss) was discovered. Measurements showed that Eris is slightly larger than Pluto and has a small moon astronomers called Dysnomia (diss NOH mee uh), shown in Figure 3. Some astronomers Dysnomia thought that because Eris is larger than Pluto, it should be considered the tenth planet. Other astronomers thought Eris there might be more objects in the Kuiper belt larger than Pluto. This meant that if Eris was considered a planet, then there could be many other objects in the solar system that would also be considered planets.

Figure 3: This photo, taken by the Keck Observatory at Mauna Kea, Hawaii, shows Eris and its satellite, Dysnomia. [Credit: WM Keck Observatory]

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The sizes of Eris, Pluto, and some of the other largest known trans-Neptunian objects are shown in Figure 444.4... All these objects, except Sedna, are part of the Kuiper belt. Sedna orbits the Sun in a very elongated orbit. It is so far from the Sun that Sedna takes more than 12,000 years to complete one orbit.

Figure 4 The sizes of Eris, Pluto, and some other large TNOs compared to Earth. [Credit: NASA, ESA]

The IAU Debates How to classify Pluto and Eris caused a great deal of discussion among astronomers. The International Astronomical Union (IAU), established in 1919, is the organization that defines the classification of objects in our solar system. The IAU also approves the names to be used for newly discovered objects. In August 2006, the IAU held a meeting in Prague, Czech Republic.

During the IAU meeting astronomers debated how to define a planet and how to classify the newly discovered objects in the Kuiper belt. As a result of these discussions, a new definition of a planet was approved.

Planets Defined The IAU now defined a planet is any object in space that satisfies three conditions. The first condition is that it is in orbit around the Sun. Second, it must have enough mass for the object to have formed nearly into a sphere. Third, the object must have cleared away other objects from the neighborhood around its orbit. This last condition means that a planet must be much larger than any other object that crosses its orbit.

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Dwarf Planets Defined The IAU also approved the definition of a new type of object called a dwarf planet. A dwarf planet is any object in space that satisfies the following four conditions. First, it is in orbit around the Sun, and second, it has enough mass for the object to have formed nearly into a sphere. The third condition is that a dwarf planet is not a satellite of another celestial body. Finally, a dwarf planet has not cleared away other objects from the neighborhood around its orbit. This last condition means that a dwarf planet is not much larger than some of the other objects that cross its orbit.

Based on these definitions, the planets of the solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is no longer considered one of the planets. Instead, it is now classified as a dwarf planet. There are two other known dwarf planets in the solar system—Eris and the asteroid Ceres (SIHR eez).

Ceres Ceres was discovered in 1801, in orbit between Mars and Jupiter. Ceres was the first asteroid discovered and, with a diameter of about 940 km, it is the largest asteroid. When it was first discovered, Ceres was considered a planet. However, many more objects similar to Ceres were discovered in the asteroid belt between Mars and Jupiter. As a result, by the middle of the nineteenth century, astronomers had reclassified Ceres as an asteroid rather than a planet. The location of the asteroid belt is shown in Figure 5.

Figure 5 The asteroid belt is a group of objects that orbit the Sun between the orbits of Mars and Jupiter. These objects are made of rock and ice and exist in various sizes.

Not everyone is excited about the change. Hundreds of astronomers and planetary scientists are unhappy with the IAU’s new planet definition. After the IAU resolution defined a planet, over 300 scientists signed a petition opposing the new definition. Only about 2,500 of the 10,000 members of the IAU attended the meeting, and of that number, only 424 voted on the planet reclassification resolution. Some also have criticized the IAU’s voting procedures, which do not allow members to vote by e-mail. However, any change to the IAU’s definition of a planet will have to wait until the next IAU General Assembly in 2009.

Exploration of Pluto To learn more about Pluto and other objects in the Kuiper belt, NASA has launched a new spacecraft to Pluto and beyond. The New Horizons spacecraft, shown in Figure 666,6,,, was launched in January 2006 and is scheduled to arrive at Pluto in July 2015. New Horizons will be the first spacecraft to visit Pluto. It will take pictures and make maps of Pluto and Charon and will measure the surface properties, temperatures, and the composition of Pluto’s atmosphere. The data gathered by this mission will help astronomers better understand how the solar system was formed. Figure 6: This is an artist’s drawing of the New Horizons spacecraft as it approaches Pluto. Pluto’s moon, Charon, and the Sun are shown in the background. [Credit: Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute]

Science in Action In the 1800s, the discovery of many new objects in the asteroid belt caused astronomers to change the classification of Ceres. Almost 150 years later, the discovery of many new objects beyond the orbit of Neptune caused astronomers to change the classification of Pluto. As new discoveries continue to be made, the definition of a planet and the classification of objects in the solar system might change again. This is just another example of how science keeps changing.

Scientific Debate Have students research the IAU, its reclassification decision, Pluto, and Eris. Encourage students to decide for themselves whether they agree on the reclassification of planets entirely, partly, or not at all.

Set up a scientific debate within your class. 1.1.1. Separate students into groups (one group of students in favor of the reclassification: one group opposed to any reclassification; and one group in favor of the reclassification, but unwilling to change Pluto). 2.2.2. Select a moderator and three student representatives from each of the groups. 3.3.3. After all students in each group have concluded their research, conduct a class scientific debate on the topic. 4.4.4. As was done by the IAU in August 2006, conclude your class debate with a vote.

Solar system table Have students construct a table similar to the one shown to organize solar system objects.

Planets Dwarf Planets TransTrans- ---NeptunianNeptunian Objects Mercury Pluto Pluto Venus Eris Eris Earth Ceres Sedna Mars Charon Jupiter Dysnomia Saturn Quaoar Uranus 2003 EL61 Neptune 2005 FY9 Nix Hydra

Keep ananan Eye ononon the IAU Have students periodically check to see if new dwarf planets have been found and/or officially named. A classroom model of the solar system could be generated and changes could be made as new objects are discovered.