SCIENCE INININ ACTIONACTION:: How Pluto became a Dwarf Planet

hat happens when new things are discovered? What if these discoveries Wconflict with long-held ideas and theories? Copernicus changed the way people thought the universe looked when he hypothesized that Earth was not at its center. Newton’s law of universal gravitation changed how people thought about the motion of planets and stars. When new observations and information become available, theories and explanations often have to be modified to explain the new data.

A planet named Ceres? In the late 1770s, the astronomer Johann Bode predicted there should be a planet between the orbits of Mars and Jupiter. His prediction was based on a numerical sequence that was derived from the distances of the six known planets from the Sun. The formula he derived was known as Bode’s law and predicted that there should be an undiscovered planet at about 2.8 astronomical units (AU) from the Sun. Bode’s law also predicted there should be a planet at about 19.6 AU from the Sun. This planet, Uranus, was discovered in 1781 by William Herschel.

The discovery of Uranus, which orbited the Sun at the distance predicted by Bode’s law, caused astronomers to search for the planet predicted to be between Mars and Jupiter. This object was discovered in 1801 and was about 2.8 AU from the Sun, just as Bode’s law predicted. It was named Ceres and for a while was known as the eighth planet. However, measurements showed that Ceres was much smaller than the other planets, and within a short time, many other smaller objects were discovered nearby. Ceres is shown in Figure 1.

Figure 1 Ceres is shown in this photo taken by the Hubble Space Telescope . [Credit: NASA, ESA]

Science in Action © Glencoe/McGraw-Hill Page 1

Although Ceres was the biggest of a large number of rocky objects that orbited the Sun in the asteroid belt, many more of these objects continued to be discovered. Astronomers soon decided that Ceres was not a planet. Finally, Ceres and the rest of these objects were designated as asteroids. By the end of the 1800s, hundreds of these objects had been discovered. Today, over 260,000 asteroids are known.

Pluto is discovered. Pluto was discovered by accident by Clyde Tombaugh in 1930. Earlier calculations showed small differences between the predicted and observed motions of Uranus and Neptune. Some astronomers thought these differences were due to the gravitational effects of a large planet beyond Neptune called Planet X. Tombaugh searched for Planet X in a region of the sky predicted by certain calculations and discovered Pluto on February 18, 1930. However, data showed that Pluto was much too small to be Planet X. Decades later, it was discovered that the disagreement between the predicted and observed motions of Uranus and Neptune were due to an inaccurate estimate of Neptune’s mass. The evidence for Planet X had disappeared.

In a number of ways, Pluto is not like the other planets in the solar system. It is smaller and less dense than the inner planets (Mercury, Venus, Earth, and Mars). It is much smaller than the outer planets (Jupiter, Saturn, Uranus, and Neptune), but it is more dense. Its estimated density of 2 g/cm 3 indicates a composition of about 70 percent rock and 30 percent water ice, similar to the composition of comets. Also, Pluto’s orbit is much more elliptical and is more tilted than the orbits of the other planets. These facts led many astronomers to think that Pluto was a different type of object than the other planets.

What is Pluto? In the past few decades, a number of large objects have been discovered in a region of space known as the Kuiper belt. The Kuiper belt is a disk-shaped region of space that extends from the orbit of Neptune to Dysnomia about 50 AU from the Sun. Pluto orbits the Sun at an average distance of about 40 AU, so it is part of the Eris Kuiper belt.

In 2005, an object was discovered in the Kuiper belt that was larger than Pluto. Previously known as UB313 and nicknamed Xena, it is now called Eris. Eris and its satellite, Dysnomia, are shown in Figure 2. Figure 2: This photo, taken by the Keck Observatory at Mauna Kea, Hawaii, shows Eris and its satellite, Dysnomia. [Credit: WM Keck Observatory]

Science in Action © Glencoe/McGraw-Hill Page 2

Because Eris was larger than the planet Pluto, many astronomers thought that Eris should also be considered a planet. However, other astronomers thought that Eris was one of a large number of similar-sized, Kuiper-belt objects. This meant that if Eris were designated a planet, the solar system could eventually include a large number of planets as more of these objects were discovered.

The dilemma caused a great deal of discussion among astronomers. In August 2006, the International Astronomical Union (IAU) decided to reclassify Pluto. Under a new classification of solar system objects, Pluto is now a dwarf planet.

What is a planet? According to the new IAU definition, for an object to be considered a planet it must satisfy three conditions. The first two conditions are that it orbits the Sun and is large enough to be spherical in shape. The third condition is that the object has cleared the neighborhood around its orbit. This third condition means that the object is much larger than any other object that crosses its orbit. The planets are now Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

What is a dwarf planet? Pluto is no longer considered one of the planets in our solar system. Instead it belongs to a new classification called dwarf planets. A dwarf planet also must orbit the Sun and be large enough to be round. However, a dwarf planet cannot be a satellite and it has not cleared the neighborhood around its orbit. This last condition means that a dwarf planet is not much larger than some other objects that cross its orbit. The dwarf planets are Eris, Pluto, and Ceres.

Could Pluto be a double planet? Pluto has three satellites, shown in Figure 3. The two smaller satellites, Nix and Hydra, were discovered in 2005. The largest satellite, Charon, was discovered in 1979 and has a diameter that is about one-half of Pluto’s diameter. No other satellite in the solar system is so large compared to the object it orbits. The closest is our Moon with a diameter of about one-fourth of Earth’s. The center of mass of the Pluto-Charon pair is a point in the space Nix between the two bodies. This is different from Earth and Hydra the Moon, where the center of mass is a point inside Earth. Because the center of mass of the Pluto-Charon Charon pair is outside each body, Pluto and Charon might be considered a double planet. Pluto

Figure 3: This photo of Pluto and its three satellites was taken by the Hubble Space Telescope . [Credit: NASA]

Science in Action © Glencoe/McGraw-Hill Page 3

What is the IAU? The International Astronomical Union was established in 1919. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. As part of its mission, the IAU is internationally recognized as the organization that designates the classification of objects in our solar system. The IAU also approves the names for newly discovered objects.

Why is the IAU decision getting so much attention? Not everyone is excited about the new classifications. One scientist who has spoken out against the IAU changes is Dr. Alan Stern. He is the leader of the team responsible for NASA’s New Horizons spacecraft and its mission to explore Pluto and the Kuiper belt. New Horizons , shown in Figure 4, was launched in January 2006 and is scheduled to arrive at Pluto in July 2015. Dr. Stern is opposed to the IAU’s definition of a planet and to the classification of Pluto as a dwarf planet. Hundreds of other astronomers and planetary scientists, unhappy with the IAU’s resolutions, have signed a petition opposing the new definitions. Only about 2,500 of the 10,000 members of the IAU attended the General Assembly and of that number, only 424 voted on the planet reclassification resolution. However, any change to the IAU’s definition of a planet will have to wait until the next IAU General Assembly in 2009.

Figure 4: This illustration is an artist’s rendition of what the spacecraft New Horizons might look like as it studies Pluto and other objects in the Kuiper belt. [Credit: Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute]

What does it all mean? As new discoveries are made, the definition of a planet and the classification of objects in the solar system might change again. Scientists will continue to develop their understanding of the universe as they refine older hypotheses and form new hypotheses to explain their observations. This is just another example of how new discoveries can cause scientific theories to change.

Science in Action © Glencoe/McGraw-Hill Page 4 SCIENCE IN ACTION: Student Activities

Scientific Debate Have students research the IAU, its reclassification decision, and public response. 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. 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. Select a moderator and three student representatives from each of the groups. 3. After all students in each group have concluded their research, conduct a class scientific debate on the topic. 4. As was done by the IAU in August 2006, conclude your class debate with a vote.

Make a Model Research the diameters of Earth’s Moon, the four Galilean satellites of Jupiter, Titan (a satellite of Saturn), Pluto, Ceres, and Eris. Draw models of each object so that all can fit on one sheet of paper. Then determine a scale to represent each object’s distance from the Sun. Using this scale, place each planet, dwarf planet, large satellite, the asteroid belt, and the Kuiper belt in its position from the Sun. Use a roll of calculator paper or paper towels to generate a model of our solar system. Place the Sun at one end of the roll and place each item listed above in its proper place.

What’s in a name? Research the names Eris and Dysnomia . Have students write a journal entry explaining why the IAU chose them for the farthest known dwarf planet and its satellite.

Keep An Eye On 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.

Science in Action © Glencoe/McGraw-Hill Page 5