dwarf planets

pluto

The reclassification of Pluto to the new category dwarf planet stirred up a great deal of controversy. How the classification of Pluto has evolved is an interesting story in science. From the time it was discovered in 1930 until the early 2000s, Pluto was considered the ninth planet. When astronomers first located Pluto, the telescopes were not as good, so Pluto and its moon, Charon, were seen as one much larger object (Figure 1.1). With better telescopes, astronomers realized that Pluto was much smaller than they had thought. Pluto and its moon, Charon, are actually two objects. Better technology also allowed astronomers to discover many smaller objects like Pluto that orbit the Sun. One of them, Eris, discovered in 2005, is even larger than Pluto. Even when it was considered a planet, Pluto was an oddball. Unlike the other outer planets in the solar system, which are all gas giants, it is small, icy, and rocky. With a diameter of about 2,400 km, it is only about one-fifth the mass of Earths Moon. Plutos orbit is tilted relative to the other planets and is shaped like a long, narrow ellipse. Plutos orbit sometimes even passes inside Neptunes orbit. From what youve read above, do you think Pluto should be called a planet? Why are people hesitant to take away Plutos planetary status? Is Pluto a dwarf planet? Pluto has three moons of its own. The largest, Charon, is big enough that the Pluto-Charon system is sometimes considered to be a double dwarf planet (Figure 1.1). Two smaller moons, Nix and Hydra, were discovered in 2005. But having moons is not enough to make an object a planet. Pluto and the other dwarf planets, besides Ceres, are found orbiting out beyond Neptune. Click image to the left or use the URL below. URL:

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ceres

Ceres is by far the closest dwarf planet to the Sun; it resides between Mars and Jupiter. Ceres is the largest object in the asteroid belt (Figure 1.2). Before 2006, Ceres was considered the largest of the asteroids, with only about 1.3% of the mass of the Earths Moon. But unlike the asteroids, Ceres has enough mass that its gravity causes it to be shaped like a sphere. Like Pluto, Ceres is rocky. Is Ceres a planet? How does it match the criteria above? Ceres orbits the Sun, is round, and is not a moon. As part of the asteroid belt, its orbit is full of other smaller bodies, so Ceres fails the fourth criterion for being a planet.

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makemake

Makemake is the third largest and second brightest dwarf planet we have discovered so far (Figure 1.3). With a diameter estimated to be between 1,300 and 1,900 km, it is about three-quarters the size of Pluto. Makemake orbits the Sun in 310 years at a distance between 38.5 to 53 AU. It is thought to be made of methane, ethane, and nitrogen ices. Largest Known Trans-Neptunian Objects. Makemake is named after the deity that created humanity in the mythology of the people of Easter Island.

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eris

Eris is the largest known dwarf planet in the solar system it has about 27% more mass than Pluto (Figure 1.3). The object was not discovered until 2003 because it is about three times farther from the Sun than Pluto, and almost 100 times farther from the Sun than Earth is. For a short time Eris was considered the tenth planet in the solar system, but its discovery helped to prompt astronomers to better define planets and dwarf planets in 2006. Eris also has a small moon, Dysnomia, that orbits it once about every 16 days. Astronomers know there may be other dwarf planets in the outer reaches of the solar system. Haumea was made a dwarf planet in 2008, so the total number of dwarf planets is now five. Quaoar, Varuna, and Orcus may be added to the list of dwarf planets in the future. We still have a lot to discover and explore. Click image to the left or use the URL below. URL:

what is a planet

In 2006, the International Astronomical Union decided that there were too many questions surrounding what could be called a planet, and so refined the definition of a planet. According to the new definition, a planet must: Orbit a star. Be big enough that its own gravity causes it to be shaped as a sphere. Be small enough that it isnt a star itself. Have cleared the area of its orbit of smaller objects.

dwarf planets

The dwarf planets of our solar system are exciting proof of how much we are learning about our solar system. With the discovery of many new objects in our solar system, astronomers refined the definition of a dwarf planet in 2006. According to the IAU, a dwarf planet must: Orbit a star. Have enough mass to be nearly spherical. Not have cleared the area around its orbit of smaller objects. Not be a moon.

instructional diagrams

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questions

what qualities define a planet?

a) orbits a star

b) is big enough to have its own gravity causing it to be shaped as a sphere

c) has cleared the area of its orbit of smaller objects

-->  d) all of the above

what is not characteristic of a dwarf planet?

a) orbits a star

b) is big enough to have its own gravity causing it to be shaped as a sphere

-->  c) has cleared the area of its orbit of smaller objects

d) all of the above

pluto was thought to be one of nine planets in our solar system from 1930 to 2006.

-->  a) true

b) false

why was pluto thought to be a planet when it was first observed through a telescope in 1930?

-->  a) pluto and its moon, charon, appeared as one much larger object.

b) pluto was larger, but it has been struck by so many meteorites that its gotten smaller.

c) pluto was thought to be spherical but it was found not to be.

d) pluto was found to be a star.

why is pluto no longer considered to be a planet?

a) it is very unlike the other outer planets.

b) it is smaller than one dwarf planet and earths moon.

c) it is part of the kuiper belt with 200 million other objects.

-->  d) all of the above.

although it is in the asteroid belt, ceres is not an asteroid because it is spherical.

-->  a) true

b) false

ceres has been considered

a) the 10th planet.

b) the largest object in the asteroid belt.

c) a dwarf planet.

-->  d) all of the above

dwarf planets are all made of

a) gases and ices.

b) rocks and metal.

c) methane, ethane and nitrogen ices.

-->  d) none of the above.

which is the largest known dwarf planet in the solar system?

a) ceres

b) pluto

-->  c) eris

d) makemake

the final totals are 8 planets and 5 dwarf planets in the solar system.

a) true

-->  b) false

diagram questions

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