SCIENCE IN ACTION:
ACTION:
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
enabled astronomers to make more accurate
estimates of Pluto’s size and mass. According to the
new data, Pluto had a diameter of about 2,300 km.
This meant that Pluto was less than half the size
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
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.
Nix
Hydra
Charon
Pluto
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
thought that because Eris is larger than Pluto, it should be
considered the tenth planet. Other astronomers thought
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.
Dysnomia
Eris
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 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.
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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 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.
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SCIENCE IN ACTION: activities
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. 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.
Solar system table
Have students construct a table similar to the one shown to organize solar system objects.
Planets
Dwarf Planets
TransTrans-Neptunian 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 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.
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