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Chapter 26
ASTRONOMY 8CONNECTION
What Happened to
Pluto?
On August 24, 2006, the International Astronomical Union (IAU)
announced that Pluto had been dropped as the ninth planet in
the solar system and downgraded to “dwarf planet” status. This
decision made headlines across the globe. Many ordinary citizens
were left wondering why. Who were these IAU people? Had
something about the planet changed?
Pluto hasn’t been hit by an asteroid or pulverized by some other
extraterrestrial disaster. In fact, Pluto hasn’t changed much since it
was discovered in 1930. What has changed since then is our “big
picture” view of the different kinds of objects that exist in our solar
system.
Every three years, distinguished astronomers from all over the
world come together to share discoveries and exchange ideas at a
conference held by the International Astronomical Union (IAU). This
organization promotes and safeguards the science of astronomy, and
is considered an authority in the astronomical field.
The 2006 meeting, held in
Prague, Czech Republic,
addressed an important
issue. Thanks to advances in
telescope technology, we now
know there are hundreds, or
maybe even thousands, of
objects similar to Pluto that
were formed early in our solar
system history and that orbit
IAU General Assembly, Prague, 2006.
the Sun in the region beyond
Neptune that is known as the
Kuiper Belt. If Pluto is a planet,
what about all these other objects? Are they planets, too? How do
we define what is and isn’t a planet?
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Chapter 26 The Solar System
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Planet Redefined
By a majority vote, the IAU astronomers created a new definition for
a planet in our solar system. A planet is a celestial body that (1) is in
orbit around the Sun; (2) is large enough (massive enough) to have
its self-gravity pull itself into a round (near-spherical) shape; and (3)
has cleared its orbit of other objects. What this last part means is
that a planet is large enough that, as it revolves around the Sun, the
other objects in its orbit have either become part of the planet by
fusing with it or have collided with the planet and moved out of the
planet’s orbit.
What’s a Dwarf Planet?
The IAU astronomers defined a dwarf planet in our solar system
as a celestial body that (1) is in orbit around the Sun; (2) is large
enough (massive enough) to have its self-gravity pull itself into a
round (near-spherical) shape; (3) has NOT cleared its orbit of other
objects; and (4) is NOT a satellite (a body in orbit around a planet or
other body larger than itself). A dwarf planet cannot clear objects
out of its orbit because it is too small in mass to significantly alter
its environment. Because Pluto overlaps the orbit of Neptune, it was
disqualified as a planet and reclassified as a dwarf planet.
The Dwarf Planets
As of this writing, five solar system objects
have been classified as dwarf planets.
One of these, Ceres, is found in the asteroid
belt between Mars and Jupiter. It was
discovered in 1801. Ceres was originally called
Ceres
a planet and is listed as such in astronomy
textbooks up until about 1850. As more objects
were discovered in the asteroid belt, Ceres became classified as the
largest of many similar objects (asteroids) that orbit the Sun in the
region between Mars and Jupiter. Due to Ceres’ large size
(950 kilometers in diameter) and nearly spherical shape, it is now
known as a dwarf planet. For the moment, Ceres also retains the
asteroid label. Future IAU meetings may address whether the term
dwarf planet is a single defining category, or if dwarf planets can
keep their older designations as well.
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Chapter 26
The other dwarf planets are found in
the region beyond Neptune, so they
are sometimes called trans-Neptune
objects or TNOs. In addition to Pluto,
we have:
1. Eris, discovered in 2005. Eris has a
diameter of 3,000 kilometers—
so it is larger than Pluto. Eris is the
most distant object that we know
to be in orbit around the Sun—it is
about 10 billion miles away from
the Sun. That’s three times as
far away as Pluto!
2. Makemake (pronounced MAH-keh
Mah-keh), also discovered in 2005. This dwarf
planet has a diameter about one-third as large
as Pluto. It is named after a creator god in the
mythology of the people of the Pacific island
of Rapa Nui, also known as Easter Island.
3. Haumea (pronounced how-MAY-ah), whose discovery was
announced in 2005, doesn’t seem to fit one of the characteristics
of a dwarf planet: It isn’t even close to spherical. The IAU
described its shape as “similar to a plump cigar” in the September
17, 2008 press release which announced Haumea’s designation
as the fifth dwarf planet. It earned the designation because the
irregularity in shape is not due to insufficient gravity, but is quite
possibly due to its fast rotational speed. Haumea is named after
the Hawaiian goddess of childbirth and fertility.
Illustration not to scale. Planet images courtesy of NASA
The IAU Executive committee met in Oslo, Norway in 2008 and
approved a new name for trans-Neptune dwarf planets similar to
Pluto: The plutoids. The IAU
predicts that as more is learned
about the region of our solar
system beyond Neptune, new
plutoids will be discovered—
perhaps dozens, or possibly
even a hundred or more!
Source: International Astronomical Union Press
Release, “Pluto and the Developing Landscape
of Our Solar System,” 2008.
Plutoids: Pluto and its moons (left)
and Eris and its moon (right)
Questions:
1. What is the difference between a planet and a
dwarf planet?
2. Why is Pluto no longer considered a planet?
3.What is the purpose of the International
Astronomical Union?
Illustration of planet candidates and IAU Assembly photo courtesy of The International Astronomical
Union/Martin Kornmesser. Plutoids photo courtesy of IAU, NASA/ESA Hubble Space Telescope, H. Weaver
(JHU/APL), A. Stern (SwRI), the HST Pluto Companion Search Team, and M. Brown.
Artist’s illustration of Makemake (above right). Makemake is the dwarf planet identified as 2005 FY9 ,
and Haumea is the dwarf planet 2003 EL61 (see illustration of dwarf planet candidates below).
Unit 9 Matter and Motion in the Universe
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