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The Gravitation and the Waltz of the Planets
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Changing Pluto’s
Status as a Planet
SUPPLEMENT
WHAT DO YOU THINK?
luto has been removed from the pantheon of
planets and relegated to the status of a “dwarf
planet,” leaving eight surviving planets. While
this change requires everyone to re-learn the number of
planets, it places Pluto into a new category that will eventually help astronomers better understand all the types of
bodies that orbit the Sun.
Changing categories in astronomy is not new. Imagine
that the year is 1780. Six planets are known: Mercury,
Venus, Earth, Mars, Jupiter, and Saturn. Now move ahead
a year: Uranus has just been discovered, requiring people to
learn that there are seven planets, not six. Fast forward
nineteen years. It is New Year’s Day, 1801 and the
Swiss/Italian amateur astronomer Giuseppe Piazzi discovers the eighth planet, which he named Ceres (Figure 1). It
is located between Mars and Jupiter. In the following year,
German astronomer H. Wilhelm Olbers discovers the ninth
planet, Pallas, in the same region of the solar system. The
discovery of Pallas is followed within five years by the discovery of planets ten and eleven, Juno by German
astronomer Karl Harding and Vesta (Figure 2) by Olbers.
These bodies also orbit between Mars and Jupiter.
P
1
How elliptical is Pluto’s orbit
compared to the orbits of
the other traditional planets?
Compared to the orbits of
asteroids?
22 How big is Pluto compared to the
Earth? Compared to our Moon?
3
Should Pluto should be considered a “planet?”
FIGURE 2 Asteroid Vesta, between Mars and Jupiter. Credit:
Ben Zellner (Georgia Southern Univ.), Peter Thomas (Cornell Univ.) and
NASA.
The fifth such body, Astraea, was discovered between
Mars and Jupiter in 1845 by amateur German astronomer
Karl Hencke. It was significantly smaller than any previously-discovered planet, as were all but one1 of the next 18
planets discovered by 1852. Furthermore, all these smaller
FIGURE 1 Ceres, the largest asteroid in orbit between Mars
and Jupiter. Credit: NASA, ESA, J. Parker (Southwest Research
Institute), P. Thomas (Cornell University), L. McFadden (University of
Maryland, College Park), and M. Mutchler and Z. Levay (STScI).
1 The
one exception was giant Neptune, discovered in 1847 out beyond
Uranus.
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bodies were located in the same general region of the solar
system, between Mars and Jupiter. Seeing a physically
meaningful pattern emerging, some journals began listing
the bodies smaller than Vesta (namely, Astraea and all the
subsequently-discovered small bodies) as asteroids, or
equivalently, minor planets or planetoids. The first two
names are still in common use today.
Prior to 1855, the sizes of Ceres, Pallas, Juno, and
Vesta had been grossly over-estimated. In that year, another measurement scheme was used that actually underestimated their sizes. Based on these smaller sizes, these four
bodies were added to the list of asteroids, leaving eight
planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune. The categorizing served to support
the idea that the smaller objects, the asteroids, are fundamentally different than the eight planets 2.
INSIGHT INTO SCIENCE
Putting similar objects in the same category or
classification is often the first step to understanding
how they formed and how they evolve. The process
of classification and subsequent explanation occurs in
every realm of science. It has yielded insights into: the
evolution of life, properties of different chemical
elements, the formation and evolution of stars, and
geological processes on Earth, among many other
things. As the other properties of objects like Pluto
become better known, theories that explain their formation and distribution will be developed and tested
by observations and computer simulations.
1 2
Modern observational techniques have finally established the sizes of the four largest asteroids as being
between the original and their 1855 sizes. Nevertheless,
they are all very small compared to even the smallest of the
eight planets, Mercury. Indeed, the largest of the asteroids,
Ceres, is only a quarter the diameter of our Moon.
Zoom ahead to 1930. In that year Pluto is discovered
by American astronomer Clyde Tombaugh. It was then at
the edge of the known solar system with an orbit that sometimes took it closer to the Sun than Neptune. Indeed,
Pluto’s orbit is much more elliptical than that of the eight
planets, but similar to the ellipticities of many asteroid
orbits. Unlike the asteroids, however, it is not located
between Mars and Jupiter. Its size was decidedly unusual,
too. With a diameter 18 percent that of Earth, just less than
half that of Mercury, just two thirds that of our Moon, and
2 We
honestly don’t yet know all the differences between planets and asteroids or between the various asteroids.
FIGURE 3 Pluto and its moon Charon. Credit: Dr. R. Albrecht,
ESA/ESO Space Telescope European Coordinating Facility; NASA.
just four times that of Ceres, Pluto didn’t fit well as a planet or as an asteroid. It was grouped with the eight planets,
despite debate about whether it was more like them or like
the ever-growing number of known asteroids.
While asteroids continued to be discovered throughout
the twentieth century, the status of Pluto as a planet
unthreatened until 1978, when Pluto was discovered to
have a moon, Charon (Figure 3), almost as large as it was
(2300 km for Pluto versus 1190 km diameter for Charon).
For comparison, the Earth and our Moon, which are the
next most-similar pair of planet and moon, have diameters
of 12,800 km and 3500 km, respectively. Pluto and Charon
are a pair of nearly-equal mass bodies orbiting each other,
which is unique in the solar system. The status of Pluto as
a planet came into further question as similar-sized bodies
such as Sedna and Eris were discovered in the same remote
area of the solar system.
Over a hundred thousand asteroids and bodies orbiting
beyond Neptune have now been identified. Their sizes and
locations suggest that they are all quite different from eight
of the nine traditional planets, the exception being Pluto.
Pluto’s size is more similar to the larger asteroids than to
any planet and its orbit is more similar to those of other
objects orbiting beyond Neptune than to the orbit of any
other planet. Pluto has finally been reclassified to better
reflect these connections. This was done in the summer of
2006 at a meeting of the International Astronomical Union,
the group charged with classifying and naming objects in
space. That meeting was filled with tension, as numerous
competing definitions of planets and other solar system
objects were proposed and rejected. In the end, the following definitions were adopted:
A planet is a celestial body that (a) is in orbit around
the Sun, (b) has sufficient mass for its own gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium (i.e., the object is nearly spherical), and (c) it
has cleared the neighborhood around its orbit (of smaller
bodies).
Changing Pluto’s Status as a Planet
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While (a) is self-explanatory, (b) and (c) deserve some
explanation. If an object has its shape because of the electromagnetic bonds between its atoms, then it can have
essentially any shape. Rocks look like rocks, potatoes look
like potatoes, and you and I look as we do because of such
bonding. However, if an object has enough mass, then the
gravitational attraction between its particles are strong
enough to reshape the body, pulling down high places and
pushing up low ones, until the object becomes nearly spherical. The resulting balance between the force of gravity
pulling inward and the pressure created by the matter
pressing on itself pushing outward is called hydrostatic
equilibrium.There are about three dozen objects in the solar
system that meet this criterion, including Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, our
Moon, the asteroid Ceres, several moons of each of the
large planets, Pluto’s moon Charon, and several of the
Pluto-like objects orbiting beyond Neptune that have been
discovered, including Sedna and Eris. Conversely, there are
over a hundred thousand objects in the solar system that
look more like potatoes than anything else. These include
most of the moons and asteroids, plus all the comets.
The third condition on being a planet, (c) above, means
that the object must also have enough gravitational attraction to pull onto itself, or to fling far away, the myriad
smaller pieces of debris that orbited in its neighborhood.
The inner eight planets satisfy all three criteria, but observations reveal numerous other objects still in Pluto’s vicinity.
That body does not have enough gravitational attraction to
clear its surrounding area and so by the new criteria for
planets, Pluto is not one.
Nevertheless, Pluto and numerous other objects are
still in the solar system, so they need to be classified.
Because we know very little about the chemical compositions of these bodies (including Pluto), we don’t yet know
if they are a single group of objects or several distinct classes of objects. For example, some of them may be composed
mostly of rock and metal, while other may be mostly rock
and ice; some may have sheaths of ice, while others have
rocky surfaces. Until such details are known, all the other
spherical objects orbiting the Sun that have not cleared
their neighborhood in space of debris and that are not satellites3 of other bodies are classified as dwarf planets. Pluto is
a dwarf planet, but its moon Charon is not since Charon
orbits the slightly larger body, Pluto. The asteroid Ceres is
now a dwarf planet (Figure 4).
Finally, the panoply of non-satellite, non-spherical
objects in the solar system are called small solar-system bodies. Neither this name nor dwarf planets is very satisfying.
FIGURE 4
the smallest planet, with Pluto, now classified a “dwarf planet.”
Scales of the solar system. All these bodies are
presented to scale. The two insets compare the size of Mercury,
3 i.e.,
moons.
Credit: International Astronomical Union/NASA.
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INSIGHT INTO SCIENCE
3
Reasonable People Can Disagree. While most
astronomers believe that Pluto is better grouped with
similar-sized objects like Sedna and Eris, some
astronomers believe that Pluto, Sedna, and Eris should
be considered as planets, rather than as the largest
of the dwarf planets. As discussed in the chapter,
names assigned to groups of objects often help provide
insight into how they form and develop. Future
observations of all the objects in the solar system will
tell whether Pluto and similar bodies are more like
the present eight planets or more like the larger of
the bodies traditionally called asteroids, with
which they have now been grouped.
The four inner planets, Mercury, Venus, Earth, and Mars,
are much smaller than the four outer planets, so it is easy
to confuse the inner “terrestrial” planets with “dwarf”
planets. Just as the name planetoid was dropped in favor of
asteroid, the two names, dwarf planets and small solar-system bodies, may eventually be changed to better reflect the
objects they represent.
As has occurred several times since the eighteenth century, people now have to go through the uncomfortable
process of learning new names for old astronomical objects.
Clearly, changing Pluto’s designation to “dwarf planet
(134340) Pluto” does not change it physical properties.
Indeed, astronomers hope that the new designations are
on the way to better representing Pluto’s properties, along
with the properties of the other small objects orbiting the
Sun. While the renaming will be confusing for a while, it
will eventually help clarify our perception of the solar
system.
WHAT DID YOU THINK?
1
How elliptical is Pluto’s orbit compared to those of
the other traditional planets? Pluto’s orbit is much
more elliptical than those of the eight other traditional planets. Compared to the orbits of asteroids?
Pluto’s orbit is similar in ellipticity to that of many
asteroids (as well as more distant objects, like
Sedna and Eris).
2
How big is Pluto compared to the Earth? Pluto has
18% the diameter of the Earth. Compared to our
Moon? Pluto is only two thirds the diameter of
our Moon
3
Should Pluto should be considered a “planet?”
Because it is more similar to smaller bodies, many
astronomers find it more instructive to classify Pluto
as a “dwarf planet,” along with such bodies as
Ceres, Sedna, and Eris.