There is nothing so far removed from us to be beyond our reach, or so far hidden
that we cannot discover it...
To this end, we will explore and pose questions.
What is a Planet?
The ancient Greeks looking up at the night sky realized that some of the points
of light moved against the background of stars. They called these lights "PLANETES",
meaning wanderer. For thousands of years this scientifically inexact word was
sufficient to distinguish other objects in the sky from the stars.
As the power of telescopes increased our ability to see into the depths of
space, our understanding of the solar system evolved. By the middle of the 19th
century, we listed 15 planets in the solar system: Mercury, Venus, Earth, Mars,
Ceres, Pallas, Juno, Vesta, Astraea, Hebe, Iris, Jupiter, Saturn, Uranus and
Neptune. Within 50 years we had concluded that an additional term was needed to
properly describe what we had discovered, the asteroid belt.
At the beginning of the 20th century, Ceres, Pallas, Juno, Vesta, Astraea, Hebe
and Iris were referred to as asteroids and we believed that our solar system
included the eight planets clearly visible through our telescopes and at least
one more, yet to be found.
Pluto was discovered in 1930 and was hailed as the ninth planet, even thought at
the time some astronomers did not think the term planet accurately described the
By 2005 many believed we had identified the 10th planet in our solar system.
Nicknamed Xena, latter officially designated Eris, this distant icy object
slightly larger than Pluto rekindled the debate over "what exactly is a planet?"
As we continue to explore our solar system and our technology continues to
improve, we are seeing more and more objects on the distant fringe of our solar
system that need to be defined and classified. Much like other sciences, such as
biology, were one may discover a new species, classification is a common and
needed process which groups like objects for purposes of comparison and further
study. And as in all sciences, classification can and should change based on new
The International Astronomical Union, a renowned organization dedicated to
promoting and safeguarding the science of astronomy, recently took up the debate
over "what is a planet?"
While many accept the definition issued by the IAU, scientists worldwide
continue to debate the issues. And there are many national and international
science organizations who have not weighed in. In this first attempt at
scientifically defining a planet, the IAU said that a planet must have these
It must be a body that is in orbit around the sun;
it must have sufficient mass so that its own gravity pulls it into a nearly
and the object must clear away other objects in its neighborhood.
The resolution further defined a new category called Dwarf Planet, which has
these four following traits:
It too is in orbit around the sun;
it also must have sufficient mass so that its own gravity pulls it into a nearly
however, it has not cleared its neighborhood; and it cannot be a moon.
The second IAU resolution made Pluto the prototype of the newly created category
called Dwarf Planet. Pluto as well as Eris, are not dominant enough for their
gravitational fields to have incorporated or shoved aside all of their
neighbors. In fact, based on current knowledge, they are only two bodies in a
large field of thousands to millions of similar objects known as the Kuiper
To better visualize the location of the Kuiper Belt, let's look at how far it is
from the sun. The Earth is roughly 93 million miles from the Sun and the Kuiper
Belt lies between 2.8 and 4.6 billion miles from the Sun. In fact, it takes more
than four hours for the Sun's light to travel this distance. How far is this
Let's imagine a road that took us from the Sun to the end of the Kuiper Belt.
Now let's imagine you're in a sports car. Imagine speeding down the road at 100
miles an hour. You would pass the Earth after 106 years of driving; You would
come upon Mars after travelling 162 years; reach Jupiter in 552 years; and pass
Saturn after 1,011 years on the road. Finally, at 3,181 years, you would hit the
beginning of the Kuiper Belt; drive by Pluto during year 4,187; And at long last
you would arrive at the outer reaches of the Kuiper Belt, after being on the
road for 5,302 years straight.
It is not surprising then that we have only catalogued a very small percentage
of the Kuiper Belt, and that only in the past 10 years. The scope for
exploration is vast. In the future, Pluto Eris and untold other objects in the
Kuiper Belt which have yet to be discovered, might be included in a new
classification system as dwarf planets.
Although the IAU was the first to try to scientifically define a planet, many
astronomers disagree with the definition as it classifies a planet in large part
by what it is near, and not by its properties. Based on the IAU ruling, if the
Earth were in the Kuiper Belt, it would not meet the current IAU definition of a
planet. Also, Jupiter, with its shared orbital asteroids known as the Trojan
asteroids, hasn't cleared its orbital path, so it too might not be considered a
The topic of defining a planet is still being vigorously debated. What we know
is that our technology will continue to improve and with it our understanding of
the universe. We should be flexible in our interpretation of what we do know, as
what we do not know will always surpass it.