What Is A Planet?


Uploaded by ScienceAtNASA on 28.05.2010

Transcript:




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.
and at least one more, yet to be found. Pluto was discovered in 1930
and was hailed as the ninth planet, even though at the time
some astronomers did not think the term planet accurately described the new find.
By 2005 many believed we had identified the 10th planet
in our solar system. Nicknamed Xena, later 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 knowledge. 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 three traits
: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 round shape; 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 round shape; 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 Belt
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 really? 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 traveling 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 planet. 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.