Pluto’s
Demise Remains Controversial
By Dr. Syed Amir
Bethesda, MD
In
late February on a starry night in 1930, a young
student, Clyde Tombaugh, barely 24-years old, was
peering down a telescope at the Lowell Observatory
located at Flagstaff in the Arizona desert when
he made some remarkable observations that led to
the discovery of Pluto. Yet, ever since its discovery
almost 76 years ago, the status of Pluto as the
ninth planet has remained a subject of relentless
controversy, one that has only been exacerbated
by time.
This long-standing controversy aside, Pluto’s
discovery makes a fascinating story. Growing up
on an agricultural farm, Tombaugh had inherited
a passion for astronomy and a zeal for stargazing
from his father, and as a boy had constructed his
own rudimentary telescope to observe Jupiter and
Mars. His ambition was to study astronomy at a university,
but his family did not have enough money to support
his quest for higher education. He therefore started
to look for alternative opportunities to fulfill
his aspirations.
Tombaugh got his opportunity when he went to work
at the Lowell Observatory. The celebrated American
astronomer, Percival Lowell, who founded the Lowell
Observatory in 1894, had been intrigued during his
life time by his finding that the orbits of two
known planets, Uranus and Neptune, seemed to deviate
from their predicted course. He theorized that the
observed perturbations were caused by the gravitational
pull of another planet yet to be discovered. He
named this unknown planet, X. Following his death
in 1916 other astronomers continued to look for
the presence of the hypothetical planet in the sky.
It was not an easy undertaking and the planet X
defied for many years all attempts to find it.
When Tombaugh was interviewed for a job at the Lowell
observatory, the scientists who interviewed him
were impressed by his youthful enthusiasm and promptly
offered him a position. He was assigned the task
of surveying the heavens for the elusive planet,
photographing the constellations on different nights
and then meticulously studying the photographic
plates for any evidence of the unknown planet.
In their search, the astronomers were hoping to
take advantage of the differences in the relative
movements of stars and planets. Stars move little
relative to each other over a short time, but planets
orbit the sun, shifting their positions significantly
relative to stars in the night sky. The two planets
located at the edge of our solar system, Uranus
and Neptune, are so far away from us that they are
visible only as spots of light in the darkness.
The only hope the young astronomer had for finding
the new planet in the neighborhood of Uranus and
Neptune was to be able to identify a speck of light
in the distant sky that shifted its position, as
opposed to numerous others that did not move at
all.
After persevering for a year and help with serendipity,
Tombaugh was finally able to locate the planet X
by comparing its image with thousands of stars all
of which he had captured on his photographic plates.
Paradoxically, while the new planet was found in
the region where the astronomer Lowell had predicted
that it would be, the initial prediction itself
was based on an incorrect premise. Later calculations
showed that there was no true deviation in the orbits
of Uranus and Neptune, as Lowell had long believed;
he had merely made an error in his orbital calculations.
The new planet was named Pluto, after the Greek
god of the underworld, following the suggestion
made by an eleven-year girl from Oxford, England,
who was interested in classical mythology. The exotic
name was nicely consistent with Pluto’s cold,
frozen character and its remote location. It was
the only planet ever discovered by an American.
With the addition of Pluto, the number of planets
in the solar system went up from eight to nine.
But even as school textbooks and astronomical documents
were getting updated, and children were struggling
to learn the names of the nine planets, Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
and now Pluto, the status of Pluto as a planet was
steadily coming under attack. Initially believed
to be of much larger size, larger than mercury,
Pluto turned out to be of much smaller size, as
more powerful and sophisticated instruments became
available to study it. The original calculations
of Pluto’s size were erroneous, because the
telescopes available at the time could not separate
Pluto from its large satellite, Chiron, giving the
illusion of a larger body. In fact, Pluto has a
diameter of only 1,440 miles, or less than one-fifth
the size of our earth. Its distance from the earth
is estimated to be 2.67 billion miles, and it is
so far away from the sun, 3.7 billion miles, that
when viewed from its surface the sun looks like
a dim star.
As the full realization of Pluto’s true size
and density emerged, it re-ignited the controversy
as to why it should continue to be classified as
a planet and not considered a comet. Comets are
small celestial bodies that are made mostly of ice
and stellar debris and abound in the outer regions
of the solar system. Pluto, based on available information,
is believed to have an icy surface that consists
of 98% nitrogen, mixed with small amounts of methane
and carbon monoxide. Its small size has also been
problematical, since a number of moons that orbit
planets are known to be larger than Pluto.
To settle the question of whether Pluto is a planet
or a comet, the International Astronomical Union
(IAU) appointed a committee to make a final recommendation
about its status. The seven-member committee met
in June and July this year, and returned with a
recommendation designed to please everybody. They
proposed the creation of a new subcategory of small
planets to be called Plutons. They further recommended
that Pluto be placed in this special category to
distinguish it from the eight established planets.
However, the new proposal failed to carry a majority
support of the astronomers and astrophysicists.
At its meeting in Prague where 2,500 astronomers
gathered on August 24, 2006, IAU, after much acrimonious
debate, decided to strip Pluto of its status as
a planet. They settled on a new definition of planets,
according to which a planet must have an orbit that
is clear of all debris and smaller celestial bodies.
Pluto did not qualify, as it travels mostly through
a ring of icy chucks and debris, the so-called Kuiper
belt, and its orbit is crossed by the much larger
planet, Neptune.
The decision of IAU, however, has not settled the
controversy -- far from it. Recently, more than
300 planetary scientists and astronomers in the
United States signed a petition protesting the IAU’s
decision and expressing their disagreement with
the new definition of a planet. They are planning
to have their own international conference next
year and propose to reopen the debate. Thus, the
final chapter on the status of Pluto may not yet
have been written.
Meanwhile, Pluto’s demotion to the status
of a dwarf planet not only dismayed school children
around the world who seem to have special affection
for it, it also saddened the widow of Tombaugh,
the discoverer of Pluto who died in 1997 at age
90. Tombaugh’s ashes are currently being carried
by the spacecraft, New Horizons, which will rendezvous
with Pluto in the year 2015 and will scatter his
remains on the former planet’s surface. Alas,
Mrs. Tombaugh who is 93 years old is unlikely to
be alive to celebrate the event.
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