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Pluto debate
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Planet
X?
In the year of 1930 the planet
Pluto was originally discovered .Once it was found, its faintness
and failure to show a visible disc cast doubt on the idea
that it could be Lowell's Planet X. Lowell had made a forecast
of Pluto's position in 1915 which had turned out to be fairly
close to its actual position at that time; but Ernest W. Brown
accomplished almost immediately that this was a coincidence,
and this view is retained today. Lowell had also made earlier,
different predictions of Planet X's position beginning in
1902.
In the
subsequent decades estimates of the Plutonian mass and
diameter were the matter of debate as telescopes and
imaging systems improved. The consensus gradually favored
smaller masses and diameters as time passed. In fact,
one observer waggishly pointed out that if the trend
were extrapolated, the planet seemed to be in danger
of vanishing altogether, a observation which proved
possibly prophetic in light of later debates over Pluto's
status as a "planet". |
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In an endeavor to reconcile
Pluto's small apparent size with its identification as Planet
X, the theory of specular reflection was anticipated. This
held that observers were measuring only the diameter of a
bright spot on the highly reflective surface of a much larger
planet which could thereby be massive without having remarkably
high density.
The ambiguity was decisively
resolved by the discovery of Pluto's satellite Charon in 1978.
This made it possible to establish the combined mass of the
Pluto-Charon system which turned out to be lower even than
that projected by skeptics of the specular reflection theory,
which was then rendered completely untenable. The conventional
figure for Pluto's diameter today makes it considerably smaller
than the Moon, with only a fraction of the Moon's mass on
account of its being largely composed of ice.
In recent times, measurements of the path of Voyager 2 have
shown that Neptune has a lower mass than previously believed
and that when this lower mass is taken into account there
is no anomalous movement of Uranus or Neptune. Thus Pluto's
discovery and Lowell's 1915 prediction were largely coincidental
as Pluto in fact has no role in what were believed to be anomalies
in Neptune and Uranus' motion. Pluto's discovery was mostly
due to the meticulousness and diligence of Tombaugh's search,
which he sustained for some time after the discovery and left
him contented that no other planet of a comparable magnitude
existed.
Although Pluto's identification
as Planet X began to be doubted soon after its discovery,
and for some decades afterwards some considered that a hypothetical
tenth planet might be the true Planet X which supposedly caused
anomalies in Uranus and Neptune's position, Pluto's identity
as the solar system's ninth planet was unquestioned until
the 1990s.
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Minor
planet?
An image of Pluto and Charon taken with a 61" telescope;
note the intricacy in resolution despite telescope size. Pluto's
planetary status is debated for the reason of its small size.
In September of 1992 scientists
began discovering hundreds of other bodies in the area of
the solar system beyond the orbit of Neptune. The second of
these, after Pluto and Pluto's moon Charon, was (15760) 1992
QB1. The sustained discovery of these objects, particularly
of plutinos, rekindled a debate that goes on to this day:
is Pluto a major planet or simply one of the largest trans-Neptunian
objects?
Trans-Neptunian objects are
well thought-out to be minor planets, so the question arose
whether to consider Pluto to be one too. This planetary sciences
debate landed in newspaper headlines, editorials, and on the
Internet in the mid- to late-1990s. The view that Pluto might
be "demoted" to non-planet status created an expressive
response in certain sectors of the public. Such news outlets
as the BBC News Online, the Boston Globe, and USA Today all
printed stories noting that the International Astronomical
Union was considering dropping Pluto's planetary status. As
a result "Save Pluto" websites sprang up, and school
children sent letters to astronomers and the IAU.
On February 3, 1999 Brian
Marsden, of the Minor Planet Center unintentionally fueled
the debate when he issued an editorial in the Minor Planet
Electronic Circular 1999-C03 noting that the 10,000th minor
planet was about to be numbered and this called for a large
celebration. He suggested that Pluto be honored with the number
10,000, giving it "dual citizenship" of sorts as
both a major and a minor planet. Between the media reports
and the Minor Planet Electronic Circulars, IAU General Secretary
Joannes Anderson issued a press release that same day, stating
there were no plans to change Pluto's planetary status. Ultimately,
the number 10,000 was assigned to an "ordinary"
asteroid, 10000 Myriostos.
The debate focuses on how
a "planet", from the Greek for "wanderer",
is a designation that depends upon an object's particular
size, formation, or orbit. Some argue that not only is Pluto
a major planet but also some moons like Titan, Europa or Triton,
or even the larger asteroids. Some argue that an astronomical
object more than about 360 km in diameter, at which point
the object has a affinity to become round under its own gravity,
should be known as a major planet; this would comprise several
moons and a handful of asteroids. Isaac Asimov suggested the
term mesoplanet be used for planetary objects intermediate
in size between Mercury, the smallest terrestrial planet with
a diameter of 4879.4 km and Ceres, the largest known asteroid
with a mean diameter of 950 km, which would include Pluto
but not most moons.
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