When Pluto was recently demoted by the International
Astonomical Union (IAU) from being a full-fledged planet to dwarf-planet
status, many people the world over were upset by the seeming arrogance of the
action. They felt that the IAU had
arbitrarily over-ruled a scientific fact they knew to be true: the Sun has nine
planets. They mourned the loss of
one of them.
I guess I had even more reason to be upset, for Pluto was
discovered during the week of my birth in 1930. In a sense, it is my birth sign. So, when my book club decided to read The
Hunt for Planet X: New Worlds and the Fate of Pluto by Dutch astronomer Govert Schilling, I plunged into
it to see why and how my birthright had been diminished. I finally decided it was all
the fault of modern electronics.
A brief history: The first five planets other than
Earth—Mercury, Venus, Mars, Jupiter and Saturn—were known to the ancient
Babylonians. Although the
telescope was invented in 1608, and some telescopic sightings of Uranus were
reported in 1690, it wasn't until 1781 that it was confirmed as the seventh planet. Likewise, Gallileo's drawings show that
he had seen Neptune as early as 1612, but it wasn't established as the eighth
planet until 1846, and that was mostly because Uranus' orbit deviated from the
one dictated by Newtonian physics. A trans-Uranian planet was conjectured as
the cause, leading to a successful telescopic search for Neptune in a
calculated location.
An earlier "eighth planet," Ceres, had been
detected in 1801, orbiting between Mars and Jupiter. It is very small, less than 1000 km (600 miles) in diameter. But soon many other, smaller objects
were discovered between Mars and Jupiter, so astronomers decided not to call
them all planets, re-categorizing them as asteroids. Adding an eighth planet to the known
solar system thus had to await Neptune's discovery in 1846. Finding Pluto took almost another
century. Its orbit is mostly
beyond Neptune's; its diameter—about 2300 km—is about half the width of the
U.S. and half the diameter of Mercury.
Finding a new body in the solar system by telescopic
observation had thus traditionally been a tortuous endeavor, involving
hand-written records or, later, huge libraries of photographic plates, together
with mind-bending manual calculations.
Then came a late-20th century break-through: mounting
electronic CCD cameras on telescopes—cameras like those in cellphones, but with
orders of magnitude more sensitivity and resolution. Using digitally stored photographs from them, a computer can
quickly detect a new solar-system object and calculate its orbit. Combined with increasingly large
terrestrial and space telescopes, such cameras have within the past several
decades found a cornucopia of objects rotating about the sun. The largest, Eris, found in 2005, is
roughly the size of Pluto. Should
Eris therefore be called the tenth planet, and other new bodies also be added
to the list?
Astronomers were now in the same position as when they
defined the asteroid belt, having to revisit the question of which objects
would be classified as planets. Like
Congress drafting a bill that will benefit only some companies without naming
them, the IAU—and with similar acrimonious debates—set about drafting a
definition of "planet" that would include the classical eight but
exclude Pluto and Eris and lesser bodies, without naming any of them. It finally resolved that to be called a
planet in our solar system, an object must satisfy three criteria:
• It must circle the sun, but not be the
satellite of a planet.
• It must be massive enough to be in hydrostatic
equilibrium under its own gravitational
force, normally meaning that it is of spherical or ellipsoidal shape.
• It must have cleared the neighborhood around
its orbit, i.e., be gravitationally
dominant in it, so no objects of comparable size are present there other than its own moons or other bodies under its gravitational influence.
Several of the eight classical
planets have objects in their orbits that have not been cleared (so-called
Trojans), but they are synchronous with the planet, locked in by its
gravitational field so as to revolve around the Sun at fixed distances in
advance or behind it. Those
planets therefore satisfy all three conditions, which is why they still qualify
as planets under the new prescription.
Pluto, Eris and Ceres pass the first two tests but fail the
third. Pluto's orbit passes
through the Kuiper belt, a region well beyond Neptune's orbit, in which more
than 100,000 objects over 100 km in diameter are believed to exist,
including many that Pluto hasn't cleared from its orbit but aren't under its
gravitational sway. Eris, which
ranges further from the Sun than Pluto—through the Kuiper belt and beyond into
the so-called scattered disk—similarly hasn't cleared its orbit. And Ceres has neither cleared other
asteroids from its orbit nor locked them into synchronism.
Bodies like Pluto, Eris and Ceres that satisfy only the
first two criteria are now called dwarf planets. Two others have been recognized by the IAU: Haumea and Makemake, both having
diameters roughly 60% of Pluto's and orbits about the same size. It is suspected that another 100
now-known bodies may qualify, and the eventual total may be as many as
200. So poor Pluto, my cherished
birth sign, has been legislated out of its former planetary grandeur.
I'm delighted, though, that Pluto retains some of its
idiosyncracy. It has a nearby
sister, Charon, half its diameter but large enough so the two jointly rotate
around a barycenter lying between their surfaces rather than inside Pluto, like
an unbalanced dumbbell—see the illustration below. (For comparison, the earth-moon barycenter is about 1700
km—1000 miles—below the earth's
surface.) Considering the external
position of their barycenter, some astronomers call Pluto and Charon a double
dwarf planet, twins so to speak, unique in our solar system; but the IAU
persists in classifying Charon as a moon of Pluto. The former designation appeals to me because of its singular
glamor, but I am solipsistically pulled in the other direction: since I don't
have a twin, how can Pluto, my birth sign, have one?
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Artist's portrayal of Pluto and Charon [source
unknown]. The added
white dot is the barycenter around which the
two rotate in common.
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At any rate, I still feel sad for Pluto. Perhaps never having achieved glory is
better than achieving it and then having it fall to others' machinations.