Download by Kendrick Frazier Pluto turns out not to be responsible for

by Kendrick Frazier
Pluto turns out not to be responsible for
peculiarities in the orbits of other outer planets.
Is a tenth planet of the sun still to be found?
11 iwo occasions a few nights apart
in May 1795, French astronomer
Joseph Lalande charted the position of what he took to be a star. Observing
that the star's position seemed to be different
the second night, he concluded that his first
observation had been a mistake. He moved
the star's position on his chart and apparently
never looked for it again. If he had, he might
have discovered the planet N e p t u n e .
Despite Lalande's erroneous conclusion,
his precise observations of N e p t u n e ' s position in 1795 have become critical to succeeding generations of planetary astronomers.
T h e orbital motion that he saw is part of the
evidence for an additional massive planet in
the solar system, out beyond N e p t u n e .
By the early 20th century, astronomers of
the stature of Percival Lowell and William H.
Pickering were devoting their professional
lives to the search for this ninth planet of
the sun. Lowell called the body he sought
Planet X; to Pickering, it was Planet O. T h e y
sought it without success.
Then, still more than half a c e n t u r y ago,
astronomer Clyde Tombaugh, fresh off a
Kansas farm, discovered the distant planet
Pluto. For many years it was considered to
be the missing mass whose gravitational attraction perturbs the orbits of the outer
planets. N o w it is known that Pluto w o n ' t
do. Everything learned about Pluto since
T o m b a u g h ' s discovery has diminished its
size; the effect of its gravity on the orbits
of the outer planets would be all b u t nil.
But while Pluto can no longer be considered
a candidate, more and more sophisticated
observations of the planets U r a n u s and
MOSAIC September/October 1981 27
associate Robert Harrington say it is "reasonably certain" that something else is out
there. They estimate that there is "only about
a 5 percent probability" that the answer to
the discrepancies they see in the planetary
orbits is something other than a perturbing
body.
But there are skeptics. Planetary searches
have considerable glamour attached to them,
and enthusiasms can easily outpace realities.
Scientists have learned from history to be
cautious on such matters.
At the Naval Observatory's Nautical Almanac Office, the division that carries out
orbital calculations, director P. Kenneth
Seidelmann finds the anomalies intriguing
but won't commit himself to the idea that
another large planet necessarily remains to
be found. Recalling that in 1977 Charles T.
Kowal discovered a minor planet—the asteroid named Chiron—in orbit between Saturn
and Uranus, he would, he says, " b e amazed
if there is not more material out there. But
I'm not sure I'd expect to find a single,
solid, large, additional object."
Neptune in the past 50 years seem to confirm
orbits perturbed as if by some massive body.
T h e perturbations that led Lowell and
Pickering to posit another planet tugging at
the outer planets have not been explained
away; all indicate something awry in the
outer solar system.
Lowell honored
" A single undiscovered planet could entirely explain the observed irregularities in
the orbits of each of the outer planets/ 7
astronomer T h o m a s C. Van Flandern of
the U.S. Naval Observatory said early in
1981. He was announcing what he and his
colleagues at the observatory are calling
" t h e renewal of the trans-Neptunian planet
search."
They use the term " t r a n s - N e p t u n i a n "
rather than " t r a n s - P l u t o n i a n " for several
reasons. The n e w search is an explicit renewal of the efforts of Percival Lowell to
find a planet of significant mass beyond
N e p t u n e . There is also debate within the
astronomical community about whether
Pluto, considering its small size, should even
be called a planet any longer; its mass is
between that of the smallest k n o w n planet,
Mercury, and large asteroids.
Further, Pluto is temporarily not the most
distant k n o w n planet; since 1979, when
Pluto's strangely elliptical orbit carried it
inside the orbit of Neptune, it has not been
the planet farthest from the sun. N o r will
it be again until 1999.
Pluto or no Pluto, Van Flandern and his
28 MOSAIC September/October 1981
Some other astronomers have even more
pronounced reservations. Brian G. M a r s d e n
of the Harvard-Smithsonian Center for Astrophysics finds it all very tentative. "There's
a lot of uncertainty about it," M a r s d e n
cautions.
T o m b a u g h himself is skeptical. He has
examined 70 percent of the sky in his 14
years of searching for outer planets (13 of
them after his discovery of Pluto), and he is
certain he didn't overlook anything as bright
as the massive planet the Naval Observatory people are talking about. " I would
guarantee that within the area I examined
there is no tenth planet," he states bluntly.
T o m b a u g h acknowledges that such a
planet could be in one of the two portions
of the sky he didn't get to examine. O n e is
in the northern sky, in a region 45 degrees
north of the ecliptic. (The ecliptic is the
plane of the orbit of earth around the sun.
It is also roughly the orbital plane of most
of the other planets.) T h e other area is the
far southern sky. At declinations of more
than 50 degrees south, that region is not
visible from Lowell Observatory's latitude
at Flagstaff, Arizona, where T o m b a u g h
carried out his search. If any undiscovered
planet is in those unexamined regions, it will
be extremely difficult to find, says T o m baugh, because it is then likely to be right
in the middle of very bright portions of the
Milky W a y .
Something awry
Today astronomers have twice as long a
span of observations of N e p t u n e as Lowell
had for his calculations. They n o w k n o w
that the best calculated orbits for all of the
outer planets—Jupiter, Saturn, Uranus, Neptune, Pluto—fail to agree with observations
to some degree. T h e n e w , lower estimate of
Pluto's mass in the calculations complicates
matters even further.
Seidelmann and his g r o u p have prepared
a series of charts c o m p a r i n g the observations and the predicted positions of both
Uranus and Neptune. In just a 10-year period,
since the early 1970s, the charts show a
noticeable diversion from the straight line
that indicates N e p t u n e ' s expected position.
"It's running off the fit already," says Seidelmann. A n d , he adds, looking at a plot of
Uranus's right ascension, " t h e r e seems to be
something funny here t o o . " Pluto also appears to be showing such residuals, or deviations, although the 51-year duration of
observations is too short to make too much
of that.
A variety of such plots all s h o w the same
kinds of discrepancies. "Historically, everybody who has been able to fit the observations of Neptune in the past has failed to
predict its positions in the future," says
Seidelmann.
The planets aren't the only solar system
objects hinting at an u n c h a r t e d influence.
Van Flandern points out apparent anomalous
influences on six long-period comets. Of
course, one has to be careful in making
too much of comet-orbit problems. As comets
approach the sun, gases previously locked
up within them heat u p , expand, and are
released like a small iet thruster, altering
the comet's orbit from w h a t gravitational
calculations predict. But, Seidelmann points
out, this thrust effect is most prevalent nearest the sun, and it also generally acts in the
plane of the comet's orbit. "For long-period
comets there appears to be some sort of effect
when they are a w a y from the sun, and it is
an effect out of the orbital p l a n e / '
All in all, something does seem to be awry.
The University of Texas's J. Derral Mulholland, a specialist in celestial mechanics, has
followed this debate and the evidence closely.
He thinks the N a v a l Observatory group is
correct in saying that the perturbations are
real and that they may be due to another
planet. " M y feeling that they're likely right
is based on what I see when I look at the
best-constructed planetary orbits," he says.
Those constructions, he notes, all s h o w discrepancies in the orbits of the outer planets.
" T h a t is a hard observational fact," M u l holland adds. "Something is going on. There
is no chance that these are artifacts. A b solutely no c h a n c e . "
Several approaches
Even granting the accuracy and reality of
the residuals in the outer planets' orbits,
that doesn't necessarily mean some undiscovered planet is responsible for them.
Seidelmann, Van Flandern, and others point
to the quest for the hypothetical planet Vulcan
inside the orbit of Mercury. T h e existence
of such a planet seemed to be indicated by
the measurement in 1859 of an otherwise
unexplained advance in the perihelion (closest
approach to the sun) of M e r c u r y ' s orbit.
Vulcan was searched for. Some observers
even thought they had found it, but the
observations were all due to defects in photographic plates or the effects of wishful
thinking. M a r s d e n refers to the various
claimed observations of Vulcan as "a particularly sorry episode in the history of astrono m y . " T h e perilhelion advance of Mercury
remained unexplained until Einstein's general theory of relativity came along and
successfully accounted for the discrepancy.
T h e Naval O b s e r v a t o r y is approaching
the observational problem from several
directions. " W e are refitting all the observational data," Seidelmann says. " W e are taking
the N e p t u n e data and trying to fit segments
rather than the whole thing. W e are seeing
if we can change the masses of the outer
planets and solve the problem. We're trying
to get a good fit of the observational data
without another object. And we're trying to
factor in improved knowledge from other
sources."
One exotic example of a possible new source
of information is the Viking spacecraft circling
Mars. Seidelmann and Van Flandern both
point out that the Viking orbiter senses the
center of mass of Mars with incredible prec i s i o n ^ a matter of several meters. As it goes
around the planet, Viking can record the
gravitational perturbation of Mars by Uranus
and other bodies. This kind of analysis should
lead to better figures for the masses and
residuals in future calculations of outer planet
orbits. " W e need a factor of three better
precision in those masses than we have now,"
says Van Flandern.
So far, Van Flandern adds, he and his
g r o u p have tried all the easy approaches
and failed. Calculations on the hypothetical
planet's orbit and location are proving to be
incredibly difficult.
" W h i l e originally I t h o u g h t we might be
able to predict its position to within 20 degrees, n o w I am fearful that the prediction
may have an uncertainty of 30 to 50 degrees,"
he says. " T h a t ' s a big h u n k of the s k y . "
But he adds, " A t any rate, it's now time to
press the calculations as far as we can."
T h e n if they get something they can call a
prediction, the search can begin.
T h e natural question to ask is: If there is
such a planet out there, why hasn't it already
been found? Van Flandern's and Harrington's
best estimates are that the planet should be
of two to five earth masses, have a highly
elliptical orbit that takes it some 80 to 100
astronomical units (multiples of the distance
from the earth to the sun) from the sun at
m a x i m u m (Pluto's mean distance is about
39 astronomical units and maximum distance
about 49), and be at a visual magnitude of
from 15 to 17 (objects of magnitude 6 or
lower are visible to the unaided eye). Pluto
was at magnitude 15.5 at the time of T o m baugh's discovery, b u t his plates were generally sensitive to stars at least as faint as
magnitude 17.
Grounds for skepticism
T o m b a u g h , w h o at 75 still puts in an
afternoon of work each day at New Mexico
State University in Las Cruces, has some
very definite ideas on the subject. After he
detected Pluto in 1930, the Lowell Observatory officials asked him to continue the search,
with the same degree of thoroughness, in
the hope of finding still more planets. He
did so for the next 13 years, when he was
interrupted by W o r l d War II. He carefully
examined two-thirds of the sky. As he says
now, "I didn't find a single thing."
In a way, this negative result after the
discovery of Pluto may be just as important
as the discovery itself: It would seem to put
some severe limits on the possibility of another
outer planet of significant size.
T o m b a u g h takes justifiable pride in the
care with which he carried out the search.
The estimated number of stars in the examined
areas totaled 44,675,000, give or take a million, he says. C o u n t i n g two plates per scan,
that's 90 million star images. "Every one of
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tricity and high inclination." The inclination of Pluto, the most highly inclined known
planet, is 17 degrees. So perhaps an undiscovered planet could reside in the areas of
the sky, far from the ecliptic, which Tombaugh did not examine.
N o t only do the calculations seem to point
to a planet with a highly inclined orbit, but
there is still another line of evidence, an
exotic theory proposed by Harrington and
Van Flandern, that also seems to them to
point well out of the plane of the ecliptic.
H a r r i n g t o n and Van Flandern point out
that the innermost of Neptune's two moons,
Triton, is the only large, relatively close satellite in the solar system having a retrograde
orbit and so highly inclined (160 degrees) to
its planet's equator. And the outermost satellite, Nereid, has twice the eccentricity of any
other satellite or planet in the solar system.
T h e y theorize that these anomalies in Neptune's satellite system, as well as the highly
inclined orbit of Pluto, can all be explained
by a single encounter of Neptune sometime
in the distant past with a planet then in a
circular orbit, beyond N e p t u n e , and having
a mass two to five times that of earth.
Wandering star
the 90 million images was seen individually
by m e , " says T o m b a u g h . "It required a total
of 7,000 h o u r s of w o r k . "
T o m b a u g h discovered not only Pluto but
some 775 asteroids (one of them named for
him) as well. He marked a total of 3,969
asteroid images and 1,807 variable stars,
and he c o u n t e d 29,548 galaxies. Concludes
T o m b a u g h : " F e w astronomers have seen so
much of the universe in such minute detail."
Besides being a remarkable achievement
in itself, the thoroughness with which T o m baugh has studied the firmament gives him
a unique perspective on the search for a
tenth planet. He declares unequivocally that
a planet the size of the one postulated by
Van Flandern and Harrington isn't in any of
the areas he examined. "I could have picked
up a planet as massive as they say, and it's
not there. I could have picked up a planet
the size of earth at a distance of 100 astro-
30 MOSAIC September/October 1981
nomical u n i t s , " he says. "I know what's
there and what's not there."
A skewed orbit
"It could be there if it's inclined 37 degrees or more north of the ecliptic and in the
area I didn't get to examine because of World
W a r II," notes T o m b a u g h . " O r it could be
in the extreme s o u t h e r n area of the sky,
south of where I could observe from Flagstaff." The unobserved southern areas are
about 50 degrees south of the celestial equator.
Analyses by the Naval Observatory
astronomers do hint that any planet causing
the Uranus and N e p t u n e residuals should
be sharply out of the plane of the ecliptic.
Harrington says an inclination greater than
30 degrees, " p e r h a p s even greater than 50
degrees," is not b e y o n d reason. " T h e more
we try to solve for the missing planet," says
Harrington, " t h e more we need high eccen-
W h a t e v e r tugged this planet out of place
(perhaps, Van Flandern suggests, a star
passing within 800 astronomical units of
our solar system—some 80 billion miles) could
have sent it off into a highly inclined orbit
beyond Pluto. He and Harrington have done
numerical experiments showing that such a
one-time occurrence could explain quantitatively not only the orbits of Triton, Nereid,
and Pluto, but also the residuals of Uranus
and N e p t u n e .
This exotic scenario is more than a little
hard for most astronomers to accept on the
basis of what is now known. Seidelmann,
for instance, says he likes to keep the question
of the Uranus and Neptune residuals separate
from his colleague's Neptune-encounter
hypothesis. Harrington and Van Flandern
insist that both lines of evidence do suggest
an undiscovered planet in a highly inclined
orbit.
Harrington thinks it likely the tenth planet
would be very southerly, perhaps somewhere
in the s o u t h e r n m o s t part of the Milky Way.
Last year he had some people at the Cerro
Tololo Inter-American Observatory in Chile
make a few photographic plates for him. He
blinked them and found nothing—hardly a
surprise for such a preliminary first effort.
Nor does he plan to examine any more plates
until and unless he and Van Flandern can
come u p with some sort of reasonable prediction of where best to look. •
A triumph of observationtal astronomy and a triumph
of celestial mechanics added planets seven and
eight to the solar system.
he modern era of planetary astronomy began the night of March 13,
1 7 8 1 , as the English a s t r o n o m e r
William Herschel was using his 15.7-centimeter reflector telescope in an extensive review of the stars. "While I was examining
the small stars in the neighborhood of H
Geminorum,"
he began his account, "I perceived one that appeared visibly larger than
the rest." Because it had a measurable diameter, it could not be a star.
H e continued his observations over the
next several nights and observed definite
motion. He thought his discovery was a
comet, but within two months other astronomers had decided correctly that it was
indeed a new planet. A Finnish astronomer,
A n d e r s Lexell, determined that its distance
from the sun was 18.9 astronomical units
(the earth is 1 astronomical unit from the
s u n ) and its orbital period was between 82
and 83 years. These were close, as it turned
out; the figures are 19.2 astronomical units
and 84 years.
Herschel had doubled the size of the known
solar system. He proposed naming the planet
after King George III of England, but the
mythological name for Saturn's father,
U r a n u s , prevailed.
In contrast to the discovery of Uranus, a
t r i u m p h of observation, the discovery of
N e p t u n e in 1846 was a triumph of the predictive science of celestial mechanics. After
U r a n u s was sighted, astronomers turned u p
several instances in which it had been recorded even earlier, without the observer
realizing at the time that it was a planet.
T h e English astronomer John Flamsteed, for
instance, had seen Uranus in 1690.
But a number of these prediscovery observations appeared to have the planet in
the wrong place, if its orbit is calculated
b a c k w a r d in time. By the 1830s it became
clear that something was amiss with the
orbit of U r a n u s . Before long the possibility
of the gravitational effect of another planet,
as large as U r a n u s , was being seriously proposed as the source of the observed anomalies.
In the 1840s, the young English astrono m e r - m a t h e m a t i c i a n John Couch Adams
a n d the French mathematical astronomer
Urbain Jean Joseph Leverrier both tackled
the problem of trying to calculate the expected position of the u n k n o w n planet.
Neither was aware of the other's efforts.
Adams had an answer by September 1845,
but he c o u l d n ' t get anyone to begin effective
observations in the designated region of the
sky. This was partly due to the notable lack
of enthusiasm of the Astronomer Royal.
Leverrier, too, had trouble getting his countrymen to look for the object. In frustration
he finally wrote to an astronomer at the
Berlin O b s e r v a t o r y , Johann Galle. Galle
received the letter on September 23, 1846,
and that night he and observatory director
Johann Encke turned their 23-centimeter
refractor to the position in the sky Leverrier
had given.
T h e y soon found an object of magnitude
eight that wasn't on the star maps. Galle
immediately wrote Leverrier, " T h e planet
whose position you have pointed out actually
exists." It had been found within one degree
of Leverrier's predicted position. Its apparent
diameter was 3.2 seconds of arc; Leverrier
had calculated 3.3 seconds. Leverrier first
wanted the planet named after himself, but
after some months he accepted N e p t u n e .
N e p t u n e might have been discovered half
a century earlier if only Joseph Lalande, the
French astronomer, had had more confidence in his o w n observations. In 1795 he
recorded as a star on M a y 8 the position of
Neptune. O n May 10 he rechecked and found
the star was not in the same position he
had marked. He assumed he had made a
mistake and dropped the subject.
For that matter, Galileo, we now believe,
observed N e p t u n e during fortuitous observing conditions in 1613, when it was
very close to Jupiter's position in the sky.
Charles T. Kowal of Caltech's Palomar O b servatory and Stillman Drake of the University of T o r o n t o reported in September
1980 that they had found this extraordinary
prediscovery observation of N e p t u n e recorded in Galileo's notebooks. D u r i n g observations of Jupiter, Galileo not only had
recorded N e p t u n e ' s position on several different nights but also noticed that it had
shifted positions.
Kowal and Drake point out that Galileo
would have found it extremely difficult to
follow up on this observation of a faint,
moving body without having an adequate
telescope mounting. N e p t u n e soon moved
out of his telescope's field of view as he was
observing Jupiter and its moons. " W i t h o u t
a graduated mounting on his telescope, and
without Jupiter to serve as a guidepost, he
would hardly have been able to find a planet
and a fixed star that are not visible to the u n aided eye, let alone determine their motions."
Lalande's observation shows N e p t u n e to
have been in a position at least seven arcseconds from where our modern calculations
indicate it ought to have been in 1795. A s tronomers have long debated whether the
apparent discrepancy is an observational
error or whether it is reliable and therefore
an indication of the gravitational influence
of an undiscovered planet. Brian M a r s d e n
of the Harvard-Smithsonian Center for
Astrophysics refers to it as a "slight discordance" and questions its reliability. Robert
Harrington of the U.S. Naval Observatory
calls it a " w h o p p i n g big residual" and b e lieves it shouldn't be too easily written off
MOSAIC September/October 1981 31
Planet nine was a triumph
of persistent blinking
and the perseverance
of Clyde Tombaugh.
t was the mathematical prediction of
Neptune's position, based on observed
perturbations in Uranus's orbit, that
finally brought planet prediction by orbit
calculation into high repute. But even before
N e p t u n e ' s discovery, an astronomer named
Peter Hansen had suggested in 1834 that a
single additional large planet still would not
account for the increasing residuals—the
remaining deviations^in the orbital longitude
of Uranus.
as incorrect analysis. Kowal takes an intermediate view.
Galileo's u n k n o w i n g observation of what
appears to have been N e p t u n e raises similar
questions. The planet was from 40 seconds
to a full minute of arc from where our best
estimates say it ought to have been, if the
distance scale Galileo was applying to Jupiter's moons (shown on the same page of his
notes) can be applied to Neptune's position.
"We don't know if he was doing it to scale/'
says Kowal of Galileo's reference to the
position of the planet. The other objects on
the page were drawn to an indicated scale,
the first time such scale measurements show
up in Galileo's notebooks, but N e p t u n e ' s
position was off the page and was indicated
by a directional line. He then included a
second drawing on the same page showing
the relationship between the fixed star he
had seen and the object we now k n o w is
N e p t u n e ; the question is whether to assume
32 MOSAIC September/October 1981
it is at the same scale as his drawing of
Jupiter and its moons. Kowal hopes to find
more prediscovery observations of N e p t u n e
in the notebooks of 18th-century European
astronomers. (Propitious observing conditions—another conjunction of Jupiter and
Neptune—occurred again in 1702.)
Kowal has been involved in the high-risk
search for a planet beyond Pluto since 1977,
probing parts of the sky covered by Clyde
T o m b a u g h but examining stars far fainter
th an any T o m b a u g h could see.
Using a 1.2-meter Schmidt telescope,
Kowal can record star images out to the
20th magnitude; T o m b a u g h reached only
the 17th. Kowal's more sensitive instruments are a mixed blessing, however, since
each increase in magnitude roughly doubles
the n u m b e r of star images to be examined.
" M y original idea was to cover a zone 30 degrees wide, centered on the ecliptic. I am
about one-third d o n e . " •
In the 1870s several astronomers carried
out searches without success, and in 1905
Percival Lowell began his serious search for
a planet beyond N e p t u n e . He used distribution of the orbits of comets and then the
residuals in the positions of U r a n u s and
N e p t u n e to pinpoint his Planet X. T h e laborious search lasted until N o v e m b e r 1916.
T h e death of Lowell, by then a discouraged
man, brought the Lowell Observatory's planet
quest to a halt.
William Pickering continued to c o m p u t e
outer planet orbits from the distribution of
cometary orbits at least through the 1920s,
even proposing one (of several) paths for his
Planet O that corresponded roughly with
where Pluto was later found. T h e search
largely rested there for a time.
Then, in 1929, two things happened: Lowell
Observatory decided to resume Lowell's quest
for Planet X, and Clyde T o m b a u g h arrived
at the observatory.
T o m b a u g h ' s family had moved from Illinois to a farm in Kansas in 1922, w h e n he
was 16. He pursued his lively interest in
astronomy with a 5.7-centimeter telescope
given to him by an uncle. He taught himself
to observe, and the more he saw of the night
sky the more he yearned to see. He and his
family couldn't afford to buy telescopes, so
he began building them himself.
The third one he made, a 23-centimeter
reflector, was the best. As he says now, "This
telescope changed my life." Using his new
eye on the heavens, he made a series of
detailed drawings of the surface of Jupiter
and sent them off to the astronomers at
Lowell Observatory so they could check
them. " T h e y were apparently impressed."
The observatory was then looking for
someone to help with the observing tasks on
its new 33-centimeter telescope. After a series
of letters back and forth, observatory director
V. M. Slipher invited T o m b a u g h to come to
Flagstaff on a few m o n t h s ' trial basis. So it
was that on January 14, 1929, three weeks
short of his 23rd birthday, he boarded a
train and headed southwest.
T o m b a u g h h a d n ' t k n o w n he was to help
with the resumption of the planet search,
but by April he was b u s y exposing the glass
photographic plates. Pairs of identical exposures of regions of the sky, 12 degrees by
14 degrees in size, were taken several days
apart. One of a pair was then aligned with
the other in a device called the blink comparator. First one plate, then the other, was
rapidly blinked into the instrument's field
of view. Distant stars would not change
their positions as seen first on one plate and
then rapidly on the other. But any object
closer to the earth than to the vault of distant
stars—and thus close enough to be part of
the solar system—would blink from visibly
discrepant positions on the otherwise identical
plates. (See " H u n t i n g by blinking," accompanying this article.)
the third plate would be examined for comparison. This quickly eliminated m a n y false
leads caused by flaws on a plate. But the
real key to finding Pluto was the t h o r o u g h ness of the blinking. T o m b a u g h divided the
plates into thin horizontal strips and moved
slowly across each, rapidly blinking each
star image. He found that three alternating
views per second worked best. "I was a
perfectionist," he recalls. T h r o u g h o u t the
fall and early winter he continued the p h o tographing and the blinking, "field by field,
strip by strip, panel by panel."
O n the morning of February 18, 1930, he
placed plates of the region of Delta Geminorum taken on January 23 and J a n u a r y 29
into the blink comparator. By four that afternoon he was about one-fourth done. He had
the guide star Delta Gem in the device's
eyepiece, scanned a few fields to the left,
and turned the next field into view.
In his newly published autobiographical
book about Pluto (Out of the
Darkness,
with Patrick Moore), T o m b a u g h relates
what happened next:
"Suddenly I spied a fifteenth-magnitude
image popping out and disappearing in the
rapidly alternating views. T h e n I spied another image doing the same thing about
three millimeters to the left. ' T h a t ' s it,' I
exclaimed to myself." He checked the individual plates. The image was to the left in the
later plate. It was indeed retrograde motion.
"Considering the interval between plates,
the parallactic shift indicated that the object
was far beyond the orbit of Neptune, perhaps
a thousand million miles b e y o n d . A terrific
thrill came over me. I switched the shutter
back and forth studying the images.. .. This
would be a historic discovery."
For the next 45 minutes T o m b a u g h rechecked everything. He found the object on
the next earlier, January 2 1 , plate and again
on three other plates taken at the same time
with a smaller camera on the same telescope
mount. " N o w I felt 100 percent sure. . . .
Checking each star on a plate pair for
such anomalies is tedious, time-consuming
work. The three senior astronomers at Lowell,
busy with their travels and many other tasks,
soon fell about 100 plates behind.
"The plates stacked u p , " recalls Tombaugh.
"So Slipher told me to do the blinking. I
shuddered when he said that, because I knew
what was on the plates. I k n e w what the task
was." Each plate had many tens of thousands
to several hundred thousand stars on it,
some had more than a million, and each image
had to be looked at individually.
Tombaugh plunged into the job with
energy but trepidation. By the time he finished, his methods were honed and he had
begun what was to be his life's work.
Blinking thorough
O n e of the first problems, recalls T o m baugh, was to avoid being fooled by asteroids.
His solution was to p h o t o g r a p h each area of
the sky only when it was at opposition
(directly opposite the earth across the sun).
At this point all bodies in the solar system
beyond earth's orbit appear from the parallax
effect to shift westward (retrograde) in the
sky, and the amount of the shift depends
on the object's distance. An asteroid, being
inside the orbit of Jupiter, would shift far
too rapidly to be an undiscovered outer planet
and could be ignored.
A second innovation was to p h o t o g r a p h
each area of the sky not twice but three
times. Whenever a suspicious object turned
up in the blinking of the first pair of plates,
MOSAIC S e p t e m b e r / O c t o b e r 1981 33
Clyde W. Tombaugh; Lowell Observatory.
34 MOSAIC September/October 1981
I walked d o w n the hall to V. M. Slipher's
office. T r y i n g to control myself, I stepped
into his office as nonchalantly as possible.
He looked up from his desk work. 'Dr.
Slipher, I have found your Planet X.' "
T h a t night was so cloudy no photographs
could be taken. T h e only person to discover
a planet in the twentieth century, just 24
years old, went down the hill to the only
movie theater in Flagstaff and watched Gary
Cooper in The Virginian.
Shrinking Pluto
W e k n o w n o w that Pluto was not the
hypothetical Planet X. It is just too tiny.
Lowell expected Planet X to be 6.6 times the
mass of earth. R. L. D u n c o m b e of the University of Texas and the Naval Observatory's
P. K e n n e t h Seidelmann recently chronicled
the story of Pluto's declining mass.
By the year of Pluto's discovery, it was
clear from the calculation that its mass was
less than earth's. In 1955 the best estimate
stood at 0.82 earth mass. Duncombe, W. J.
Klepczynski, and Seidelmann produced new
estimates of 0.18 in 1968 and then 0.11 in
1971. In 1976, D. P. Cruikshank, C. B. Pilcher,
and D a v i d Morrison of the University of
Hawaii f o u n d evidence that Pluto is covered
with methane ice and inferred that its albedo,
or reflectivity, is four times as great as had
been s u p p o s e d . This reduced the size (and
therefore the probable mass) of Pluto still
more, to 0.004 earth mass.
T h e n in June 1978 James W. Christy of
the Naval Observatory in Washington made
an unexpected discovery. He was measuring
some plates of Pluto taken with the observatory's 1.55-meter astrometric telescope at
Flagstaff, located just four miles from Lowell
Observatory, where Tombaugh had first seen
Pluto. Each of the six images of Pluto taken
on April 13 and M a y 12 showed a neverbefore-noticed elongation, sometimes above
Pluto, sometimes below. Christy had discovered that Pluto has a moon.
Further checks showed the same elongation
on plates taken earlier. In fact, five plates
taken d u r i n g a single week in June 1970
showed the elongation angle proceeding
clockwise around Pluto.
C h r i s t y proposed that the moon has a
roughly six-day orbital period, with the
orbit inclined to the line of sight such that it
is most easily seen first above Pluto and
three d a y s later below it. (The exact orbital
period is n o w assumed to be 6.3871 days,
says Christy.)
Observations taken with the Flagstaff telescope in July 1978, w h e n atmospheric conditions m a d e seeing exceptionally good,
Hunting by blinking
H u n t i n g for a planet beyond the edge
of the k n o w n solar system is a painstaking process. T h e way it is done is called
blinking, which involves: (1) photographing each section of the sky at least twice,
several days apart. Both plates should
be exposed when the part of the sky being
scanned is at opposition—directly across
the sun from the earth; (2) thorough
examination of each image on the photographic plates for any spot of light that
might have moved against the background
of more distant stars during the interval
between exposure of the first and second
plates of the pair. Hundreds of thousands
of points of light—star images—on each
plate have to be examined individually
for such a p p a r e n t motion.
The comparison of the pairs of photographic plates is done with an optical
device called a blink-microscope comparator. T h e blink comparator presents
the two plates of the pair to the eye in
rapid alternation, so that the eye sees
first one plate, then the other, in the
exact position. All distant stars will appear
as u n c h a n g i n g dots in the viewfield as
the plates alternate. Any solar-system
object, however, will be in a slightly
different position on the two plates. This
apparent motion against the background
of distant stars will show u p in the blink
comparator as an image " h o p p i n g " back
and forth as the plates alternate.
Most of this apparent motion is a consequence of the distance the earth moved
in .si.' I.I . ' i >• i • • i ••!• in- i ii ••! .!•:..; -"• cn-ii.
p h o t o g r a p h . As Pluto discoverer Clyde
Tombaugh says, "The motion on the plate
is caused by parallax. M a n y people don't
realize that the shift you are looking for
is created by the earth's orbital motion."
When the photographic plates are taken
at the opposition point, all solar-system
objects beyond earth appear to make little
retrograde orbital jogs against the backg r o u n d of distant stars. The amount of
this retrograde motion varies with the
object's proximity to earth. This provides a very quick way to distinguish
the many hundreds of asteroids that show
u p o n p h o t o s from the more distant outer
planets. Most of the asteroids are between
the orbits of Mars and Jupiter, at distances
from the s u n between 2.5 and 3.5 astronomical units. Thus they show enormous
apparent motion. A planet on the edge
of the solar system—Pluto's mean distance
is 39.4 astronomical units—would show
a much smaller displacement on the blinking plates.
As soon as Tombaugh noticed the image
of what turned out to be Pluto, a tiny
three millimeters apart on the plates he
was blinking in February 1930, and he
confirmed that the motion was retrograde,
he k n e w he had discovered a planet beyond the edge of the t h e n - k n o w n solar
system. As he recalls, "Considering the
[six-day] interval between the plates, the
[size of the] parallactic shift indicated
that the object was far beyond the orbit
MOSAIC Sentemhfir/Ortnhftr 1Qfi1 . 1 *
confirmed the discovery and allowed more
accurate calculation of the characteristics of
the moon. The excellent seeing conditions
resulted in the best photos of Pluto ever
taken. They still d o n ' t show a sharp-edged
disk, nor are they quite sharp enough to
reveal a separation between Pluto and its
m o o n , which Christy named Charon.
Christy says the observatory's Flagstaff
facility is trying to get better pictures, but
the viewing conditions of July 1978 have
yet to be surpassed. Of about 100 images
of Pluto taken since then, says Christy, about
one-third are sharp enough to show the
elongation. " W e were very fortunate to
h a v e the exceptional seeing conditions of
that July."
J. Derral Mulholland of the University of
T e x a s has been using a new electrographic
camera in an attempt to resolve the images
b u t has so far been unsuccessful, due to
the poor seeing conditions he encountered
d u r i n g recent o b s e r v i n g a t t e m p t s at
M c D o n a l d Observatory. H. J. Reitsema and
Faith Vilas of the University of Arizona
h a v e used a computer to produce a digital
image of Charon by subtracting out the image
of Pluto. In a few years, however, Pluto and
C h a r o n should go into a multi-year period
of frequent mutual eclipses, and astronomers
should be able to get the most precise information yet on the Pluto system.
Apart from the intrinsic interest in finding
a n e w satellite in our solar system, the discovery of Charon had wider significance.
As Christy's colleague Robert Harrington
points out, if we k n o w the period and mean
distance of a satellite, we can compute the
mass of the planet-satellite system. Harringt o n ' s calculation n o w gives a mass for the
two together of only 0.0024 earth masses,
with 0.0022 of that due to Pluto and 0.0002
due to Charon. Pluto had shrunk even more.
In fact, A. J. Dessler of Rice University
36 MOSAIC Sfintemher/nntnhftr1Qft1
and Christopher T. Russell of the University
of California at Los Angeles wrote a whimsical paper in 1980 noting the downwardplunging trend of the estimates of Pluto's
mass. Entitled " F r o m the Ridiculous to the
Sublime: T h e Pending Disappearance o{
Pluto," the paper concluded that "Pluto is
simply evaporating with time." W h e n this
was published in the American Geophysical
Union's publication EOS, the authors were
surprised to learn that some readers took
them seriously. Perhaps nothing should
surprise anyone about Pluto anymore.
T h e n e w estimate of mass resulting from
the discovery of Pluto's moon was the clincher, if one was needed. As Harrington says,
"Pluto is obviously not Lowell's Planet X
nor William Pickering's Planet O, nor any
of the other planets that have been hypothesized to explain the discrepancies between
the observed and predicted motions of Uranus
and N e p t u n e . "
T h o m a s Van Flandern of the Naval O b servatory concurs: "Pluto is too small to
account for m a n y of the observed irregularities in the motions of the outer planets."
Ironically, it appears that Lowell's accurate
calculation of the location of Planet X was
a fluke. Astronomers now agree that it was
Clyde Tombaugh's extraordinary search procedures that led to the discovery of Pluto.
Pluto was found, says Brian Marsden, "not
because it was predicted but because it was
there." A n d there is still no real clue to the
cause of the perturbations in the orbits of
Uranus and N e p t u n e . •
Kendrick
interested
sciences.
Frazier is a science writer long
in astronomy and the geophysical
The National Science Foundation has contributed to the support of its share of the
research discussed in this article through its
Solar System Astronomy
Programs,
PET
I wanted to drop you a note telling you
that I have been using copies of the
article "Seeing the Brain at W o r k . " It
appeared in the M a y / J u n e issue of Mosaic,
Volume 12, N u m b e r 3.1 have been
passing out copies to patients and their
families who are considering participation
in our PET research. I have also sent
copies to physicians who have referred
patients to me or have asked about
the procedure. It has proven helpful and
effective in this capacity.
National
Norman Foster
Institutes of Health
Bethesda,
Maryland