By, John Noble Wilford
Something out there beyond the farthest
reaches of the known solar system seems to
be tugging at Uranus and Neptune. Some
gravitational force keeps perturbing the two
giant planets, causing irregularities in
their orbits. The force suggests a presence
far away and unseen, a large object that may
be the long-sought Planet X.
Evidence assembled in recent years has led
several groups of astronomers to renew the
search for the 10th planet. They are
devoting more time to visual observations
with the 200-inch telescope at Mount Palomar
in California. They are tracking two
Pioneer spacecraft, now approaching the
orbit of distant Pluto, to see if variations
in their trajectories provide clues to the
source of the mysterious force. And they
are hoping that a satellite-borne telescope
launched last week will detect heat
“signatures” from the planet, or whatever it
is out there.
The Infrared Astronomical Satellite was
boosted into a 560-mile-high polar orbit
Tuesday night from Vandenberg Air Force
Base, CA. It represents an $80-million
venture by the United States, Britain and
the Netherlands. In the next six or seven
months, the telescope is expected to conduct
a wide-ranging survey of nearly all the sky,
detecting sources not of ordinary light, but
of infrared radiation, which is invisible to
the human eye and largely absorbed by the
atmosphere. Scientists thus hope that the
new telescope will chart thousands or
infrared-emitting objects that have gone
undetected – stars, interstellar clouds,
asteroids and, with any luck, the object
that pulls at Uranus and Neptune.
The last time a serious search of the skies
was made, it led to the discovery in 1930 of
Pluto, the ninth planet. But the story
begins more than a century before that,
after the discovery of Uranus in 1781 by the
English astronomer and musician William
Herschel. Until then, the planetary system
seemed to end with Saturn.
As astronomers observed Uranus, noting
irregularities in its orbital path, many
speculated that they were witnessing the
gravitational pull of an unknown planet. So
began the first planetary search based on
astronomers’ predictions, which ended in the
1840’s with the discovery of Neptune almost
simultaneously by English, French and German
astronomers.
But Neptune was not massive enough to
account entirely for the orbital behavior of
Uranus. Indeed, Neptune itself seemed to be
affected by a still more remote planet. In
the late 19th century, two American
astronomers, William H. Pickering and
Percival Lowell, predicted the size and
approximate location of the trans-Neptunian
body, which Lowell called Planet X.
Years later, Pluto was detected by Clyde W.
Tombaugh working at Lowell Observatory in
Arizona. Several astronomers, however,
suspected it might not be the Planet X of
prediction. Subsequent observations proved
them right. Pluto was too small to change
the orbits of Uranus and Neptune; the
combined mass of Pluto and its recently
discovered satellite, Charon, is only
one-fifth that of Earth’s moon.
Recent calculations by the United States
Naval Observatory have confirmed the orbital
perturbation exhibited by Uranus and
Neptune, which Dr. Thomas C. Van Flandern,
an astronomer at the observatory, says could
be explained by “a single undiscovered
planet.” He and a colleague, Dr. Robert
Harrington, calculate that the 10th planet
should be two to five times more massive
than Earth and have a highly elliptical
orbit that takes it some 5 billion miles
beyond that of Pluto – hardly next-door but
still within the gravitational influence of
the Sun.
Some astronomers have reacted cautiously to
the 10th-planet predictions. They remember
the long, futile quest for the planet Vulcan
inside the orbit of Mercury; Vulcan, it
turned out, did not exist. They wonder why
such a large object as a 10th planet escaped
the exhaustive survey by Mr. Tombaugh, who
is sure it is not in the two-thirds of the
sky he examined. But according to Dr. Ray
T. Reynolds of the Ames Research Center in
Mountain View, CA, other astronomers “are so
sure of the 10th planet, they think there’s
nothing left but to name it.”
At a scientific meeting last summer,
10th-planet partisans tended to prevail.
Alternative explanations for the
outer-planet perturbations were offered.
The something out there, some scientists
said, might be an unseen black hole or
neutron star passing through the Sun’s
vicinity. Defenders of the 10th planet
parried the suggestions. Material falling
into the gravitational field of a black
hole, the remains of a very massive star
after its complete gravitational collapse,
should give off detectable x-rays, they
noted; no X-rays have been detected. A
neutron star, a less massive star that has
collapsed to a highly dense state, should
affect the courses of comets, they said, yet
no such changes have been observed.
More credence was given to the hypothesis
that a “brown dwarf” star accounts for the
mysterious force. This is the informal name
astronomers give to celestial bodies that
were not massive enough for their
thermonuclear furnaces to ignite; perhaps
like the huge planet Jupiter, they just
missed being self-illuminating stars.
Most stars are paired, so it is not
unreasonable to suggest that the Sun has a
dim companion. Moreover, a brown dwarf in
the neighborhood might not reflect enough
light to be seen far away, said Dr. John
Anderson of the Jet Propulsion Laboratory in
Pasadena, CA. Its gravitational forces,
however, should produce energy detectable by
the Infrared Astronomical Satellite.
Whatever the mysterious force, be it a brown
dwarf or a large planet, Dr. Anderson said
he was “quite optimistic” that the infrared
telescope might find it and that the Pioneer
spacecraft could supply an estimate of the
object’s mass. Of course, no one can be
sure that even this discovery would define
the outermost boundary of the solar system.
New
York Times, January 30, 1983
