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
.......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.