After the discovery of Neptune in 1846, eager
astronomers combed the Solar System for signs of
another planet that might lie beyond this gas giant.
They had good reason to believe one might exist:
Neptune’s discovery had directly resulted from French
mathematician Urbain Le Verrier’s hypothesis that
perturbations in the orbit of Uranus were caused by the
gravitational pull of a yet-undiscovered planet. Le
Verrier predicted the position of this new planet and
sent his calculations to German astronomer Johann
Gottfried Galle. With these coordinates, Galle
discovered Neptune the very next day—exactly where Le
Verrier had predicted. But there remained some
discrepancies in the gas giants’ orbits even with the
discovery of the blue planet, so the hunt for yet
another distant planet in the solar system forged
onward.
Percival Lowell, an early twentieth century
astronomer, exhibited a particular obsession with this
search for a ninth planet. Lowell christened this
hypothetical new planet “Planet X,” and, in his 1915
Memoir on a Trans-Neptunian Planet, concluded that
Planet X must have a mass roughly seven times that of
Earth—about half that of Neptune—and a mean distance
from the Sun of 43 AU.* When Lowell died suddenly in
1916, his observatory, led by the efforts of his widow,
persisted in the search for the elusive Planet X.
Fourteen years later, an ambitious budding
astronomer from a Kansas farming family made the first
major discovery in the search for Planet X. In 1929,
twenty-three-year-old Clyde Tombaugh arrived at the
Lowell Observatory and was given the task of
systematically imaging the night sky. Tombaugh captured
sections of the sky in pairs of images taken two weeks
apart. He then placed the paired images into a machine
called a blink comparator, a microscope that
superimposes two photographic plates, “blinking”
rapidly between them and creating a time-lapse illusion
of the movement of any planetary body. In February
1930, after searching for almost a year and examining
nearly two million stars, Tombaugh discovered a moving
object on photographic plates taken in the previous
month. The object lay just six degrees from one of the
two locations for Planet X that Lowell had suggested,
and it seemed as though, at long last, Lowell had been
vindicated.
This supposed new planet was named Pluto, in
part to honor Percival Lowell, as his initials make up
the first two letters of the word. However, it did not
take long before astronomers began debating Pluto’s
status as a planet. Observations showed that Pluto was
six times dimmer than Lowell had predicted. It also had
a far more elliptical orbit than any other planet in
the solar system. In 1978, Pluto was found to be too
small for its gravity to affect the gas giants, a
discovery that resulted in a search for a tenth planet,
which was eventually abandoned in the early 1990s when
the Voyager 2 spacecraft found that irregularities in
Uranus’s orbit were actually due to a slight
overestimation of Neptune’s mass.
In 1992, the discovery of numerous small, icy
objects similar in size and orbit to Pluto led to a
more vocal debate over whether it should remain a
planet or whether Pluto and its asteroid neighbors
should be given their own separate classification. In
2006, the International Astronomical Union ultimately
reclassified Pluto and its friends as “dwarf planets,”
leaving the Solar System, once again, with only eight
planets. It is not likely that Pluto will ever be
called an official planet again, either. Mike Brown,
who discovered Eris, a dwarf planet larger than Pluto,
in 2005, speculates that there are likely thousands of
other “rocks” like Pluto orbiting in the Kuiper belt
outside of Neptune.
Today, the astronomical community is largely in
agreement that Planet X as it was originally conceived
does not exist, but Planet X as a concept has been
revived by a number of astronomers to explain other
anomalies observed in the outer Solar System. In
popular culture, Planet X has become a stand-in term
for any undiscovered planet in the Solar System,
regardless of whether or not it fits into Lowell’s
original theory. Thus, real or not, Planet X remains a
fixture in the astronomical universe.
