Name: Grant W Gearhart, Kathy Tice, B Randolph, Steve Mueller and
Pamela A McDermott
I would like a little information on Planet X. I would like to know where
it is located.
Planet X was postulated to exist beyond the orbit of Pluto to account for
apparent gravitational perturbations in the orbits of Neptune and Uranus.
It was these perturbations that led to the search for Pluto, but after Pluto
was found it was determined that its mass is not sufficient to fully account
for those perturbations. Despite many years of searching, no planet X has
been found. Nowadays most astronomers do not believe that Planet X exists;
the apparent perturbations, they believe, are due simply to errors in
Many scientists once thought that there must be a planet beyond Neptune and
Uranus to account for a variation in their orbits. Once Pluto was discov-
ered, its mass was not large enough to account for this variation. As
observations have gotten better with the help of things like the Voyagers,
scientists now think that those unexplained variations were really observa-
This is a really good example of the process of science: always willing to
question, apply new data, make new hypotheses...
As a historical note, the term "Planet X" was coined by the astronomer
Percival Lowell in 1915. It referred to a hypothetical planet beyond
Neptune that would account for the (apparent) gravitational perturbations in
the orbits of Uranus and Neptune not due to the other known planets. Clyde
Tombaugh discovered Pluto in 1930, but it was found that Pluto was not
nearly massive enough to produce the perturbations. So Pluto was not Planet
X, and the search continued, without success. In recent years, with the
benefit of more and better data (including measurements by the Voyager
probes) most astronomers have come to believe that the apparent discrepan-
cies in the orbits of Uranus and Neptune were due to observational error in
the older data.
Is there really a planet X? The real question is what kind of measurements
could you design to see if there is one. One could be the amount of gravity
pull that would be present if there were a planet on the opposite side of
the Sun. Could it be detectable? There have been probes that have gone out
far from the Earth and what kind of camera would we need for them to detect?
A big effect should be felt on certain comets and I do not think that anyone
saw anything strange about recent comets. The effect on their orbit should
be observable. None of these kind of effects have been observed and on that
basis most would conclude that this idea has no merit. Can you think of an
experiment that would show its presence?
Is there any new evidence to this theory? Steve, your question is a good
one. I have seen no evidence that is convincing that there is a 10th
planet. Even is there was it would probably be very small and not very
interesting. While people have been looking I do not feel any one has
convincing evidence. Most people are looking for things like dark matter
and black holes to see if the universe is open or closed.
Samuel P Bowen
Click here to return to the Astronomy Archives
Update: June 2012