Download A Possible Ninth Planet

Astronomy and Space articles
by Martin George of the Launceston Planetarium
6 February 2016
A Possible Ninth Planet
This August will mark the tenth anniversary of Pluto being demoted from planetary status.
Up until then, for three quarters of a century we held that there were nine planets in the
Solar System. With the changes in the definition of the word 'planet' that excluded Pluto
in 2006, the count went down to eight, and Neptune reclaimed its title as the most distant
of the Sun's planets.
However, some new research has strongly suggested the possibility that there is another
planet lurking around out there. If it exists, it would be much farther out than Neptune,
and finding it will be rather like looking for a needle in a haystack. If astronomers are
correct, it probably averages at least 20 times Neptune's distance from the Sun, and it
may be detectable only with the world's largest telescopes.
Neptune, imaged in 1989 by NASA's
Voyager 2 spacecraft, is the most distant of
the Sun's known planets. Could there be
another, much more distant, one?
IMAGE: NASA
In astronomy, there are many cases of 'discoveries' of objects that can't be seen, or
haven't yet been seen, directly. Indeed, some could never be seen in the normal way.
Black holes are one example: we can infer their presence by detecting x-rays from the
superheated material surrounding them. Another example is the detection of planets
moving around other stars by the gravitational effects that they have on the stars
themselves.
In this case, observations of a group of six objects orbiting beyond Neptune have
revealed something most interesting: their orbits have certain similarities that suggest the
Astronomy and Space articles
by Martin George of the Launceston Planetarium
6 February 2016
presence of a so far unseen and very distant planet, at least 10 times the mass of the
Earth.
It's all to do with gravity.
We often think of the planets and various other objects as orbiting the Sun in an orderly
fashion because of the Sun's gravity. However, all objects have gravity, and it can be
said that every object in the Universe affects every other object because of the
gravitational attraction between them.
The situation is very complex, and in our Solar System the planets have the capability of
affecting the size, shape and orientation of the orbits of everything else.
There is a lot of mathematics in this, and it is far from easy. Although it is a relatively
simple task — for astronomers — to calculate the effects on other objects of a planet of
known mass in a well-defined orbit, working the other way is far less simple.
That is, looking at the orbits of certain objects and trying to deduce the mass, orbit and
current position of an unseen planet that has caused this situation is an uphill battle.
In the excitement of this new suggestion, I am reminded of a similar situation that was
faced by astronomers in the nineteenth century when the motions of the planet Uranus
were not as predicted.
Uranus had been discovered by chance in 1781, when William Herschel spotted it using
his home-made telescope while observing from his garden in England.
However, it deviated from its calculated orbit, and mathematicians John Adams in
England and Urbain Leverrier in France undertook the mammoth task of trying to
determine the mass, orbit and current position of an unseen planet whose gravity may be
affecting Uranus.
Eventually, Leverrier's calculations were used by astronomers at the Berlin Observatory,
who spotted the new planet on the night of 23-24 September 1846. This is the planet
that came to be called Neptune.
Despite the current task being far more complex than the one face by the nineteenthcentury astronomers, it has been suggested that we may find the new 'ninth planet' within
a few years. However, it will take some finding: depending on where it currently is along
its orbit, it may be so distant that only the world's largest telescopes are capable of
detecting it!
Article by Martin George, Launceston Planetarium, QVMAG. Reproduced with permission of the Mercury newspaper.