by Mark Thompson
9 November 2012
Could a large object at the edge of the solar system
be causing a sudden
drop-off in Kuiper Belt population?
(Source: G. Bacon (STSci)/NASA)
Recent discoveries of exoplanets are
raising the question whether astronomers have found all the
planet-like objects in our own system.
The announcement of the recent discovery of
exoplanet Alpha Centauri Bb is a
testimony to how far planetary detection techniques have come over
the last few decades.
It brings the total of confirmed exoplanets - or "extra-solar
planets" - to a staggering 825. However, the search for planets in
our own solar system has subsided since the pioneering days at the
end of the 18th century with the discovery of Uranus and almost one
hundred years later with the identification of Neptune.
The idea of another planet, dubbed 'Planet X', inspired astronomers
to keep searching for yet another 100 years in a hunt that was full
of twists and turns.
The hunt for Planet X began in 1781 when British astronomer
Sir William Herschel was studying stars in the constellation
of Taurus and noticed one star seemed slightly fuzzy or nebulous in
appearance. A few days later it seemed to have moved position - he
concluded it was a comet. Further study revealed it was actually a
planet - Uranus - the seventh planet in our solar system and beyond
the orbit of Saturn.
Detailed observations of Uranus' movement revealed an orbit that
seemed to be influenced by another, even more distant, object.
Mathematicians studying the data
predicted the position of an eighth planet before it was officially
discovered. Visual confirmation of Neptune's existence was announced
Using the same techniques to study the orbital characteristics of
both Uranus and Neptune revealed they were both still being tugged
at by the gravitational force from another unknown object.
The search for the ninth planet in the
solar system began and it was American astronomer Percival Lowell
who identified possible candidates.
And then there
Some years after Lowell's death in 1930, Pluto was identified by
Clyde Tombaugh (an astronomer working at Lowell Observatory) and
was believed to be the final member of the solar system's planetary
However, the 1978 discovery of Pluto's
moon Charon reopened the Planet X
Through accurate measurements of Charon's orbit, the mass of Pluto
could be deduced. Ultimately it showed that the 'ninth planet'
couldn't possibly have affected the orbits of Uranus and Neptune as
observations appeared to show.
The renewed interest in Planet X was short lived as the Neptunian
flyby by Voyager 2 in 1989 revealed its mass was less than thought.
Reapplying this knowledge showed the
outermost "ice giant" planets were behaving exactly as they should
and the orbital perturbations were down to observational error. It
seems the myth of Planet X had finally died...
This could have been the end of the Planet X saga, but recent
the Kuiper Belt - a region of icy
minor planets located in the outermost reaches of the solar system -
suggest this may not be the case.
It would be reasonable to expect the millions of frozen lumps of
rock would gradually decrease with distance from the sun, but at a
distance of 48 Astronomical Units (beyond the orbit of Pluto) they
seem to drop off suddenly, at the so called "Kuiper
Maybe Planet X is responsible for this strange unexpected feature in
the outer edge of our solar system... or maybe not.
Pioneer spacecraft heading out of
our solar system haven't detected any substantial planets that might
cause the 'cliff,' but space is vast; the chances of a spacecraft
happening to fly past a previously undiscovered world would be
Also, ground-based observatories and space telescopes (like NASA's
Wide-Field Infrared Survey Explorer) have turned up little evidence.
But the jury is still out...
Until an answer is found for the Kuiper
Cliff, the ghost of Planet X will remain as a tantalizing, but