Hunting for the real 'Planet X'
Ghostly planet 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 in 1846.
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 were nine
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 family.
However, the 1978 discovery of Pluto's moon Charon reopened the Planet X debate.
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 studies of 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.
The Kuiper cliff
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 Cliff."
Maybe Planet X is responsible for this strange unexpected feature in the outer edge of our solar system ... or maybe not.
The Voyager and 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 highly unlikely.
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 tantalising, but unlikely, explanation.