The Mystery of Planet-X
The understanding of our Solar System has been in a state of flux for centuries. In the early days it was thought to consist of just five planets: Mercury, Venus, Mars, Jupiter and Saturn. This grew to six once it was recognised that the Earth was a planet but that’s where it stopped for centuries. The discovery of Uranus in 1781 set things on the move again. Careful study of its orbit led to the discovery of Neptune in 1846 which ultimately led to the discovery of Pluto in 1930. The growth of the Solar System didn't stop there though as the search for the mystical Planet-X has baffled astronomers for decades!
The hunt for Planet-X goes back to the discovery of Uranus in March 1781. On March 13, of that year, Sir William Herschel was engaged in a series of observations to measure the parallax (apparent shift of position due to the movement of Earth) of fixed stars. Whilst observing stars in the constellation Taurus he found an object that he noted as "either a nebulous star or perhaps a comet". He looked for it again four days later on March 17 only to find that it had moved and so concluded it must be a comet.
Following his announcement of a comet discovery, many astronomers across Europe took time to study the new object. Due to the absence of a nebulous coma or tail it soon became apparent that the comet was in fact a planet. Herschel acknowledged this “by the observation of the most eminent astronomers in Europe, it appears that the new star, which I had the honour of pointing out to them in March 1781, is a primary planet in our Solar System”. Over the following years the movement of Uranus around its orbit was carefully observed and it was found that it seemed to be wandering from its expected path. These perturbations could only be caused by the gravitational influence of another, more distant planet.
By applying Newtonian mechanics to the orbit of Uranus, the French mathematician Urbain Le Verrier was able to calculate the possible location of the undiscovered planet giving astronomers an area of the sky to focus on. The discovery of Neptune in 1846 has since been attributed to both Le Verrier and John Couch Adams, a British mathematician and astronomer who had been working independently of Le Verrier. Careful study of the orbits of Uranus and Neptune showed further perturbations. It seemed that there was yet another planet out there in the depths of the Solar System gently tugging at these two gas giants. Planet-X was still at large.
It was left to an astronomer called Percival Lowell, founder of the Lowell Observatory in Flagstaff Arizona, to try and find the mysterious Planet-X which was causing disturbances of Uranus and Neptune. He set about the search and homed in on a number of possible locations for the ninth planet. However he died without knowing his team had captured images of Pluto on two occasions a year before he died. It was an astronomer working at the Lowell Observatory some years later who finally positively identified Pluto. After almost a year of comparing photographic plates taken a few nights apart, Clyde Tombaugh spotted a faint moving object, Pluto. It was believed the Solar System was complete and that Planet-X had finally been found.
The case for Planet-X was re-opened again in 1978 with the discovery of Charon, the faint moon of Pluto. Its discovery allowed astronomers to accurately calculate the mass of Pluto and when it was found to be just 0.2% the mass of the Earth it was realised that it was far too small to cause the observed perturbations in Uranus' orbit. This was all to be short lived though as the Voyager flyby of Neptune in 1989 meant that the mass of Neptune could be accurately calculated. Determining the mass of Neptune from the Voyager data reduced the might planet's mass by just 0.5% and reapplying this new value to the orbit of Uranus nicely accounted for the perturbations in its orbit. Finding an accurate figure for the mass of Neptune finally laid rest to the need for Planet-X for Uranus and Neptune were moving exactly as they should be. Finding Pluto, it seems, was just luck!
It seems then that Planet-X never existed and was proposed simply to account for unexplained variations in the orbit of Uranus. Does this mean that our Solar System is complete and we will never discover another major planet in the depths of the outer Solar System? The chances seem very unlikely now and even the Voyager and Pioneer probes which are heading out into interstellar space have not detected the presence of a previously unidentified giant planet.
That was back in 1992 and you might think that was the end of the story. However over recent years there has been a slow revival in the Planet-X concept due to a number of observed characteristics in the outer Solar System. The most recognised of these is the so called Kuiper Cliff. The Kuiper Belt is found beyond the orbit of Neptune extending out to around 55 Astronomical Units (8.2 billion km). It can be thought of as a larger version of the asteroid belt between Mars and Jupiter although in composition there are many more frozen elements such as water, ammonia and methane due largely to the greater distance from the Sun. Pluto and a number of the other 'trans-Neptunian' objects are now considered to be Kuiper Belt objects.
It is reasonable to assume that the existence of Kuiper Belt objects would gently decrease with distance from the Sun however in reality the belt seems to suddenly terminate at a distance of about 48 Astronomical Units. This so called 'Kuiper Cliff' is thought to be the result of an unidentified planet orbiting the Sun at this distance, perhaps the size of Earth. Such an object would not only explain the abrupt termination of the belt but would also explain why a few objects seem to have been ejected into different orbits.
It seemed that the idea of the mysterious Planet-X as an unidentified giant in the darkness of the outer Solar System has been laid to rest. However the concept is enjoying a revival as a rather more generic explanation for any gravitational anomalies at the edge of the Solar System.