The object which can claim to hold this title is probably one of the comets which passed close to the Sun many years ago and has returned to the furthest limits of the solar system never to return again. If we limit our choice to objects which can be observed at a great distance then we can restrict our selection to objects which are 'permanent' members of the solar family. We must then consider the planets, periodic comets and asteroids.
Pluto is the planet whose orbit takes it further from the Sun than any of the other planets. Pluto's orbit is a much more elongated ellipse than those of the other planets, so for 20 years out of its 249 year orbit, it is in fact closer to the Sun than Neptune. This happened, most recently, between 21 January, 1979 and 11 February, 1999. Pluto will next pass inside Neptune's orbit in 2226.
The possibility of there being another planet, the so-called Planet X, has been suggested. The main scientific reason for another planet was that there were unexplained perturbations to the orbits of Uranus and Neptune. A recent analysis of the motions of these two planets has shown that the supposed perturbations are not real and that there is no need for another planet and, indeed, no evidence for the existence of one.
Many periodic comets are known with orbits that lie inside the planetary system. Halley's comet is one of these. There are only two comets which have been observed at more than one close approach to the Sun which move further away from the Sun than Halley's comet. These are P/Comet Swift-Tuttle and P/Comet Herschel-Rigollet.
Halley's comet has been seen at many returns; there are written accounts of all of its returns during the last two millenia. Halley has a period of 76 years and a maximum distance from the Sun of 35.3 AU.
P/Comet Swift-Tuttle has recently (September 1992) been rediscovered. It has been identified with previous comets seen in 1737 and 1862. It has a period of 135 years and a maximum distance from the Sun of 51.7 AU.
P/Comet Herschel-Rigollet has been seen twice in 1784 and in 1939. It has a period of 155 years and a maximum distance from the Sun of 56.9 AU.
There are many comets which have only been seen to make one approach to the Sun. Many of these have orbits which are very elongated. With an orbit of this shape it is often very difficult to distinguish between an orbit which is closed (meaning that the comet will eventually return towards the Sun) and one which is open (will never return). These have orbits that take them out to more than 1000 AU, but these large distances are very inaccurate.
Comets are believed to arise from a cloud of many millions of objects (known as the Oort cloud) in circular orbits around the Sun at distances of about 50,000 AU. These objects would then represent the outermost reaches of the solar system.
Comets, although they are known to travel to these outer reaches of the solar system can only be observed when they are close enough to the Sun to reflect enough light to allow them to be seen. At large distances from the Sun the Sun's radiation is not strong enough to evaporate the ices that form the comet's solid core and so the comet has no coma or tail. The solid cores are only about 10 kilometres across and so appear very faint at large distances. Until 1994 no comet had ever been seen further than about the distance to Saturn but in January Halley's comet was observed at a distance of about 15 AU.
Distant small bodies in the Solar System
Most of the asteroids lie in a belt between the orbits of Mars and Jupiter. It has been suspected that some of the matter from which the Sun and planets were formed exists in a ring of small bodies much further away from the Sun than the planets.
A minor body, named 1992 QB1, was discovered in September 1992. It was further from the Sun than any planet in the solar system. Its discoverers were discussing the novels of John Le Carre at the time and suggested the name Smiley. Unfortunately there is already a minor planet with that name and so 1992 QB1 will remain the name of this distant object. It is a very faint star-like object, visible in only the largest telescopes. It is assumed to be an almost black object with a diameter of less than 200 kilometres. Little is known of its orbit but it is about 37-59 AU from the Sun. It is thought to be one of these `left-overs' from the formation of the solar system. If so, it is likely to be one of thousands of such objects or perhaps even billions. The problem of finding objects like these is that they are extremely faint 1992 QB1 is 6 million times fainter than the faintest object visible to the naked-eye.
In March 1993 a second object, named 1993 FW (or unofficially Karla, after George Smiley's antagonist), has been reported. It is very likely that this object is at a similar distance. Since then several more such objects have been found (two of them with the Isaac Newton Telescope). These have distances from the Sun of between 34 and 45 AU. The Hubble Space Telescope has been used to look for comet sized objects at these sorts of distances and the observers have reported that many such objects are present, confirming that there is a belt of small objects associated with the larger ones found earlier. This is almost certainly the Kuiper belt which was predicted to exist from theoretical analysis of periodic comets with short periods. These objects are thus more likely to be cometary objects than asteroids.
In October 1996 a group of astronomers from the University of Hawaii discovered what is now believed to be the furthest observable object in our solar system, named TO66. It is one of the brightest trans-Neptunian objects known to date. It is at a mean distance of 45 AU from the Sun and is about 1.5 million times fainter than the faintest star visible by naked eye.
Since the discovery of Pluto, it has occasionally been suggested that there is a tenth planet whose orbit lies at an even greater distance from the Sun. This conjecture is referred to as Planet X. Irregularities in the orbit of Uranus resulting from the gravitational pull of an unknown body led to the discovery of Neptune in 1846. Similar irregularities were thought to have been detected in the orbit of Neptune, which caused the discovery of Pluto in 1930. However it was realised that Pluto was too small to cause these fluctuations, suggesting to some astronomers that there are other large trans-Neptunian bodies. A variety of possible planets were suggested with widely varying masses and orbits.
In recent years, the spacecraft Pioneer 10 and 11 and Voyagers 1 and 2 travelled outside the solar system and were used as 'probes' for gravitational forces from possible unknown planets without success. The Voyagers also gave more accurate measurements for the masses of the known outer planets. If these updated masses are used, the need for a 'Planet X’ disappears.
Probably the most amazing theoretical trans-Neptunian object is the hypothesised Nemesis. This is a predicted companion star to the Sun, with an elliptical orbit between 20 000 and 90 000 AU from the Sun and a period of 30 million years. This object would be very dark and faint, possibly a brown or black dwarf. The evidence for this star is the apparent 30 million year period of global catastrophes and consequent mass extinctions of species such as the dinosaurs, The theory suggests that Nemesis disturbs the Oort cloud, displacing a shower of comets and sending large numbers towards the inner Solar System where one or more collisions with the Earth take place. However, the IRAS satellite surveyed the entire sky at infrared wavelengths without detecting an object of this kind, providing strong evidence that Nemesis does not exist.
Another Tenth Planet?
On 13 October 1999, two independent research groups based at the Open University in the UK and Louisiana University in the USA suggested the existence of a planet in the Oort cloud. The new planet would be at a distance of 30000 AU from the Sun and have an orbital period of six million years. Its position is deduced from the orbits of a number of comets (13 by the UK group and 82 by the US astronomers) which may all have been deflected into the inner Solar System by a single massive body. If it exists, the new planet has a mass several times that of Jupiter, is too faint to be detected with current telescopes and moves very slowly across the sky. The two research groups disagree on the shape of the orbit of the new body. If the new planet is real, then it may be observed by the next generation of space-based infrared telescopes.