Background to Planet X
The idea that at an unseen planet might lurk beyond the known planets is an old one. Adams and Leverrier used unexpected irregularities in the motion of Uranus to deduce the existence of an unseen planet and predicted its position with such accuracy that it was easy to find once looked for (by Galle in 1846). We call this planet Neptune, of course, and it is 30 AU from the sun (Uranus is at 20 AU) and contains 17 Earth masses of material.
Later, Percival Lowell tried to play much the same game using unexpected deviations in the motion of Neptune. He inferred the existence of a still more distant planet and set Clyde Tombaugh to look for it. Tombaugh found Pluto in 1930. Unfortunately, Pluto soon proved to be too small (it contains only 0.002 Earth masses) to measurably perturb massive Neptune, leading to the suspicion that the massive "Planet X" remained undiscovered.
This notion lingered through the 20th century and sometimes resurfaces even today. However, it is now understood that the deviations in the motion of Neptune modelled by Lowell and others were simply errors of measurement, and that there is no evidence for unexplained gravitational deflections of the planets by a Planet X.
What, then, of Pluto? How could it have been found by Tombaugh on the basis of a faulty prediction by Lowell? The answer seems to be that Pluto was discovered by Tombaugh simply because he was a determined observer with a definite aim in mind. It was not predicted by Lowell in any meaningful way. One unfortunate result of this accidental discovery is that Pluto became labelled a planet (because Tombaugh was searching for a planet he assumed that was what he had found) whereas we now appreciate it more clearly as the largest known Kuiper Belt Object.
Could There Be a Planet X?
Yes. Planet X could exist provided it is
1 Far enough away and/or small enough to exert no measurable gravitational deflections on the planets, comets or spacecraft in the outer solar system.
2 Far enough away and/or small enough to have escaped detection by any of the all-sky surveys having sensitivity to moving objects that have been conducted to date.
What this means is that a planet of Earth's mass could exist undetected if it were more than a few 100 AU away, and even a Jupiter (300 Earth mass planet) could exist at distances only slightly greater. The sun could have a companion brown dwarf or even a star if far enough away! It's a nice thought but it will be very tough to do anything about it unless we are lucky. The Pan STARRS telescope now under development in Hawaii will provide the best constraints in the forseeable future. Will we get lucky? Stay tuned to find out.
Is 2003UB313 = Planet X?
No. It is another big KBO, like Pluto. See 1000 km KBOs
There is no convincing evidence for Planet X but "absence of evidence is not evidence of absence". Such an object could exist provided it is sufficiently far away.
David Jewitt: Last updated 2005 Aug