| PLANETS IX and X |
|---|
Background to Planets IX & 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 is 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. Lowell's prediction was bogus.
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.
Is There Evidence for a Planet IX or X?
Nothing convincing, yet (2021). Trujillo and Sheppard in 2014 noticed an apparent alignment of the orbits of distant KBOs, and speculated that they might be hearded by an unseen planet. Others have jumped on this idea, wrapped it in elaborate dynamical simulations, and found some interesting consequences if such a planet indeed should exist. All that matters, though, is the observational evidence, and that remains roughly neither more nor less convincing than it was in 2014. The problem is that there are not many aligned objects, and the observations are 'biased' by the way they are taken (basically, you can't look equally in all directions with a telescope on the Earth, so some directions are naturally preferred).
Could There Be a Planet IX or X?
Yes. An unseen planet 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 LSST telescope 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 KBO, like Pluto. See 1000 km KBOs
What is the Difference Between Planet IX and X?
It's confusing. Pluto used to be "Planet 9" (or IX in Roman-speak) until it was found to be much more like the Kuiper belt objects near it than like any of the other planets. It got bumped down to "dwarf planet", although nobody really knows what that means. The current "Planet 9" is an object that might or might not exist...people are looking for it now. Planet X (or 10, but the X is often used because it has an element of mystery about it, and sounds better) is another hypothetical body that people have mused about for a long time. Planet 9 currently refers to the unknown object that might be causing the alignment of distant KBOs, if that alignment is real. Planet X refers to the unknown distant object that has long been recognized by internet conspiracists as the home of the Reptoids.
Summary
There is no convincing evidence for an unseen planet but "absence of evidence
is not evidence of absence". Such an object could exist provided
it is sufficiently far away.
David Jewitt: Last updated 2021
