No Planet X

Knowing that the mass of the Pluto—Charon double planet is small—only one part in 400 of the terrestrial mass—it was clear that the apparent wanderings of Uranus and Neptune required an alternative explanation. Many have seized upon the notion that indeed the discovery of Pluto was by chance, even if part of a deliberate search, and there must be another massive body out there, a Planet X.

This idea is too simple, though. The situation here is similar to when Le Verrier tried to explain the motion of Mercury using hypothetical bodies near the Sun. No single unknown object could explain how Mercury moved and, while popular imagination fo-cussed upon a planet Vulcan, Le Verrier himself knew that several intramercurial bodies would be necessary. Turning to the cases of Uranus and Neptune, again no single unobserved body could explain the apparent anomalies. There would need to be not only a Planet X, but also a Planet XI, XII, XIII, XIV, XV, and so on.

Are these indeed the minor planets now being spotted regularly out beyond Neptune? The answer is no—for several reasons.

One is that they are simply not big enough, being smaller even than Pluto. Another, paradoxically, is that there are too many of them. When there are many objects, all separately producing gravitational tugs—and we think that there are some millions of minor planets in that belt concentrated between 30 and 60 astronomical units—their effect is smeared out, and no distinct wobbles to the motions of the outer planets would be produced.

This was all a bit of a tease to astronomers, the solution eventually being reached only in the early 1990s. When Voyager 2 flew by Uranus in 1986 and Neptune in 1989, radio tracking of the bending of its trajectory by the gravitational attractions of those bodies allowed researchers to determine the planetary masses with unprecedented precision. And they showed the previous values to be wrong.

When the measurement of the masses of planets using observations of natural satellites was discussed above, I did not mention Uranus and Neptune. The masses of those planets had indeed been evaluated in that way, each of them possessing a flotilla of moons, but remember that they are a long way from us. One could time the orbits quite accurately, by observing eclipses perhaps, but measurements of the sizes of their circumplanetary loops must be inherently inaccurate from this distance. However, not only did Voyager 2 pass close by those planets, but also the radio tracking could be carried out with great precision.

When the spacecraft data were analyzed, it was realized that the previous masses for Uranus and Neptune were each slightly wrong, by a fraction of 1 percent, one too high and one too low. The improved evaluations for the masses were plugged into the numerical models for the whole Solar System, and when that was done there no disagreement remained between the observed plan etary positions and the theoretical positions from the computed ephemeris. The earlier theoretical positions were wrong simply because they were based on slightly incorrect planetary masses.

By 1992 the last nail was hammered into the coffin of Planet X. The observed motions of the outer planets are consistent with there being no other planet comparable in size to the Earth anywhere within 100 astronomical units. This just shows again that the discovery of Pluto was a fluke, resulting from inaccurate data. No one was to blame for this; one must remember that there is never absolute certainty in science, only limits of confidence that depend on the accuracy of the information in hand at any time. Lowell and his colleagues started looking for Pluto due to wishful thinking, rather than a sober analysis of the situation, and so it was actually a happy chance that the planet was found. If Tombaugh had not spotted it in 1930, someone else would have done so before too many years were out.

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