Predicting the planets

The ancients knew the planets Mercury, Venus, Mars, Jupiter, and Saturn (and Earth, although they did not think of it as a planet). Some of them offered reasons why there were these and no more. Pythagoreans, for example, argued on the basis of number properties. Much later, Johannes Kepler claimed that the number of planets is fixed by the number of the five regular solids. The world was astounded, then, when William Herschel discovered a seventh planet, Uranus, in 1781.

In fact, Uranus had been seen by other astronomers for years. It can be seen with the naked eye and had even been included on star maps. Since no one expected a new planet, however, it had gone unidentified as one. Herschel was the first observer whose telescope was good enough to reveal the "star" as a disk. (All stars except the Sun are so far away that they appear as points in even the largest telescopes.)

Herschel's discovery was the result of his systematic searching of the sky, his good telescope and good vision, and a certain amount of luck. The other two planets, Neptune and Pluto, that have been discovered since were both predicted. Nevertheless, the same qualities, especially the luck, were vital in finding each.

In 1821 Alexis Bouvard used all the known sightings of Uranus to calculate its orbit. The calculations from earlier sightings could not be made to agree with later ones, however. Something was wrong.

Something was wrong with Mercury's orbit, also. Like that of Uranus, the orbit of Mercury could not be made to fit with Newton's laws. Either Newton was not quite correct or there must be other bodies, most likely planets, near enough to Mercury and Uranus to affect their orbits. Most astronomers felt strongly that Newton must be correct and that other planets would be found.

Among the first to calculate the position of such a planet was Urbain Le Verrier in 1845. He predicted a planet inside the orbit of Mercury. He even named the planet--Vulcan. A few astronomers had been searching for such a planet already. Even with the help of Le Verrier's calculations, no planet was found.

In 1843 John Couch Adams determined the position of a planet outside the orbit of Uranus. But when he offered his calculations to the astronomer royal, George Airy, Airy paid no attention. Airy was one of the few astronomers who thought the irregularities were caused by defects in Newton's theory.

In 1845, although disappointed in the search for Vulcan, Le Verrier also turned to the planet beyond Uranus. He produced essentially the same result as Adams had. Unlike Adams, however, Le Verrier took his calculations to someone who was willing to conduct the search. Johann Gottfried Galle immediately found a planet where Adams and Le Verrier had said it would be, although at a different distance. Both calculators had relied on Bode's law to locate the planet, but the "law" is apparently the result of chance and does not apply to the new planet, which Le Verrier named Neptune.

Neptune had actually been seen earlier, but not recognized. The earliest sighting came on January 27, 1613, when Galileo recorded two stars in his field of view. One was Neptune and the other a genuine star. When he observed the pair again the following night, Galileo noted that the two "stars" appeared to be farther apart. Had Galileo followed up on the observations, he would have been the first to discover a planet.

When the orbit of Neptune was calculated from these observations, it became clear that Le Verrier and Adams were also lucky. Neptune could not account for all of the deviations in the orbit of Uranus after all. Furthermore, Neptune's orbit also deviated.

Scientists set out to find the next planet, which they believed would resolve these problems. The most dedicated was Percival Lowell, but he was unable to find a planet either by calculation or observation. Lowell even built his own observatory.

After Lowell's death in 1916, the search continued at Lowell Observatory. Finally, Clyde Tombaugh located a "star" on a pair of photographs that was in a slightly different place from one day to the next. He had found Pluto.

As usual, a lot of luck as well as systematic work went into the discovery. However, finding Pluto did not solve the problems with the orbits of Uranus and Neptune. For one thing, Pluto is too small. Many astronomers today think that Pluto, instead of being the smallest planet, is not a planet at all, but a kind of cross between a comet and an asteroid known as a Kuiper Belt Object. There are very many other Kuiper Belt Objects, although Pluto seems to be the largest. In 2001 the odd orbit of Comet 2000 CR₁₀₅ suggested to Brett Gladman of the Observatoire de la Côte d'Azur in Nice, France, that a planet the size of Mars or larger might lurk in the Kuiper Belt--or that it was there at some time in the past. Another possible explanation for the comet's path would be that Neptune had a different orbit in the past and that while in that orbit it had disturbed the comet's orbit.

By the way, in 1915 Albert Einstein developed a new theory of gravity, one that superseded Newton's. In Einstein's theory, Mercury's orbit was perfectly all right. There was no need for a planet to perturb it.