Almost all of the planets in our Solar System have been discovered visually, by astronomers – including Pluto (no longer officially a planet), although in that case it took the added assistance of some photographic plates. The exception is Neptune, which at over 2.8 billion miles distance is more than 30 times as far from the Sun as the Earth is. Neptune’s existence, peculiarly, was first mathematically predicted by Urbain le Verrier and John Couch Adams during the 1840s, and only afterward located by a Berlin astronomer in 1846. As well as an astronomical peculiarity, the discovery was marked as a significant victory for Newton’s law of gravity, which had been used to locate the planet.
UNRECOGNIZED
Actually, the historical “fact” that Neptune was discovered by astronomers is slightly misleading. The planet is so far away that it can never be seen with the naked eye, but it is relatively easy to find with a telescope. For this reason, it can be found in the notes and records of several early astronomers – Galileo, Jerome Lalande, and, in the 19th century, John Herschel – who all seem to have dismissed it as just another star. John Herschel was the son of William Herschel, credited with the 1781 discovery of the next-outermost planet, Uranus.
Later, after the actual discovery of Neptune, Herschel acknowledged that he had spotted Neptune in 1830, noted its blue colour, but had not realized that it was a planet rather than a star.
TROUBLE WITH URANUS
The discovery of Neptune actually began as astronomers tried to figure out a different puzzle: the strange orbit of Uranus. By the 1820s, observers such as Alexis Bouvard realized that Uranus’s orbit did not seem to follow the precise path predicted by Sir Isaac Newton’s revolutionary law of gravity. Instead, it suffered from measurable anomalies, veering too far away from the Sun.
It was possible that Newton had simply been wrong about the effects of gravity, especially at distances as great as the orbit of Uranus (20 times as far from the Sun as the Earth). However, it was also possible, argued John Adams of Cambridge, that Newton had been entirely correct about gravity, but that something very heavy was pulling Uranus slightly away from the Sun. A new graduate of Cambridge in 1843, Adams set as his first postgraduate project the exploration of Uranus’s orbit. Over the next two years, using notations supplied by the Greenwich Observatory, Adams estimated that a “New Planet” must exist well outside Uranus’s orbit, of sufficient mass to pull the seventh planet away from the sun.
Adams was not the only person working on the problem, however. In Paris, Urbain le Verrier had also decided to tackle the problem, almost simultaneously, and also published his findings in 1845 and 1846. Le Verrier was unable to estimate either the mass of the new planet or its precise orbit, but he was able to indicate its actual position in the sky.
The reason for this imprecision arises from the simple lack of data. Both Adams and Le Verrier could tell that an object must be affecting Uranus’s orbit, and could tell what direction it was in at any particular time. Had they known either how heavy the new planet was or how far away it was, they would have been able to solve the puzzle entirely. However, not knowing one, they could not figure out the other: the difference in Uranus’s orbit could be accounted for by a relatively lightweight planet close by, or by a very heavy planet far away.
CONFIRMATION
The result was a rapid search in both France and England, which Le Verrier appears to have won. Both he and Adams proposed particular areas of the sky to search for the planet: Adams’s calculations turned out to be incorrect in this regard, whereas Le Verrier was able to accurately predict the new planet’s orbital position as well as its mass.
Finally, in the fall of 1846, the Berlin Observatory received Le Verrier’s new estimates. Johann Galle promptly turned his telescope to that section of the sky, and, with the assistance of a student, began comparing their star charts, knowing that – unlike a star – a planet’s position in the sky could be expected to change noticeably over a brief period of time. In the end, it took only a matter of hours of actual astronomical work to confirm that Neptune was indeed a planet. Stunned, Galle excitedly reported his results back to Le Verrier.
Officially, both Adams and Le Verrier were credited with the discovery of Neptune, since they had independently proposed its existence, more or less simultaneously. Historically, Le Verrier probably had more of an impact than Adams: it was his data which turned out to be more accurate, and which was used by the Berlin Observatory to actually find the planet.
NAMING NEPTUNE
Of course, having two discoverers complicated matters when it came to assigning a name to the new planet, which for a time was known simply as Le Verrier’s Planet. Le Verrier himself was the one to propose the name Neptune; British astronomers favoured the name Oceanus, inaccurate (since we now know Neptune has no oceans to speak of) but both reflecting the surprising observation that the entire planet was blue. Le Verrier subsequently abandoned the “Neptune” moniker, but this was the term ultimately accepted later in the year of its discovery, 1846.
