Leverrier, Adams, and the Mathematical Discovery of Neptune

Overview

The planet Neptune was discovered in 1846 following laborious calculations by Englishman John Couch Adams (1819-1892) and Frenchman Urbain Leverrier (1811-1877). These astronomers, attempting to explain deviations noted in the orbit of Uranus, independently and nearly simultaneously predicted the location of Neptune, which was then located with little trouble by German astronomer Johann Gottfried Galle (1812-1910). This discovery was viewed as a triumph of mathematics and physics, winning acclaim for both Adams and Leverrier.

Background

Through most of human history mankind lived in a solar system with seven "planets"—Mercury, Venus, the Moon, Mars, Jupiter, Saturn, and the Sun—that revolved around the Earth. The work of Galileo (1564-1642) and Nicolaus Copernicus (1473-1543) showed this notion to be false, but the Copernican solar system still ended at the orbit of Saturn until William Herschel (1738-1822) discovered an additional planet, Uranus, in 1781. Strictly speaking, Herschel was not the first person to see Uranus; at least 20 "pre-discovery" observations were uncovered following its official discovery, but it was not recognized as a planet. As a sixth magnitude object, Uranus is barely visible to the naked eye, so it must have been seen intermittently by ancient stargazers. Herschel was simply the first to recognize it for what it was.

Occurring less than 100 years after Isaac Newton (1642-1727) published his theories of gravitation and motion, astronomers immediately used Newton's laws and the celestial mechanics of Johannes Kepler (1571-1630) to check the observed motion of Uranus against predictions. Expected to confirm Newton, these observations instead showed discrepancies that worsened with time. The dilemma posed was that either Newton's laws were incorrect or some force was pulling Uranus from its predicted path. Even when accounting for the gravitational attraction of other major planets the discrepancy was not fully resolved. The only known force that could account for the noted discrepancies was another planet even further from the sun than Uranus. Finally, in 1846, two mathematicians, Leverrier of France and Adams of England, nearly simultaneously determined that another planet must be causing the noted discrepancies. Their independent calculations agreed closely with respect to the new planet's distance from the sun, orbital period, mass, and location in the sky.

Both Leverrier and Adams had difficulty persuading their colleagues to search for the new planet. Adams ran into opposition from Sir George Airy (1801-1892), the Astronomer Royal of Great Britain, who questioned whether it was possible to discover a planet simply by performing calculations. At the same time, Leverrier's unpopularity with French astronomers led to repeated refusals to search the sky at the calculated coordinates. Finally, Leverrier persuaded Galle, a German astronomer, to search for the new planet while, in England, Airy agreed that Professor James Challis (1803-1882) should try to locate it. Aided by a set of new and unpublished star charts of the ecliptic (the plane in which the planets lie), Galle and his assistant Heinrich d'Arrest (1822-1875) found the new planet in less than an hour on their first night of observations. Further observations confirmed the discovery.

It is interesting, in retrospect, to note that both Leverrier's and Adams's solutions for Neptune's orbit were incorrect. They both assumed Neptune to lie further from the sun than it actually does, leading, in turn, to erroneous calculations of Neptune's actual orbit. In fact, while the calculated position was correct, had the search taken place even a year earlier or later, Neptune would not have been discovered so readily and both Leverrier and Adams might well be unknown today except as historical footnotes. These inaccuracies are best summarized by a comment made by a Scientific American editor:

Leverrier's planet in the end matched neither the orbit, size, location or any other significant characteristic of the planet Neptune, but he still garners most of the credit for discovering it.

It is also worth noting that, after Neptune's mass and orbit were calculated, they turned out to be insufficient to account for all of the discrepancies in Uranus's motion and, in turn, Neptune appeared to have discrepancies in its orbit. This spurred the searches culminating in Pluto's discovery in 1930. However, since Pluto is not large enough to cause Neptune and Uranus to diverge from their orbits, some astronomers speculated the existence of still more planets beyond Pluto. Hence, Pluto's discovery, too, seems to be more remarkable coincidence than testimony to mathematical prowess. More recent work suggests that these orbital discrepancies do not actually exist and are due instead to plotting the planets' positions on the inexact star charts that existed until recently.

Impact

Neptune's discovery resonated on several levels in the world of the mid-1800s. 1) It expanded the size of the solar system by a factor of 50%; 2) It served to validate Newton's and Kepler's laws of physics and celestial mechanics; 3) It heated up the Anglo-French scientific rivalry; and 4) It made the universe seem more understandable and predictable.

Of Leverrier's part in this discovery, the reigning French astronomer Francois Arago (1786-1853) said:

In the eyes of all impartial men, this discovery will remain one of the most magnificent triumphs of theoretical astronomy, one of the glories of the Académie and one of the most beautiful distinctions of our country.

Another of Leverrier's colleagues proclaimed ". . . he discovered a star with his pen, without any instruments other than the strength of his calculations alone."

The comments are indicative of the scientific impact of this discovery; Neptune was found because it was calculated that it had to be there, and it was. This was taken to indicate that scientists were finally starting to understand the laws that made the cosmos work. Scientists took this as validation of Newtonian physics, Keplerian celestial mechanics, and the sheer power of science properly applied. It is also noteworthy that Neptune is the only planet discovered in this manner. Uranus, the first telescopically discovered planet was found as the result of a search, but one that was not mathematically directed. A position was calculated for Pluto, but Pluto was found only after a laborious search and was not nearly as close to the calculated position as was Neptune.

The discovery of Neptune raised additional questions about the place of God in the universe. The discoveries of "deep time," fossils, and Uniformitarianism by geologists reduced the authority once held by the Book of Genesis as an accurate accounting of the early history of the Earth. Similarly, the discovery of Copernican astronomy, the mapping of the heavens, and the more recent discoveries of the sheer size and age of the universe seem to leave less necessity for an almighty Creator to form, maintain, and rule the universe. Instead, the universe seemed to become increasingly mechanistic as the powers of scientific prediction and explanation increased.

In scientific circles, this successful prediction led to continuing efforts to mathematically "discover" still more planets. Noting discrepancies in Mercury's orbit, Leverrier calculated the orbital parameters of an inner planet he named Vulcan. In spite of repeated sightings of candidates, it was eventually determined that Vulcan does not exist. In a similar vein, many astronomers have determined orbits for planets beyond Neptune, suggesting from one to four trans-Neptunian planets might exist. In fact, the paths of the Pioneer and Voyager space probes were tracked as they left the solar system to determine if any of the probes were gravitationally influenced by large unknown planets in the outer solar system. To date, no studies have shown the existence of a single large planet beyond Neptune, although a number of trans-Plutonian asteroids have been located.

The events of 1846 significantly increased the status of science, rather than religion, as a model of explanation. The Enlightenment of the eighteenth century had started such a process long before Neptune's existence was even postulated, let alone confirmed. However, the manner in which Neptune's position was predicted and confirmed served notice that science was beginning to assume increasing importance in its ability to predict and explain the workings of our planet and universe.

The impact to the average person was less significant, but still noticeable. One indication of this is the headline of the London Times on October 1, 1846: "Leverrier's planet found." Both England and France found reason to exhibit national pride in the achievements of their scientists and, in fact, relations between these two nations were strained much more than relations between Adams and Leverrier until it was decided to credit both men with Neptune's discovery. It is also noteworthy that, even today, publications of the Royal Greenwich Observatory bemoan the fact that Neptune's discovery must be shared with someone from another country and examine the events that kept Britain from having sole credit for the discovery.

P. ANDREW KARAM

Further Reading

Kippenhahn, Rudolf. Bound to the Sun. New York: W. H. Freeman and Company, 1989.

Miner, Ellis D. Uranus, the Planet, Rings, and Satellites. New York: Ellis Horwood Publishers, 1990.