The Discovery of Neptune
by Samuel Atkins
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One of the first close up images taken of Neptune by the Voyager 2 space probe in 1989, 143 years after it was first discovered. Image credit: NASA / JPL
Neptune’s discovery was not blind luck. It’s way too far and dim to have been seen with the naked eye and the sky is much too big for someone to have found this small dot in a vast sea of dots. So how did they find it? Well, we humans can be pretty clever when we want to be, and we have a lot of knowledge about how the heavens move around us based on centuries of observation and testing hypotheses. It is on the backs of those who came before that quick-minded people of the mid-1800’s were able to make Neptune the first planet to be discovered through pure mathematical prediction.
Uranus was discovered in 1781 by German-British astronomer William Herschel, becoming the most distant planet in the known solar system which had suddenly grown double in size. It was the first planet to be ‘discovered’ as the five naked eye planets had been known of since antiquity. By this time, we had a fairly decent grasp on the orbital mechanics of planets thanks to physicists like Johannes Kepler and Isaac Newton. The orbit of a planet around the Sun is created by a combination of the Sun’s gravity pulling the planet inward and the planet’s lateral motion moving it around the Sun and preventing itself from ultimately falling into it. The speed of the planet in its orbit generally depends on its distance from the Sun, with a further planet moving slower around the Sun than a closer planet. A consequence of this is that planets will travel at different speeds with faster, inner planets overtaking slower, further planets.
The motion of planets can be affected not just by the gravity of the Sun but by the gravity of other planets as well. In the case of our solar system, the two most massive planets, Jupiter and Saturn, have the most significant effect. We also know that the closer that planets get to each other, the stronger the gravitational force between them becomes. Whenever Jupiter passed close to Uranus in its orbit, which happened roughly every 14 years, Uranus was observed to “wobble” in its orbit. The same would happen when Saturn passed by in late 1805, which is only about a third as massive as Jupiter but also only two thirds of the distance. These disturbances on Uranus’ orbit by the gravity of other planets are called perturbations. These interactions are commonplace throughout the solar system.
This simulation shows the orbits of Uranus (inner circle) and Neptune (outer circle) around the Sun (center star). As Uranus catches up to Neptune in its orbit, Neptune's gravity pulls it along and makes it move faster. Once Uranus passes Neptune, it starts to move away from it and Neptune's gravity continues to pull back on it, slowing Uranus down. The purple arrow extending from Uranus represents the strength and direction of pull that Neptune's gravity has on Uranus' trajectory. Video credit: Miki P
In 1821, with Uranus about halfway through its first revolution around the Sun since its discovery, French astronomer Alexis Bouvard had made enough careful observations of the planet to develop and publish an astronomical table which charted the predicted future positions of the planet based on Newton’s laws. Over the next few decades, astronomers noticed some significant irregularities, even after accounting for perturbations from other known planets. Specifically, Uranus was off in how far along it was in its orbit, sometimes further ahead and later trailing behind where it should have been. The planet was also off in how far it was from the Sun, sometimes found further than it should have been. There was much debate over what was causing this. Some questioned if they had found the limits of Newtonian physics so far from the Sun, while others insisted that there must have been some unknown variable.
It was hypothesized that there must have been another massive object, such as another planet, beyond Uranus with enough gravity to affect its trajectory. But the math needed to add up. How massive must an object be and where must it be positioned to cause the specific deviations that were measured in Uranus’ orbit? Two men in particular are known to have made these precise calculations independently but at about the same time: John Couch Adams in England and Urbain le Verrier in France.
Left: Berlin Observatory at Linden Street, where Neptune was first observed. Image credit: Carl Daniel Freydanck
Right: The 9-inch refractor telescope that Galle used to locate Neptune in 1846. Image credit: tm-md
John Couch Adams, an undergrad at the University of Cambridge, sent his calculations to a skeptical John Challis at the Cambridge Observatory in September 1845, who was reluctant to join the frenzied search for the eighth planet. While Adams was sure of his calculations, Challis was unable to find it. Next month, Adams attempted to correspond with George Airy in Greenwich, but they failed to reach a clear understanding.
Le Verrier sent his own calculations to the new Berlin Observatory in Germany. They received the letter on September 23, 1846, and on that very night, Johann Galle took aim at the region of the sky that le Verrier specified using the observatory’s 9-inch-wide refractor telescope. He found Neptune between the star Deneb Algedi in the Capricornus constellation and the star Iota Aquarii in the Aquarius constellation, within just 1° from Saturn and le Verrier’s prediction (pictured above).
With the planet’s position confirmed, it was retroactively discovered that Neptune had been observed multiple times before, unknowingly. Galileo laid eyes on the planet twice in December 1612 and January 1613, mistaking it for an unassuming faint star located near Jupiter. Most heartbreakingly, John Challis had spotted Neptune twice during his search just one year prior, but due to not having up-to-date star charts, he failed to identify the planet. He later expressed remorse for not taking the pursuit more seriously as he’d missed out on an historic opportunity. On the bright side, it vindicated John Couch Adams’ math as being mostly correct (though he was about 12° off), though he gave full acknowledgment to le Verrier and he held no hard feelings towards Challis or Airy.
While this wouldn’t be the last time gravitational oddities sent several scientific fields into a frenzy, the discovery of Neptune ended up being a triumph of Newton’s laws rather than a refutation of them. It demonstrated that with the proper application of scientific knowledge and mathematics, you can quite literally predict the future and find the unfindable. It is a testament to the ingenuity of humans both past, present and, hopefully, future.