Neptune: A Ceaseless Discovery

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

Neptune is the only planet in our solar system that can’t be seen with the naked eye. Discovering it was no easy task. But even more difficult than Neptune’s discovery was finding other details regarding it. The flyby mission of Voyager 2 did answer many important questions concerning Neptune, but it raised a few questions as well.

An illustration showing Voyager 2's flyby mission of Neptune.
Voyager 2 helped us discover that Neptune has some of the fastest winds in the solar system. (Image: Vadim Sadovski/Shutterstock)

Exploring Neptune has always been a challenge, but even discovering it wasn’t easy. Neptune is the only planet in our solar system that can’t be seen with the naked eye. Although Uranus needed telescopic observations to be confirmed as a planet, it can be spotted with the naked eye. Neptune cannot. It’s too faint because it’s so far away. So how was Neptune discovered?

In the early 1820s, Alexis Bouvard made predictions for the orbit of Uranus based on gravitational interactions to be expected between Uranus, the Sun, and the other known planets. However, later observations showed small discrepancies between the predictions and the actual orbit. A helpful theory came from Mary Somerville, who wrote in her famous science book, On the Connexion of the Physical Sciences, that the orbital motion discrepancies for Uranus may be due to an undiscovered planet.

Her writings inspired British mathematician John Couch Adams to start looking for an unknown planet and to refine predictions of where it might be based on the Uranus orbit discrepancies. Adams made calculations and tried to interest others in a systematic search, but no further progress was made.

Learn more about exploring the Earth-Moon system.

The Calculations of Urbain Le Verrier

A portrait of French mathematician Urbain Le Verrier.

Urbain Le Verrier calculated the location of undiscovered Neptune. (Image: Magnus Manske/Public domain)

Now, around the same time, a French mathematician and astronomer, Urbain Le Verrier, developed his own calculations for where the undiscovered planet should be. In 1846, Le Verrier convinced a German astronomer, Johann Galle, to look for a planet where Le Verrier predicted it. And there it was, only 1° away from his prediction.

The Credit for the Discovery

The discovery of Neptune was one of the great triumphs of scientific prediction and confirmation. Which of course meant it was also shrouded in controversy. Who should get credit for the discovery? Somerville, Bouvard, or others who got the search going in the first place? The discoverer, Galle, or the predictors who offered a precise location of where to look: Le Verrier, or possibly Adams?

Galle couldn’t have done it without predictions by Le Verrier. By contrast, if Galle had used Adams’s predictions instead of Le Verrier’s, he never would have found Neptune. On the other hand, Le Verrier would never have convinced Galle to look for a planet if he hadn’t shown how close his predictions were to Adams!

So, Adams, and the British, insisted they deserved credit as well, even though Adams’s predictions were 12° away from Le Verrier’s. Ultimately, both Adams and Le Verrier were given credit for the discovery along with Galle, mostly for political reasons rather than scientific ones.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.

The Naming of Neptune

The decision to name the planet ‘Neptune’ also involved some strife. Galle wanted it named ‘Janus’. A British astronomer who had neglected to notice Neptune among his own observations suggested the name ‘Oceanus’.

Supporters of Le Verrier suggested naming the planet ‘Le Verrier’. Recognizing there was no support for that, Le Verrier himself proposed ‘Neptune’, which finally became the accepted name. The naming of Neptune also finally convinced the British Nautical Almanac to abandon their half-century practice of referring to Uranus as ‘the Georgian’.

Learn more about mighty Jupiter, the ruling gas giant.

The Journey of Voyager 2 toward Neptune

Voyager 2 approached Neptune in August of 1989; scientists had to plan the entire encounter well ahead of time. Voyager 2 was over four billion kilometers from Earth for the Neptune flyby. Any commands the scientists on Earth wanted to send to the spacecraft would take almost four hours traveling at the speed of light to reach Voyager.

This also meant that any data collected by Voyager would take four hours to reach Earth. Voyager was traveling almost 100,000 kilometers per hour as it made its closest approach to Neptune, so the core part of the flyby itself would last only hours. There wasn’t going to be any leeway to adjust trajectories or make last-minute decisions. The flyby itself would be largely over before the first signals had time to get back to Earth!

Learn more about how our Sun defines our solar system.

The Discoveries Made by Voyager 2

The Voyager 2 flyby of Neptune established that even similar planets can harbor drastically different features. For example, instead of the weirdly calm atmosphere of Uranus, Neptune turns out to have some of the fastest winds in the solar system. Neptune also has our solar system’s first and only example of a large spherical moon, Triton, which was captured rather than formed from an accretion disk around the planet.

The Discovery and Naming of Neptune Rings

A wide angled view of the rings of Neptune, taken by the Voyager 2.
Voyager 2’s flyby in 1989 found five rings around Neptune. (Image: Unknown author/Public domain)

Voyager 2’s flyby in 1989 also found five rings around Neptune. Three of the rings are narrow, and the other two are much broader. The outermost ring, the one farthest from the planet’s location, is aptly named for John Couch Adams. The Adams ring turns out to have unusual clumpy features that were observed before Voyager 2 arrived at Neptune.

Scientists initially thought there were partial ring arcs surrounding Neptune rather than a full ring. But Voyager data proved that these arcs were actually just clumps in a complete ring. A ring named for Le Verrier is appropriately brighter and closer to the planet. The ring named for Galle, the first person to see Neptune, is the closest ring of all.

Questions Raised by Voyager 2’s Ice Giants Mission

Although Voyager 2’s mission has answered a lot of questions concerning the ice giants, it has raised a few questions too. Why is the internal heat flow of Uranus evidently so low? What’s happening in the deep interiors? What does the rest of Triton look like? NASA is investigating what sorts of missions would be possible, with launch dates in the late 2020s and 2030s. Odds are that a mission to only one of the ice giants will be financially feasible.

This has led scientists to start thinking about which ice giant, Uranus or Neptune, should be visited. Both planets offer compelling cases for a mission and provide unique opportunities. Uranus would offer us the chance to examine a sideways rotating planet and understand the seasonal dynamics of such a world. Neptune would offer an active atmosphere, and Triton, a unique round moon captured from the Kuiper belt.

Either way, investigation of one of the ice giants will also provide info about the other, so fingers crossed that such a mission concept becomes a mission reality.

Common Questions about Neptune: A Ceaseless Discovery

Q: Who first discovered Neptune and when?

Neptune’s discovery was attributed to Urbain Le Verrier, Johann Gottfried Galle, and John Couch Adams since they all played important roles in its discovery.

Q: What discoveries have been made on Neptune?

Despite being similar to Uranus, Neptune surprisingly has some of the fastest winds in the solar system. Neptune also has an irregular satellite called Triton. Neptune is also confirmed to have rings.

Q: Who named Neptune?

The name Neptune is derived from the Roman god of sea. However, the planet’s name, Neptune, was proposed by Urbain Le Verrier, one of the discoverers of Neptune.

Q: Is Neptune a gas giant?

Neptune is a solid planet core surrounded by a mixture of thick water, ammonia, and methane. It is also known as an ice giant.

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