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Starts With A Bang

Neptune’s discovery 175 years ago was our first success finding dark matter

Gravitation, all on its own, can reveal what’s present in the cosmos like nothing else.
In April of 1990, the Voyager 2 spacecraft flew by Neptune, snapping a series of incredible images of our Solar System's outermost planet. 150 years prior, nobody knew that our Solar System would wind up containing 8 planets, but a few scientists suspected, from the evidence of Uranus, that it might be out there. (Credit: Time Life Pictures/NASA/The LIFE Picture Collection)
Key Takeaways
  • Neptune is the first planet whose existence and properties were predicted in advance of its discovery.
  • Its discovery at the Berlin Observatory the very night a letter arrived predicting its existence goes back to September 23/4, 1846.
  • The original “dark matter,” its discovery continues to hold scientific lessons for our modern times.

On September 23, 1846, astronomers Johann Galle and Heinrich d’Arrest discovered our 8th planet: Neptune.

As observed in 1998 by the infrared 2-Micron All Sky Survey (2MASS), Uranus and Neptune appear blue and have their own lunar systems orbiting them. While Uranus was discovered by pure chance in 1781, Neptune was expected, and found precisely where the mathematics indicated it should be in 1846. (Credit: Two Micron All Sky Survey (2MASS), UMASS/IPAC/Caltech, NASA and NSF, Acknowledgement: B. Nelson (IPAC))

Unlike Uranus or Pluto, however, the finding wasn’t purely serendipitous.

These images of Neptune, from October 7, 2017 with the Hubble Space Telescope, shows the presence of clouds, bands, and varying colors and temperatures across Neptune’s upper atmosphere. Neptune completed its first complete revolution since its discovery in 2011, and now journeys on its second.(Credit: ESA/Hubble and NASA, Acknowledgement: Judy Schmidt.)

Neptune’s existence and position were predicted prior to discovery: a theoretical triumph.

Kepler’s second law states that planets sweep out equal areas, using the Sun as one focus, in equal times, regardless of other parameters. The same (blue) area is swept out in a fixed time period. The green arrow is velocity. The purple arrow directed towards the Sun is the acceleration. (Credit: Gonfer/Wikimedia Commons, using Mathematica.)

From the 1600s, Kepler’s and Newton’s Laws described planetary motion precisely.

Although this is a modern, infrared view of our Solar System’s 7th planet, it was only discovered in 1781 through the serendipitous observations of William Herschel. Although numerous others had seen it before, Herschel’s observations led to Uranus’s identification as a new outer planet. (Credit: ESO)

In 1781, Uranus’s discovery gave us a new testing ground.

A very old orrery of the planets and moons in the solar system. An examination of this points to an origin in the first half of the 19th century: well after the discovery of Uranus and some of its major moons, but before the discovery of Neptune. (Credit: Armagh Observatory, College Hill.)

After multiple decades, something was clearly amiss.

dark matter
For decades, Uranus was observed to move too quickly (L), then at the correct speed (center), and then too slowly (R). This would be explained within Newton’s theory of gravitation if there were an additional, outer, massive world tugging on Uranus. In this visualization, Neptune is in blue, Uranus in green, with Jupiter and Saturn in cyan and orange, respectively. It was a calculation performed by Urbain Le Verrier that directly led to Neptune’s discovery in 1846. (Credit: Michael Richmond of Rochester Institute of Technology.)

Uranus initially moved quicker than predicted, then as anticipated, and, finally, too slowly.

Uranus, shown at right, appeared to orbit in violation of the laws of planetary motion. Rather than suggest a modification to the laws of gravity, simply adding in an undiscovered mass with the right parameters beyond Uranus (like Neptune, at left) could explain the observed anomalies in its orbit. (Credit: NASA/Voyager 2)

A spectacular possible explanation arose: the existence of an additional massive outer planet.

The orbital dynamics of the planets matched the law of gravity extremely well, with the relative newcomer, Uranus, providing the biggest outlier. Determining the mass, position, orbital distance and inclination of a potential planet beyond that caused these orbital perturbations was a herculean task. (Credit: NASA/JPL-Caltech/R. Hurt)

The induced gravitational acceleration could explain Uranus’s observed motion.

John Couch Adams, left, and Urbain Le Verrier, right, were the two theoreticians who worked to deduce the presence and position of a potential outer planet responsible for the anomalous apparent motion of Uranus. The discovery of Neptune in 1846 brought glory to Le Verrier, but was a missed opportunity for both Adams and all of UK astronomy. (Credit: Samuel Cousins of Thomas Mogford painting (L), Goupil & Cie/public domain (R))

Two theoreticians independently sought to mathematically locate the unseen world.

From 1845 to 1846, British astronomer John Couch Adams, also working on the problem of Uranus’ orbit, proposed no fewer than 5 potential locations for a hypothetical new planet, but detection remained elusive due to mistakes on both the theorist’s and the observers’ ends. Le Verrier made his first and only prediction in 1846, which led to a near-immediate observational discovery. (Credit: J. Lequeux, Le Verrier – Magnificent and Detestable Astronomer (2013))

The United Kingdom’s John Couch Adams made six separate predictions during 1845/6.

Neptune was discovered way back in 1846, but was predicted by two men competing to discover it: John Couch Adams and Urbain Le Verrier. Adams had no notable observational support, and today the then-observatory director James Challis is remembered largely as a charlatan. Today, the two main rings of Neptune are known as the Adams and Le Verrier rings. (Credit: NASA/Voyager 2)

However, George Airy and James Challis botched the observational side, yielding no discoveries.

Urbain Le Verrier, depicted here, was an extremely talented mathematician with an interest in astronomy. In 1845, the famed physicist François Arago compelled Le Verrier to take up work on the problem of Uranus’ orbit. By 1846, Le Verrier had a solution. (Credit: Henri Chapu (monument); Herbert Hall Turner (photo))

Contemporaneously, France’s Urbain Le Verrier composed one prediction on August 31, 1846.

The New Berlin Observatory, as drawn in this 1838 painting, was the location where Urbain Le Verrier’s letter predicting the position of Neptune first arrived on September 23, 1846. Later that evening, Johann Galle and his assistant, d’Arrest, conclusively discovered the solar system’s 8th planet. (Credit: Carl Daniel Freydanck/Leibniz-Institut für Astrophysik Potsdam)

On September 23, his letter arrived at the Berlin Observatory.

Although, through Galle’s telescope at the Berlin Observatory, Neptune appeared only as a small, faint, blue disk, it did not show up on previous recorded sketches of that very region of sky, as d’Arrest proposed. On September 23, 1846, the 8th planet in our solary system, Neptune, was discovered. (Credit: NASA/Voyager 2.)

That very night, using d’Arrest’s strategy, Neptune appeared within 1° of Le Verrier’s prediction.

The grave of Urbain Le Verrier commemorates his tremendous contributions to astronomy, while history is likely to remember him as Arago did: as the discoverer of a planet with the mere stroke of a pen. (Credit: Astrochemist/Wikimedia Commons)

As François Arago noted, Le Verrier became “the discoverer of a planet with the point of his pen.”

dark matter
Today, exoplanets that cannot be directly seen or imaged can still be detected through their gravitational influence on their parent star, which causes a periodic spectral shift that can be clearly observed. (Credit: E. Pécontal)

Gravitation always reveals massive astrophysical presences, from exoplanets to gravitational lenses to dark matter.

dark matter
This image showcases the massive, distant galaxy cluster Abell S1063. As part of the Hubble Frontier Fields program, this is one of six galaxy clusters to be imaged for a long time in many wavelengths at high resolution. The diffuse, bluish-white light shown here is actual intracluster starlight, captured for the first time. It traces out the location and density of dark matter more precisely than any other visual observation to date. (Credit: NASA, ESA, and M. Montes (University of New South Wales))

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.


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