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)

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.

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)