On this day in 1846, astronomers officially discovered Neptune—the eighth and farthest known planet in our Solar System. Unlike the other planets, Neptune wasn’t first spotted with the naked eye or through regular telescopic observations. Instead, it was predicted through mathematics before being confirmed by telescope, making it the first planet ever discovered in this way.
A Distant Ice Giant
Neptune orbits more than 30 times farther from the Sun than Earth does, taking about 165 Earth years to complete a single orbit. In fact, Neptune only completed its first full orbit since discovery in 2011! With a diameter that makes it the fourth-largest planet in the Solar System, Neptune is about 17 times the mass of Earth.
This “ice giant” is composed mainly of hydrogen, helium, and icy substances such as water, ammonia, and methane. Methane is what gives Neptune its striking deep blue color, which inspired astronomers to name it after the Roman god of the sea.
A Planet of Extremes
Neptune is one of the coldest places in our Solar System, with upper atmosphere temperatures averaging -353°F. It’s also the windiest planet—its methane clouds race across the planet at over 1,200 miles per hour, nearly five times faster than Earth’s strongest hurricanes.
A day on Neptune lasts about 16 hours, and its tilted axis means it has seasons like Earth—but each one lasts more than 40 years due to its long orbit around the Sun.
Rings, Moons, and Mysteries
Neptune has six known rings and at least 16 moons. The largest, Triton, was discovered just 17 days after Neptune itself. Despite frigid surface temperatures of -391°F, Voyager 2 revealed geysers blasting icy material miles into the sky—making Triton one of the most intriguing moons in the Solar System.
Exploring Neptune
Neptune is faint and distant, making it difficult to study from Earth. Thanks to the Hubble Space Telescope and modern ground-based telescopes, we now have a clearer view of this icy world. Voyager 2 remains the only spacecraft to visit Neptune, flying by in August 1989 and giving us our first close-up look.
Though no missions are currently approved, scientists continue to dream about returning. One proposal, the Neptune-Triton Explorer, could launch in the 2040s, offering new insights into this distant and mysterious world.