After reaching opposition with Earth on September 26, Neptune became the most distant planet in our solar system, offering the best viewing conditions of the year, although it remains too faint to be seen with the naked eye. At this time, Earth is positioned between the Sun and Neptune, making the planet appear opposite the Sun in the sky. The distance between Neptune and Earth is approximately 28.88 astronomical units, or about 4.32 billion kilometers.
On this occasion, we highlight the blue planet, the farthest planet in the solar system, which was discovered in 1846 thanks to mathematical calculations that preceded its observation by telescope, and which hides in its depths a world of raging winds, changing storms and perhaps diamond.
Neptune was not discovered by direct telescopic observation, but by mathematical calculations. After the discovery of Uranus in 1781, astronomers noticed that its movement did not exactly match the expected position, so they hypothesized the existence of another planet whose gravity was affecting its orbit.
A cutaway of Neptune (left) shows the theorized interior structure of the planet, including the region where the hydrocarbon atmosphere may produce diamond rain (light blue) that falls onto the core (black center). A recent experiment observed the formation of diamonds from hydrocarbons under conditions like those found in the interiors of Uranus and Neptune. Credit: Greg Stewart/Stanford Linear Accelerator Center National Accelerator Laboratory
Ironically, Galileo had already observed Neptune in 1612 and 1613, but mistook it for a star due to the limitations of his telescope. Thus, Neptune became the first planet discovered based on mathematical predictions.
An icy planet with fierce winds and faint rings
Neptune is located at an average distance of about 4.5 billion kilometers from the sun, and it needs about 165 years to complete one orbit around it, so only about one Neptunian year has passed since its discovery.
It has a diameter of approximately 49,500 kilometers, about four times the diameter of Earth, and is classified as an "ice giant" along with Uranus. Its atmosphere is composed primarily of hydrogen and helium, with some methane giving it its distinctive blue color.
One journey revealed a changing world
No spacecraft has ever come as close to Neptune as Voyager 2, launched by NASA in August 1977 and arriving at the planet on August 25, 1989, at a distance of about 4,800 kilometers from its cloud tops. At that time, it was the farthest planet in the solar system beyond the orbit of Pluto.
The mission revealed a giant dark spot nearly the size of Earth, but it later disappeared, while new storms appeared in different locations, indicating that Neptune's atmosphere is constantly changing.
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