Discovery of Neptune
Neptune, the 8th and final planet from the Sun, is the only planet in our solar system that is not visible to the naked eye.
As a result it was the first planet to be discovered by mathematically calculating its position based on its gravitational influence on other solar system bodies.
However Neptune was “seen” much earlier than we credit its discovery. The planet can be detected using even very low-powered telescopes, but it is such a great distance that it appears fixed in the night sky, particularly as it enters retrograde.
It is for this reason that Galileo, the first thought to have seen Neptune, mistakenly categorized it as a star when he observed it around 1612.
It wasn’t until 1821 when astronomers began to look more closely at Neptune. In that year Alexis Bouvard published an astronomical table that predicted the orbital path of Uranus, Neptune’s nearest neighbor.
However, follow-up observations of the pale world revealed that something was influencing its motion, making it veer off course. Bouvard, confident in his work, concluded that another object was gravitationally influencing Uranus’ motion.
Several calculations were offered to pinpoint the position of this mystery planet, the most accurate, ultimately, performed by Urbain Le Verrier, but astronomers at the largest observatories had little interest in doing the leg work.
But a student at the Berlin Observatory suggested to Johann Gottfried Galle, a principle astronomer at the observatory, that they could simply point the telescope in the direction of where the planet was predicted to be and compare what they saw to a recent star chart created of the region. By looking for discrepancies they might be able to located this new planet.
That first night of observation, they were able to locate and identify Neptune. As it happened, they had actually observed it twice before, but had overlooked it, believing it to be a star, and not bothering to check its position against a star chart.
Naming of Neptune
As with the discovery of Uranus, the revelation of a new planet set off a race to name the bluish orb.
Verrier, upon claiming the right to name the planet since it was his calculation that ultimately lead to its discovery, offered the name Neptune. (Even falsely claiming that the French naming authority had approved his suggestion.)
Soon after, Verrier himself changed his mind, wanting to name the new planet after himself. However, there was a significant refusal from British authorities, since their push to have Uranus named after its discoverer, Sir William Herschel, had been similarly shot down.
Therefore Neptune, the Roman God of the Sea, eventually won out.
Structure of Neptune
Neptune is structured very similarly to its nearby cousin, Uranus. The core, containing about 1.2 times the Earth’s mass, is composed of rock, iron and nickel.
The core is surrounded by a thick, liquid mantel ranging in temperature from 2,000 – 5000 kelvin. Some models place the mass of the mantel at nearly 15 times that of Earth.
Interestingly, a mantel of this type is often referred to as “icy”, despite the incredibly high temperatures. This is owed to the fact that because of the high pressure, the mantel exists in a highly conductive semi-liquid ocean.
The outermost part of Neptune is the atmosphere that gets ever more thin as it moves away from the core. At nearly 20% of the planet’s radius (and approaching 10 Earth masses), the atmosphere is made up of various gasses, including methane ammonia and water.
Storms on Neptune
The most famous storm in the solar system is the Great Red Spot on Jupiter, however it is not unique. Neptune has a very similar feature.
Moons of Neptune
Only 17 days after Neptune was discovered, astronomer William Lassell found the planet’s largest moon, Triton. Unlike every other moon in our solar system, Titan has a retrograde motion, leading scientists to believe that it is likely a captured Kuiper Belt object and didn’t form in orbit around the planet.
In addition to Triton, Neptune has an additional 12 planets. Though even combined the remaining moons are less than 1% of Titan’s mass. And Titan is the only one with enough mass to have formed into a spheroidal shape.
With the exception of Triton, none of the moons were discovered until after 1949, with most of the being discovered by Voyager 2 in 1989.
Like the other Jovian planets (Jupiter, Saturn, Uranus and Neptune) Neptune has rings. Though, unlike Saturn the rings around the rest of the worlds are difficult to see. Typically they require special instrumentation and telescopes to make out.
Neptune has four identified rings. They are mainly composed of ice and rock particles, perhaps remnants of an early moon or other object that was broken apart by the planet’s gravity.
Neptune by the Numbers
- Equatorial Radius: 24,764 ± 15 km (3.88 Earth Radii)
- Mass: 1.0243×1026 kg (17.15 Earth Masses)
- Gravity (at Equator): 11.15 m/s2 (1.14 times Earth’s gravity)
- Orbital Period: 60,190 Earth Days (~165 Earth Years)
- Mean Orbital Radius: 4.5×109 km (~30 times the Sun to the Earth)
- Rotational Period: 0.67 Earth Days (16 hours, 6 minutes, 36 seconds)
- Number of Moons: 13
- Number of Rings: 4