Do you know what today is? It’s the day on which Neptune has finally completed one full orbit of the sun, since its discovery in 1846. One orbit in just under 165 years.
This fascinating video (courtesy NASA, ESA, and G. Bacon and M. Estacion (STScI)) puts that one Neptunian orbit into an Earth timeline perspective
Neptune is a giant ball of ice and gas some 17 times the mass of Earth, orbiting the sun beyond Uranus. Ordinarily, a discovery this astonishing would have been the lead story in every media outlet in the world — but it’s pretty much guaranteed that no living human heard even a rumor about the announcement.
That’s because we automatically think of the word year in very local terms. Technically, a year is the time it takes any planet to complete one orbit of its star. Where we live, that adds up to 365 days and small change. But for Neptune, discovered in 1846, a single orbit takes a bit less than 165 Earth years — and the planet has now completed exactly one of those Neptunian years since it was first spotted by the German astronomer Johann Gottfried Galle at the Berlin Observatory. It has thus at last returned to the same point in the solar system where it was when it was first identified.
Before Neptune
Uranus was the research in the eighteenth century – the seventh planet of the solar system, until then considered the last – that led astronomers to believe that there could be another planet in the solar system even more distant.
The British astronomer William Herschel and his sister Caroline discovered Uranus in 1781, expanding the boundaries of the solar system, but soon realized that its orbit did not behave as predicted, according to the laws of Kepler and Newton.
In 1821, French astronomer Alexis Bouvard, studying Uranus, thought that perhaps another planet might be exerting some kind of attraction changing its movement, but there was a delay of 20 years before the first calculations were done.
The Frenchman Urbain Le Verrier and the Briton John Couch Adams, both mathematicians and astronomers, independently predicted where this “mysterious” planet was supposed to be by calculating how the gravity of a hypothetical object could affect the orbit of Uranus.
Le Verrier, who was the director of the Observatory of Paris, sent a note to the German astronomer, Johann Gottfried Galle, in which he discussed the possible location of the object. After two days of observation, on the 23rd of September 1846, finally Galle identified Neptune as a planet, to less than one degree from the position calculated by Adams and Le Verrier. It takes Neptune 164.79 Earth-years to go full circle.
NASA considers the discovery one of the major astronomical achievements since the Newtonian theory of gravity, contributing to better understanding the universe. However, Galle was not the first to see Neptune.
In December 1612, the astronomer Galileo Galilei had the privilege of seeing it while watching Jupiter and its moons. However, as his notes reveal – which pointed in exactly the position of Neptune – the Italian scientist mistook it for a star.
The discovery of Neptune doubled the size of the known solar system as the planet is 4.5 billion miles from the sun, 30 times farther than Earth.
This video (courtesy of NASA, ESA, G. Bacon, and Z. Levay (STScI)) shows a speeded up view of Neptune rotating, using images taken every four hours by the Hubble Space Telescope
The nomenclature of the new planet was also the subject of dispute among scientists, who wanted to christen it with their own names. The scientific community has chosen Neptune, Roman god of the sea, a mythological name in line with the other planets.
All that was a big deal for science in general. Ever since Neptune’s discovery, astronomers have used strange motions in objects they can see to deduce the existence of ones they can’t. Dark matter, for example, the invisible stuff that dominates the mass of the universe, was first detected by its gravitational influence on visible stars. Dark energy, the force that makes the universe expand faster every year, was found by clocking the speeds of distant galaxies. We know that giant black holes lurk at the center of the Milky Way and other galaxies because stars and gas clouds are whipping around too fast to be explained by anything else.
In 1989, Voyager 2 became the first spacecraft to observe the planet Neptune. Passing about 4950 km above Neptune’s north pole, Voyager 2 made its closest approach
Celebrating Neptune’s Birthday
In honor of the occasion, the people who run the Hubble Space Telescope have taken a set of four anniversary photos of the big blue planet, one every four hours or so to capture a full rotation (Neptune’s year may be much longer than ours, but its day lasts only 16 hours).
The sound of Neptune
It’s now early winter in Neptune’s northern hemisphere — a season that will last 40 Earth years or so. The planet’s solid surface is invisible, shrouded by a thick atmosphere made mostly of hydrogen and helium. White clouds of methane ice swirl through the Neptunian air — a feature nobody expected when Voyager 2 took the first and last closeups of the planet in 1989. Neptune also has a faint ring, but it’s a poor cousin to Saturn’s magnificent multicolored tutu.
A look at the distant planet Neptune, and its mysterious moon of Triton
Sometimes the technique backfires. Pluto was found because there seemed to be something pulling on Neptune, just as Neptune pulled on Uranus. But while the recently demoted dwarf planet is in the right place, it is far too tiny to have any such influence. It turned out that the anomalies in Neptune’s orbit weren’t really there after all.
Now and then, scientists can’t agree on whether there’s an effect or not: a couple of astronomers think some comets are moving strangely due to a still undiscovered giant planet far beyond Neptune, but most of their colleagues doubt it’s true. All the same, using the visible to find the invisible is one of the most powerful tools astronomers have for understanding a cosmos that hides so much of itself from view. And the discovery of Neptune, exactly one Neptunian year ago this week, marked its debut.
Filed under: Neptune
