's First Issue, the Solar System Grew by a Planet

Scientific American

3 days ago

In astronomy, 180 years is a very long time—maybe not for the goings-on in the universe but certainly for our understanding of it.
When Scientific American published its very first issue 180 years ago this month, our view of the cosmos was substantively different. We had no idea of the scale of the universe or even if anything existed outside our Milky Way galaxy. We didn't know how stars were born, what powered them or where comets came from—or that supernovae were even a thing.
Closer to home, astronomers were wildly guessing about how our solar system formed and how Earth's moon came to exist. Heck, we didn't even know how many planets were in the solar system!
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To be fair, we still don't. But our understanding of our sun's family was still pretty sketchy in August 1845, and it was scarcely a year later that our solar system would grow by an entire planet.
For all of antiquity, Saturn was the most distant planet known to humanity. It wasn't until 1781 that German-British astronomer William Herschel reported seeing a slowly moving 'comet' in the constellation Taurus as he scanned the skies with his telescope. It took two years before orbital calculations showed it was not a comet at all but instead a giant planet orbiting the sun beyond Saturn. Uranus, the first new planet ever discovered, was found by accident.
Over the ensuing decades, though, astronomers saw that Uranus was misbehaving. Using the mathematical equations governing gravity and orbits, they calculated the shape of Uranus's orbit and used that to predict where the planet should be in the sky. Observations indicated that the actual position of Uranus significantly deviated from what was predicted, however. Sometimes it 'pulled ahead' of the calculated location, and sometimes it lagged behind.
Many astronomers wondered these anomalies were caused by another planet lurking unseen beyond Uranus, which itself was, at best, barely visible to the naked eye; a planet farther out would be much dimmer and could have easily escaped detection.
But where was it? The sky is huge when you're trying to search for a dim point of light over thousands of square degrees; remember, back then, skywatchers only had their telescopes and eyes. No cameras or detectors were available. Searching for a faint, distant world was like looking for a planetary needle in a cosmic haystack.
The mathematics of orbital mechanics offered a shortcut, though. If you assumed a given orbit for the planet, then its position over time could be roughly calculated by its effect on Uranus. This sort of 'X marks the spot' calculation can be done in moments on today's computers, but in the mid-19th century it was done by hand, and the word 'tedious' hardly describes the scope of the work.
Still, in the 1840s, independently of each other, two men attempted exactly this. John Couch Adams, a British mathematician and astronomer at the University of Cambridge, worked on the calculations in his spare time starting in 1843. He reported his findings to James Challis, director of the Cambridge Observatory at the time, and to England's Astronomer Royal George Biddell Airy, both of whom treated it chiefly as an interesting bit of math rather than a guide for finding a potential planet. In their defense, however, Adams's calculations were incomplete and not yet suitable to be put into action.
At the same time, French astronomer Urbain Jean Joseph Le Verrier was also working on calculating the presumptive planet's position. He announced his results at a public meeting of the French Academy of Sciences on June 1, 1846. I'll note that Le Verrier only disclosed his calculated locations for the planet on the sky, not his estimates for its mass or orbit.
Still, this was enough to cause a minor panic across the Channel when Le Verrier's news reached Cambridge, with Airy realizing the similarity to what Adams was working on. Because discovering the first new planet in 65 years was a matter of great scientific and national pride, Challis went to the telescope and began an urgent, earnest search. Like the calculations themselves, this was a tedious undertaking that involved scanning the sky and comparing what was seen with hand-drawn, not entirely accurate star maps. Making matters even worse, Adams had been working on new solutions to the planet's location and his calculations were flawed, so Challis was looking in the wrong part of the sky.
On August 31, 1846, Le Verrier made another presentation to the academy, this time also reporting the putative new world's calculated mass and orbit. Three weeks later, assistant astronomer Johann Gottfried Galle at the Berlin Observatory read of Le Verrier's work. Assisted by a student named Heinrich Louis d'Arrest, Galle took to the observatory's 24-centimeter telescope on the evening of September 23 to look for the planet. Using better star maps than the British had, they sighted the world in the early morning hours of September 24, less than a degree from the position Le Verrier had predicted. As the story is told, Galle read off the coordinates of stars he saw through the eyepiece, and at one point d'Arrest excitedly shouted, 'That star is not on the map!'
Thus, Neptune was discovered.
Le Verrier is credited for the discovery work, though Adams, upon insistence from the British at the time, is generally also given co-credit. This is controversial because it's not clear just how accurate Adams's results were— see the article 'The Case of the Pilfered Planet,' by science historians William Sheehan, Nicholas Kollerstrom and Craig B. Waff, in the December 2004 issue of Scientific American for details.
Still, while Uranus was found by chance, Neptune was found by math (with a helping hand from happenstance).
Ironically, that night in September 1846 was not the first time it had ever been observed. Galileo took copious notes when, centuries earlier, he first turned his crude telescope to the sky; we now know he saw Neptune in 1612 and 1613 but mistook it for a star. (Too bad; had he figured it out, he would've been famous.) Neptune had been spotted many other times before as well but passed over for the same reasons. In a very cruel irony, records reveal that Challis himself saw Neptune twice in August 1846 but failed to notice its true nature.
I've observed Neptune many times through my own small telescope; it's a wan aqua dot, barely discernable from a faint background star. Still, seeing it myself—knowing those photons took many hours to fly across billions of kilometers of space only to fall into my telescope and onto my retina—has been a thrill. Of course, I've had a huge advantage over Galle, with modern star maps and software that have told me exactly where to look, but that has only shone a spotlight on what an achievement the discovery was almost 180 years ago.
And what of the 18 decades since? The universe is vastly larger than we then imagined in 1846, and we can now find Neptune-like planets orbiting other stars by the hundreds. We've also discovered thousands more objects orbiting the sun beyond Neptune, including Pluto. It's almost routine. As for Neptune itself, we've observed it with an array of space telescopes and even sent a space probe, Voyager 2, to fly past the enigmatic giant planet, allowing us to see its array of bizarre moons and weather patterns up close.
Scientific American has been there along the way, too, with its first issue in August 1845 nearly coinciding with the discovery of the last known major planet of the solar system. Researchers have taken immense steps in unlocking even deeper secrets of the cosmos over the past 180 years, and during that time, this magazine has played a major role in informing the public about their findings. I'm proud to be a part of this long-running adventure.