Pluto as seen by New Horizons on July 13, 2015 from a distance of 768,000 km (476,000 miles) from the surface. Image credit: NASA/JHUAPL/SWRI

Jul 14, 2015 Finding Pluto

Sen—Pluto is in the news, as well it should be. For the first time in human history we’re getting a close look at it. But it’s not the first time in history we’ve seen it! It was discovered in 1930… but even that wasn’t the first time it was actually seen. And to really understand it, we have to go back a wee bit further, to 1781.

For thousands of years, five planets were known to humans: Mercury, Venus, Mars, Jupiter, and Saturn. These lights in the sky moved amongst the background stars (the word “planet” is derived from the Greek word for “wanderer”). We now know the Earth is a planet as well, so you can fairly say there were six in total.

But in March of 1781, astronomer William Herschel was scanning the skies with a 15-cm telescope he built himself, looking for double stars to add to his catalog, when he stumbled upon an object that showed an obvious disk. It also moved, though slowly, and it was determined to be a planetary object outside the orbit of Saturn. He dubbed it Georgium Sidus—Latin for “George’s Star”—after his sponsor King George III, but other astronomers protested, and it was eventually given the name Uranus, sticking with the usual nomenclature of ancient Greek/Roman gods.

Over the decades, though, Uranus displayed a problem: It wasn’t where it was supposed to be. Using the equations derived by Newton and others to predict planetary positions, Uranus kept drifting from the mathematical predictions. It was proposed that another planet as yet unseen may exist further out which was tugging on Uranus. In 1846, mathematician Urbain Le Verrier predicted where it would be, and sent a letter to Johannes Galle at the Berlin Observatory. Galle found Neptune that very night, about a degree from the predicted position!

But there was still a problem: Even accounting for the masses of Uranus and Neptune, neither planet were exactly where they should be. They stubbornly moved off from predictions. Was there another planet?

The search for the so-called Planet X was on. This mission was picked up by Percival Lowell, an American astronomer who had made a name for himself promoting the idea that Mars had canals (and therefore intelligent life) on it. Lowell started a program using a dedicated camera that would take images of the sky night after night; someone would then painstakingly compare the images to look for moving objects.

In 1930, Clyde Tombaugh hit paydirt: A moving object well past Neptune. Planet X was found!

But there was a problem. Given its brightness and distance, it was too small, no more than Earth-mass. That was far too lightweight to affect Uranus and Neptune. It wasn’t until the 1980s that this problem was finally solved; when Voyager 2 flew past the two planets, it found their masses were about a half percent off previous estimates. When the new, more accurate masses were put into the planetary motion equations, Uranus and Neptune were found to be just where they were predicted to be.

In other words, Pluto was found by error!

Well, maybe that’s a tad unfair. It was found through sheer effort, elbow grease and determination. But the reason anyone was searching for it to begin with was due to an error.

As it happens, Pluto had been seen many times prior to Tombaugh’s discovery in 1930. This is common in the search for fainter objects; they appeared in earlier photos or drawings, but hadn’t been noticed. Pluto was photographed at least 20 years prior to 1930, several times in fact. But it was so dim and moving so slowly no one noticed.

Incidentally, estimates of its mass dropped with time as well. When its moon Charon was discovered, astronomers could get Pluto’s mass from the orbital period of the two around each other, and it was found to be only about 0.002 times the Earth’s mass, far less massive even than our Moon.

Because it was small, astronomers wondered if other objects like it existed out past Neptune, in what is called the Kuiper Belt region of the Solar System. In 1992 the first such object was found, called 1992 QB1. In 2005 astronomer Mike Brown found Eris, a Kuiper Belt Object (or KBO) more massive than (and very nearly the same size as) Pluto. It’s possible bigger objects exist in the icy depths beyond Neptune; but given that we haven’t found any after years of searching makes it likely Pluto is the biggest, or at least one of the biggest.

In the 85 years since its discovery we’ve learned a lot about the distant world—including its likely composition, and the fact that it has a total of five moons—and it’s been the subject of great debates (and heated arguments). Now though, literally today, our learning will take a huge jump, as big as the discovery of Pluto in the first place. The New Horizons spacecraft is passing just 12,500 km over Pluto’s surface, and we’ll get the first close-up photos of it. What we’ve seen so far is tantalizing, but what we’ll be seeing soon will be amazing.

We have a long history of exploring strange, new worlds. And don’t think Pluto is the last! There are thousands more KBOs out there, just waiting for us to take a look. Far from being the final frontier, Pluto is the first we’re seeing of a new frontier.