As I wrote a few weeks ago, we are in the midst of a planetary bounty. You can now observe all the naked-eye planets — Venus, Mars, Jupiter, and Saturn, and even elusive Mercury — over the course of a single night.
If you have a telescope of practically any size, you can add Uranus and Neptune. Add a telescope as big as a corn silo and a good set of star maps, and you can add dwarf-planet Pluto to the list.
Seeing a passel o’ planets always reminds me of the toughest question I repeatedly got when I worked at Perkins Observatory.
Let’s set the scene. The lecture hall was crowded with, say, about 90 fourth graders. During the question and answer period, I fielded the usual questions about aliens and black holes, two issues that spanned the generations of fourth graders we hosted at Perkins.
Toward the end of the visit, one of the kids would inevitably and excitedly ask, “Who discovered the first planet?”
Black holes and aliens were easy, but the planet question perplexes me to this day. I usually stumbled through an answer something like this: “I don’t know because it happened so early in human history that people didn’t write things down. So nobody knows who discovered the first planet.”
But that’s a cop-out. The problem is that to answer the “who,” you also have to answer the “which one.” And the latter question is just as difficult as the first one.
Why? The definition of a planet has changed significantly over the years. The Greeks first used the word “planetes,” or “wanderer,” to refer to any astronomical object that moved, seemingly at will, against the starry background. Two millennia later, we think of a planet as a relatively large spherical object that orbits the sun.
Also, the word “discovery” means different things to different people, as we shall see.
If that fourth grader, now grown up, asked me the same question, I might answer, “British astronomer William Herschel when he discovered Uranus on March 13, 1781.” Maybe.
Using a telescope he crafted himself, Herschel was in the process of recording every star he could see. All at once, he noticed a star barely visible to the unaided eye. Over the course of observations spanning several days, he noticed that it moved.
Herschel thought he had discovered a comet. However, other astronomers calculated the orbit of the object, and it had a path more like a planet than a comet. In effect, Herschel had to be convinced by other astronomers that he had indeed discovered the first new planet since ancient times.
Because of its faint, naked-eye visibility, several astronomers as far back as Hipparchus in the second century BCE had observed Uranus. None of them was its discoverer. Herschel’s discovery counts because he was smart enough to notice that Uranus was not a star.
On the other hand, if Herschel’s observation of Uranus was a discovery, it was an accidental one. He was mapping the stars, and he stumbled upon one that moved.
In its strictest scientific sense, discovery involves making a prediction based on available data and verifying it. Herschel’s “discovery” fails on that count.
This year’s Nobel Prize in physics provides a contemporary example of prime scientific discovery. In the 1960s, Roger Penrose and Stephen Hawking used Albert Einstein’s theory of gravity, known as general relativity, to predict the existence of black holes, places where stellar material had collapsed to a point so dense that even light could not escape its enormous gravity. They did not get a Nobel Prize.
Over a period of decades, separate teams led by Reinhard Genzel and Andrea Ghez observed the orbits of stars around the center of our Milky Way galaxy. We will never be able to see a black hole. But the orbits of stars around it prove conclusively, if indirectly, that the black hole must be there.
The Nobel Prize was given jointly to Penrose, Genzel, and Ghez because Penrose’s theoretical conclusions were finally backed up with definitive evidence. (Hawking was omitted because the award is only given to living people and can only be shared by three people anyway.)
According to that stricter definition, the prize for the first true planetary discovery might go to Johann Titus, Johann Bode, and Giuseppe Piazzi for the discovery of Ceres.
Borrowing an idea from Johann Titus, Johann Bode described a mathematical harmony in the planets’ distances from the sun. Moving outward, each planet is about twice as far from the Sun as the one before it.
The Titus-Bode Law worked for the distances to all the planets known at the time. However, the sequence was missing the planet between Mars and Jupiter. As Bode wrote, “Can one believe that the Founder of the universe had left this space empty? Certainly not.”
The hunt was on. On Jan. 1, 1801, Giuseppe Piazzi observed the missing planet, which was subsequently named Ceres.
So Ceres was declared the eighth planet (including Earth in the total). Over the next few years, three more planets — Pallas, Juno and Vesta — were spotted. As British astronomer John Bonnycastle wrote, “… (T)he sun is now well known to be placed in the centre, and to have 11 primary planets moving round him, each in its own path or orbit… . The names of these planets, according to their distance from the centre of the middle point of the sun, are Mercury, Venus, Earth, Mars, Vesta, Juno, Pallas, Ceres, Jupiter, Saturn, and Uranus… .”
By mid-century, hundreds of “small planets” had been discovered. Something had to be done for fear that 19th century fourth graders would have nervous breakdowns memorizing the names of all those planets.
The astronomical community slowly began to use the term “asteroid” to refer to objects like Ceres. They were essentially creating a new category of solar-system objects. Ironically, Uranus’s discoverer, William Hershel, had suggested the new category as early as 1802, less than a year after Ceres had been discovered.
Since then, tens of thousands of asteroids have been discovered by astronomers in the gap between Mars and Jupiter, now known as the Asteroid Belt. But none of the others have the size and gravitational oomph to form themselves into spheres. Thus, Ceres is the only dwarf planet in the asteroid belt.
Over the centuries, Ceres was demoted from a planet to an asteroid and later promoted to a dwarf planet. I suppose Piazzi’s discovery doesn’t count because Ceres is no longer considered a full-fledged planet.
However, another fundamental failing invalidates Piazzi’s claim to fame. The Titus-Bode Law was wrong. The magical mathematical harmony among planetary distances simply does not exist. The Titus-Bode Law does not predict the distance of the planet Neptune, for example.
Piazzi discovered a dwarf planet in the asteroid belt based on a false hypothesis. Like Hershel’s observation of Uranus, Piazzi’s discovery was purely accidental.
A far better case can be made for the discovery of Neptune.
In 1821, Alexis Bouvard published astronomical tables detailing the orbit of Uranus. Subsequent observations revealed that the planet wasn’t where it was supposed to be in the sky. Bouvard hypothesized that Uranus’s orbit was perturbed by another planet’s gravity pulling on it from an orbit farther out.
In 1845, Urbain Jean Joseph Le Verrier and John Couch Adams separately calculated the difference between the theoretical and actual positions of Uranus. They then calculated the place where astronomers should be able to see the supposed new planet.
On Sept. 23, 1846, Johann Galle used Le Verrier’s calculations to begin a telescopic sweep of stars near the border between the constellations Aquarius and Capricornus. He soon spotted Neptune very close to Le Verrier’s calculated position.
According to Australian physicist David Jamieson, Galileo, the first person to systematically use an astronomical telescope, has a claim to the discovery of Neptune. In 1613, Galileo observed Jupiter and its moons over several nights. He drew in his notebook a faint star near Jupiter. Subsequently, he plotted the same star in a different location and noted that it had moved. That “star” turned out to be Neptune.
Galileo had observed Neptune 233 years before Galle. Sadly, he didn’t make the logical leap between the object’s motion and its possible planetary status. He saw Neptune, but he didn’t discover it.
Here then is a perfect example of the strictest definition of discovery. Le Verrier and Adams examined the observed position of Uranus and compared it to the calculated position. They then mathematically predicted the theoretical position in the sky of the supposed planet perturbing Uranus’s orbit. Galle went out and looked, and there it was.
However, I doubt that my fourth graders would be satisfied with my answer. Instead, they imagine some early human looking up at a starry sky and exclaiming, “That’s something new! There’s something different!”
Next week, I’ll try to imagine the same thing.
Tom Burns is the former director of the Perkins Observatory in Delaware.