Defining Planets

First there were six. Then there were nine. And then there were eight. Now, are there 110?

The English word “planet’ has been in use for centuries. It was being used in the time of Old English, and even before.

So it may come as a surprise that people still haven’t decided what exactly the word “planet” means.

In olden days, there were no telescopes or dictionaries. People used to look at what they could see with their eyes, and describe them with words they knew — or make up their own. Of course, peoples’ eyes were sharper then. They could see many more things, and in more detail, than humans of today.

One of the things they looked at was the sky at night, and the many white dots of light that spread across it. Most of the dots would stay in a fixed pattern, rising and setting together like the Sun. But some of the dots — very few of them, just a handful — didn’t follow the rules. They moved differently from the rest, wandering about the pattern as they wished.

That’s why people called them wanderers—or rather, the Greek word planetai. English speakers used the word too, but the name changed a bit. It became: planet.

Eventually, people realised that the small white dots weren’t so small after all. The still ones were huge, fiery balls like the Sun, and the planets were big, round worlds like the Earth.

At first, people assumed that everything — sun, moon planets and stars — all moved around the Earth. (Initially, the Sun and Moon were called “planets” too). There were some theories that said planets circled round the Sun, that calculations were much easier if you assumed it was that way. But most people didn’t quite believe that. Then, they started looking further.

One day, two boys were playing around with lenses in a lensmaker’s shop. Purely by accident, they found that holding the lenses in a line could make far-away things look bigger and closer. The spectacle maker, Hans Lippershey, immediately saw the potential. He applied for a patent, and started selling the “Looker”.

Or so one story goes. Other versions say that Lippershey stole the idea from his assistant, or from someone else altogether. The only thing we know for certain is that he was the first to apply for a patent. Though Lippershey promoted it as something useful, to see far-away objects as if they’re close to you, the Looker became most popular as a children’s toy.

Then, one day, the astronomer Galileo Galilei decided to Look at the stars.

Suddenly, there were new stars to see— many more than had ever been seen before. Galileo saw the rings of Saturn, which he thought to be “ears”. And, he found four tiny white dots moving. But they weren’t moving around the Earth, or even around the Sun. They were moving around the planet Jupiter!

That was enough to convince him. If some dots can be moving round Jupiter, then surely it’s not so strange if planets move around the Sun?

No, it wasn’t strange. But that meant, according to the theory, that the Earth was a planet too.

Of course, it wasn’t planet in the sense of being a small dot wandering across the sky. But there were some objects that wandered around the sky, and the Earth was one of their kind. It only looked different to people because the other planets were millions of kilometres away, while the Earth was right there touching their very feet.

The meaning of “planet” had changed dramatically — and not for the last time.

Armed with new telescopes to spy on the skies, astronomers began to find and observe many new stars. They found a couple of planets too. And, they found a whole new class of objects they never even suspected.

Planet Uranus was found by accident. People didn’t expect to find new planets with their telescopes. But once this one was found, they began to search in another likely place: the huge gap between the paths of Mars and Jupiter.

In 1801, Planet Ceres was found. But was it really a planet? It turned out to be unusually small — even smaller than the Moon! And then, people started finding many similar dots, all circling the Sun in the same region. No: they couldn’t all be planets.

People gave those objects a new name: asteroids. Together they formed an “asteroid belt” around the Sun. And they were different from planets, though nobody knew why they were there. Maybe they were the remains a big planet that had broken up and scattered.

Now, the meaning of planet had been narrowed down. It didn’t mean anything moving around the Sun, but only some particular largish of things.

The describing of Ceres was sorted out soon. A different dot created a lot more confusion.

When Pluto was discovered in 1930, nobody had any doubts that it was a planet. It even had its own moon (we now know it has five). But then, people discovered another similar sized object travelling a similar path. And another. And another.

It turned out that Pluto was at the edge of the Kuiper Belt: a ring of small objects just like the Asteroid Belt but much, much further away. What’s more, Pluto was very small compared to the other planets. Soon, people began arguing about whether they should call it a planet at all.

Was Pluto a planet or not? Did the solar-system have eight planets or nine? In 2006, the International Astronomical Union, IAU, decided to settles the matter once and for all. They released a formal definition of “planet”, which went something like this:

A planet is a celestial body that (a) orbits the Sun, (b) is more or less round, and (c) has cleared the neighbourhood around its orbit.

Unfortunately, that meant Pluto was not a planet. It hadn’t “cleared the neighbourhood”, because it regularly crossed paths with other Kuiper Belt objects. Instead, Pluto was put into a different group called “plutoids” along with Ceres and a few others.

And that made a lot of people unhappy.

Do all planets orbit the Sun? What about the other stars? If they’re similar to the Sun, surely some of them will have their own planets too?

Until three decades ago, nobody could answer that question. Planets are so small compared to stars, even the most powerful telescopes couldn’t detect them. Because stars are so bright, people could only detect expoplanets indirectly, saying “this star is behaving oddly, so it must be getting pulled by the gravity of a this-big planet”.

Actually, “must be” was usually “may be”: there were many other reasons why a star could be behaving an that particular way. What’s more, people weren’t exactly looking for exoplanets. So even if they found evidence for them, they may not have realised what they had found.

It was only in 1992 that two astronomers at the Arecibo Observatory discovered what they were pretty sure was two exoplanets. They were orbiting a pulsar in the constellation Virgo, 2,500 light-years away from the Earth.

After that, more and more exoplanets have been discovered. The primary aim of the 2009 Kepler mission was to find them. Today, people know so many exoplanets that their interests have become more specific: exoplanets that are capable of supporting life.

All this is very interesting, but it highlights a flaw in the IAU definition of “planet”. The IAU defines them as things that orbit the Sun — leaving out the billions of objects that are circling other stars, or which have escaped and are drifting on their own through space. These may be many worlds out there, but, according to the IAU definition, exoplanets cannot be called “planets”.

It’s not just the possibility of life that makes exoplanets interesting. There’s also their composition, what they’re made of, and the processes going on inside their surfaces. They are all similar in many ways, and yet quite different.

But exoplanets are so small and far away, it’s hand to make out anything but the most basic details. That’s why, people study objects in our own solar-system too.

And it’s not just planets they look at.

Whether it’s the cold volcanoes of Jupiter’s moon Io, the cell-like formations on Pluto’s plains, the methane lakes on Saturn’s moon Titan, or the leaking atmosphere of Mars, there’s a lot going on in all these places, and a lot of discoveries yet to be made.

But how do astronomers find all this out, just by looking through their telescopes? They don’t. In fact, most of these explorations and studies are done not just by astronomers, but also by geologists.

Geology used to be the study of rocks. Later, it became all about how the Earth works and what goes on in its fiery interior. Then, it became not just about Earth, but about other planets as well.

And then, one geologist thought: why should astronomers be the ones to decide what a “planet” is, if geologists do work on them too? In 2017, final-year PhD student Kirby Runyon proposed a new definition of the word “planet”, using geological characteristics. “A planet”, he said, “is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.”

Or, to put it more simply,

A planet is a roundish object in space that’s smaller than a star.

Did that make Pluto a planet? Yes, because it didn’t need to “clear its neighbourhood” any more. But the new definition made Ceres a planet too, as well as moons like Titan, Callisto and Ganymede. In fact, with the new definition, there are now 110 planets in the solar-system.

The new definition looked at what a planet is actually like. It focused on the nature of the planet itself, and not where it happened to be orbiting.

But, a hundred and ten planets? How can we possibly keep track of all of them? Well — the truth is that the objects were there all along, just under a different name. All the new definition did was to remind us again of how much there is still left to explore, right in our own solar system.

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