On the 9th of January, 1992, astronomers around the world rejoiced. For the first time ever, they had definitive proof of a planet orbiting another star. These early observations were extremely limited, mainly focused on noticing the wobble of the parent star, but they opened the proverbial floodgate for exoplanet discovery. A little over two decades later, the list of known exoplanets continues to grow every day, with the number of verified planets well above a thousand, and the number of candidates exponentially more than that. All of these new planets have highlighted the rather humbling fact that, before now, we really knew nothing about the sheer number and diversity of exoplanets within the galaxy.

For a hundred years, it was assumed that other planets would roughly reflect what we see in our own solar system: A number of small rocky worlds close to the star, with a number of large gas giants circling farther out beyond the Goldilocks Zone (where liquid water can exist). The reality is quite different.

We now know, for instance, that planetary migration is common. Large gas planets, which form far away from their parent star (a requirement to keep their gas from becoming heated and stripped away during the planetary formation process), often tend to fall closer to their star over time. These “Hot Jupiters,” as they are called, often orbit extremely close to their star. This tight orbit opens up a world of geologic possibilities for the planet (and its moons). Imagine a planet like Jupiter, which has vast amounts of frozen water. As the planet drifts closer to the sun, the water ice melts and these planets develop oceans deeper than the diameter of earth. Water worlds like this have often been referenced in science fiction, but now they have become known as scientific fact.

How do we go about classifying these exoplanets when each one illustrates how little we actually know? Pluto was only recently kicked out of the planetary club, with its eviction predicated on our defining some of the most basic aspects of a “planet” (size, gravitational impact, etc.) How do we go about sub-classifying the many exoplanets we’re now beginning to find when we can barely agree on what constitutes a planet in the first place?

Should planets be classified on material composition in accordance with historical precedent (rocky worlds vs. gas giants)? This seemed to be effective at classifying the planets in our solar system, but when we know that gas giants can migrate to extremely close orbits, and their frozen gas compositions can change drastically, does this standard still hold up? The reverse of this is also true as we find an increasingly large number of “Super Earths”, planets that are rocky like earth and our inner solar system neighbors, but closer in size to the gas giants.

What about the type of star that a planet orbits? One would think that perhaps that could be the common denominator. But now we know that some planets orbit large hot stars, others orbit old cool stars, and some orbit two stars, while some have been flung out of their orbits altogether to float lonely out in space forever. Large planets evicted from their solar systems in this way aren’t even considered planets but are instead considered “sub-brown dwarfs”, somewhere in the gray zone between planets and stars.

As we have learned about exoplanets, we quickly realized that we knew practically nothing about them. What we now know has shattered our old classification system. Whatever eventually replaces it will need to be far more sophisticated and take into account the vast diversity we now know exists. The Star Trek dream of discovering Class M planets simultaneously seems further away and closer than ever before, and I for one am eager to see what happens.

Win Hansen, Production Manager
Access Innovations, Inc.