Word of the Week: Exoplanet!

Hey, Sky Fans! Here we are midweek again. I’ve had this, and a couple of related posts, on my calendar for a while now. With all the scratch-outs and changes I’ve made with other things coming up, I’ve been sort of itching to get this one off the list. Anyone else keep an editorial calendar for their blog? I’m sure you can empathize.

In June, NASA announced some new findings in its hunt for planets around other stars. This was a group of over 200 new possible, candidate, exoplanets; some of them Earth-sized. People always seem to grab me to talk about this. So, if you asked and you’re reading, thanks; this one’s for you.


Remember that old Schoolhouse Rock cartoon about “Interplanet Janet,” that galaxy girl? Every time there’s news about exoplanets, I think of that song.

The prefix exo- comes from Greek, meaning outside, and planet from the Greek word for wanderer. So, an exoplanet is an “outside wanderer.” I love the way that sounds; an outside wanderer. For this I think it’s sufficient to say a planet is something directly orbiting a star (like Mars, but not like Mars’s moon, Phobos).

An exoplanet is a planet in another star system.

Word of the Week: Exoplanet!
Word of the Week: Exoplanet!

A simple word, but a fascinating topic. As time goes on, scientists discover more and more of them. Some are found with ground-based observatories like TRAPPIST or from orbiting ones, like Kepler. The news is just about the existence of a planet and sometimes what it’s made of. It’s not about whether those planets are habitable. We’re not talking about life, not yet.

I have to confess I’m not a huge fan of the word exoplanet. This, again, has nothing to do with all the struggle over the word planet itself. I’m emotional about plenty, ask my wife and kids, but not whether Pluto is a planet. I just want consistency. Truth be told, I actually love the fact that the science community is having a hard time settling on a definition of what a planet is.

It’s the scientific method, alive and working right before our eyes. First, we knew there were six planets. Then along came Uranus, then Neptune and then Pluto, and we knew it was nine. We, as scientists, acknowledged the new data, and changed to accommodate it. We didn’t change science to accommodate us. When far-flung Eris was found, we needed to change again. The difficulty with “planet” is a metaphor for the best of humanity. We have the ability to change when new information becomes available. We’ll get there. We’ll get it right.

As far as exoplanets go, deciding to call all of those planets exoplanets means they’re separate and different from these planets here. The word seems to cordon off and segregate them.

There’s not a lot of difference between the Sun’s planets and Gleise 538’s. Sure, the details, the sizes are different, the make-up, the orbital distances are different, but to say those things are in a category other than simply “planet” kind of feels needlessly geocentric, and by extension, anthrocentric to me. When explorers from Earth set foot on the planet at Proxima Centauri, is the planet right under their feet an exoplanet? That doesn’t make a lot of sense.

Am I just complaining? Is exoplanet just a convenient one-word shorthand? I admit, maybe, and honestly, what other frame of comparison do we have? Still, I prefer “planets at other stars” or “planets in other star systems,” which allows for all kinds of systems, ones with multiple stars, not just one, like ours, or even “extrasolar planets.”

The search for exoplanets is a relatively new corner of astronomy, but they’re everywhere, and diversity we’re finding in other systems is a lot like how diverse life here on Earth is. We’ve found Mini-Neptunes (gassy, icy ones up to about 10 Earth masses), and Super-Earths (rocky, cape-wearing ones up to about 13 Earth masses; Uranus’s mass). Hot Jupiters (dibs on the band name), are incredibly close to their parent star, but have somehow managed to not get their atmospheres cooked off. They’ve all changed our thinking of how planetary systems form, including our own. Like, wow.

What do you think? Exoplanet? Planet? Good words? Bad words?

Have a great day, and clear skies, everyone!





12 thoughts on “Word of the Week: Exoplanet!

  1. I’m not a big fan of the exoplanet/planet distinction either, but I’ve reluctantly come to admit that exoplanet is a convenient shorthand. I’m willing to use it in my science writing. But when I put on my science fiction writer hat, I throw the word away. It’s too geocentric, and my alien characters would resent its usage.

    Liked by 1 person

  2. The Greek phrase ‘outside wanderer’ just sounds romantic of course, but in this day and age, as an educator I notice, and also speaking from my own experiences, students hear the word exoplanet and think something alien is going on.

    But here, you actually directed in a wonderful way how we might explain to children, adults, and anyone interested in learning about science, changes of it throughout the decades, and basically what we know today and why.

    So although I’m not quite answering your question of what I think about the word, I’m really envisioning planetarium presentation here, so that’s why I sound so overly flattering, but truly, it’s like you wrote a script.

    Wonderful entry, thanks always for the info!

    Liked by 1 person

    1. Oh, thanks Sabra Anne. I’m really glad you like the post. I’m glad you got where I was going with it. I think of a lot of the things I write from the perspective of education and, especially, presenting at a planetarium. So it means a lot to see that you see it that way, too. Thank for the compliment. It means a lot.

      Liked by 1 person

  3. I will go against the grain and assert that exoplanet is a very useful term.

    If you are frequent reader of Scott’s Sky Watch, of course you don’t need the word exoplanet. You know fully well the difference between Neptune and TRAPPIST-1b.

    However, for the public at large as well as school children, it is a great term to learn the distinction from planets in our own solar system. Most people have no clue about the order of the cosmos. They have little understanding of the difference between a planet and a star, let alone a nebula and a galaxy. Keep in mind that planets around other stars was the realm of science fiction only until fairly recently. So “exoplanet” at least offers a little bit of anchorage when you try to explain the order of the Milky Way to your friends and neighbors.

    Liked by 1 person

    1. I see what you’re saying, and you make a good point, especially about giving people who don’t know astronomy well something to grab onto; an anchorage, as you say.

      There are other ways to approach the topic, though, to less knowledgeable people. There are only eight known planets and many more planets, yet exoplanets seems somehow less than planets.

      I think I’d be more okay with it if there were a collective term for all planets, and then a differentiation made between the ones near the Sun, and the ones near TRAPPIST-1. As it is now, (to be simplistic) things that orbit stars in general are planets, but so are the things orbiting specifically the Sun, while things orbiting non-Sun stars are something else. There’s something missing.

      I’m all for giving people an extra hook to grab onto, but I don’t know how “exoplanet,” which then requires further explanation does a better job than “planet at other stars” for example.

      Liked by 1 person

      1. I sense your stance on “exoplanet” orbits very close to the Pluto matter, and generally on how planets are classified overall. 🙂

        It will be interesting to see how the term is viewed say in a decade, when planets outside our solar system should be more common knowledge than they are right now.

        Liked by 1 person

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