Sciency Words: Exoplanet

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


According to the International Astronomy Union (I.A.U.), an astronomical object qualifies as a planet if:

  • It orbits the Sun.
  • It’s round due to the pull of its own gravity.
  • It’s cleared its orbital path of asteroids or other debris (this is the part of the planet test Pluto failed.

The I.A.U.’s planet definition has caused a lot of grumbling and controversy, and not only because of Pluto. Let’s focus today on the first criterion for planethood: in order to be a planet, an object has to orbit the Sun. Not just any sun, but the Sun, as in our Sun. With a capital S.

Which means big, round objects orbiting other stars don’t qualify. The I.A.U. suggests calling them exoplanets or extrasolar planets, but they aren’t, strictly speaking, planets.

I have mixed feelings about this. On the one hand, it seems awfully geocentric of us to have one word for objects orbiting our Sun and a different word for the same type of objects orbiting other stars.

However, I have to admit having a special term for “planet orbiting another star” is kind of handy. It saves me time in conversations and cuts down on the word counts of blog posts. So I guess it’s worth knowing the official I.A.U. definition for this handy, time-saving term.

Except the I.A.U. doesn’t have an official definition for exoplanets. Why not? Let’s try adapting the current planet definition to exoplanets and see what happens.

  • An exoplanet has to orbit a star other than our Sun. (Seems okay so far).
  • An exoplanet has to be round due to the pull of its own gravity. (Our telescopes can’t visually confirm that exoplanets are round, but based on estimates of their mass we can safely assume they’re round. We’re probably still okay.)
  • An exoplanet has to have cleared its orbital path of debris. (This is a real problem because in most cases there’s no way to confirm, visually or otherwise, that an exoplanet has done this.)

There is a proposal to change the I.A.U. planet definition again, this time based on quantitative data rather than visual observations. This, by the way, is different than the geophysical everything’s-a-planet definition I wrote about previously. The geophysical definition would make Pluto a planet again; the quantitative definition would not.

Back in 2006, the I.A.U. changed the definition of planet, excluding Pluto from the planet club, because we’d learned new information about our Solar System. More new information about planets, exoplanets, and other planet-like objects has been piling up since then, which is why we keep hearing about these proposals to change the definition again.

Personally, I like the more inclusive geophysical definition, but that’s just my preference. Plenty of intelligent and well-meaning people disagree. But I think sooner or later, the I.A.U. will have to revisit this issue to ensure the definitions of planet and exoplanet match.

5 thoughts on “Sciency Words: Exoplanet

  1. I haven’t really thought this trhough, so bear with me, but it strikes me that there is a parallel with SI units here. SI units were orignally (more or less nominally) standardised ‘things’ – objects which were agreed to be the definition, so, the standard kg, the standard metre etc. Then we move to definititions based on measurements of something. I wonder if this is something that could happen to planets etc…we probably still need terms like Earth-like for public engagement, but in research etc, should we move to measurement based definitions?

    Liked by 3 people

    1. I think there’s definitely a parallel there. The I.A.U. hasn’t come up with an official definition for brown dwarfs either, and their reason for that is telling, in my opinion: we don’t know enough about brown dwarfs yet to define them. It’s probably similar for planets and exoplanets. As we learn more about them, the terms will be easier to define, and eventually we may even be able to apply specific quantitive measurements.

      Liked by 1 person

  2. I feel like we’ve talked about this before, about the (not so) subtle heliocentrism, and by extension, geocentrism of “exoplanet.” I’m just an astronomer-talking-guy, not an astronomer, but I prefer “planets at other stars” or something like that. The exo- prefix does the trick, and handles the “over there” part of it, but still it seems iffy.

    It seems redundant because a planet orbiting, say Sirius, is clearly outside out solar system. So, “An exoplanet at Sirius” seems silly. Even more so if you think of the perspective of future travelers, sending letters home after safely getting to Sirius. From their perspective, Earth is an exoplanet, unless we’re still keen on elevating the Sun and its orbiting stuffballs above all others.

    Liked by 1 person

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