Sciency Words: Planet

Sciency Words MATH

The 2015 Mission to the Solar System has finally brought us to Pluto, so for today’s edition of Sciency Words, let’s talk about the scientific term:


In 2006, the International Astronomy Union established the first ever official definition of a planet. To be a planet, an object must:

  • Orbit the Sun and not orbit anything else.
  • Be roughly spherical.
  • Have cleared most other objects or debris from its orbital path.

Pluto failed this last test. We now know that Pluto is just one of many small, icy objects in a sort of second asteroid belt known as the Kuiper belt.

Rather than demote Pluto to the ignoble status of “large asteroid,” the I.A.U. created a new category of astronomical objects called “dwarf planets.” The name comes by loose analogy with a category of small stars called “dwarf stars.” It’s worth mentioning that while dwarf stars are still a type of star, dwarf planets are not considered a type of planet.

To qualify as a dwarf planet, an object must:

  • Orbit the Sun and not orbit anything else.
  • Be roughly spherical.
  • Have NOT cleared most other objects or debris from its orbital path.

So did the I.A.U. get it right? Do these definitions of planet and dwarf planet make sense? Some astronomers would say no (don’t get too excited, Pluto fans; it’s not for the reason you think).

How are we supposed to apply these definitions to exoplanets, planets orbiting other stars? Most exoplanets cannot be observed directly. The few that are visible in our telescopes appear as just a few pixels. So how can we see if they’re spherical? How can we tell if there’s debris in their orbital paths or not?

The I.A.U. is now considering defining planets in a more mathematical way, using the following factors:

  • The planet’s mass (we can usually estimate that without directly observing the planet).
  • The planet’s orbital period (which can also be estimated without direct observation).
  • The mass of the host star (or stars).

If the mathematics work out such that an object should be able to clear its orbital path of debris within a predetermined timeframe (10% of the host star’s lifespan, perhaps), then it qualifies as a planet.

We can safely assume that any object with enough mass to clear its orbit would be also spherical, due to its own gravity. So astronomers wouldn’t have to visually confirm that an exoplanet is round.

Right now, this idea is only a proposal. But if the I.A.U. adopts this new definition of planet, it can be applied equally well to objects in our own Solar System and objects orbiting distant stars.

P.S.: Under the new definition, Pluto still isn’t a planet.

Dc02 Pluto's Not Mad


A Quantitative Criterion for Defining Planets by Jean-Luc Margot.

A New “Mathematical” Definition Proposed for What Constitutes a Planet from Universe Today.

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Today’s post is part of Pluto/Kuiper belt month for the 2015 Mission to the Solar System. Click here to learn more about this series.

11 thoughts on “Sciency Words: Planet

  1. All this angst about the definition of “planet” and whether or not Pluto is one. Small spherical objects like Pluto or Ceres are very different from a planet like Mercury. But Mercury is very different from a gas giant like Jupiter. And then you get to things like brown dwarfs, which aren’t stars, but it seems perverse to call them planets.

    The ancient definition of “planet” was “wandering star”, from the fact that they looked like stars that wandered along the ecliptic. Every definition since then seems like a hopelessly arbitrary attempt at saving appearances.

    Liked by 1 person

      1. I hadn’t heard that one. Just looked up what they want to call them. Sub-brown dwarfs? And apparently there’s controversy on what to call Jupiter sized objects that orbit brown dwarfs. Sigh. They’ll probably come up with unique names for everything to maximize complexity.

        Liked by 1 person

      2. I hadn’t heard there was controversy about objects orbiting brown dwarfs. But if planets have to orbit stars, then I guess objects orbiting brown dwarfs can’t be planets. So what are we supposed to call them? Moons?


      3. According to the wikipedia article on sub-brown dwarf, the debate is between calling them a planet or a sub-brown dwarf. Apparently that debate also even applies to some large gas giants in distant orbits (over 1000 AU) of stars.

        Liked by 1 person

    1. I don’t like it either. I’ve kind of gotten used to it, but it just feels more natural to me to refer to Pluto as a planet. And I feel the same way about the other dwarf planets too: Eris, Ceres, Haumea, etc.


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