Is Pluto a planet?

Is Pluto a planet?

My heart says yes, my head says no.

Pluto is an oddball, of that everyone can agree, but is it a planet?

At first glance, the concept of a planet seems pretty straight forward: Big round thingy swinging around a star, but there’s more to it than that, as there’s lots of stuff orbiting stars, including comets and asteroids.

Pluto’s small on the planetary scale, and therein lies the problem.

Pluto is smaller than our Moon. In fact, Pluto is smaller than several other moons in our solar system, including Titan.


In addition to this, there are several other objects in the Kuiper belt, like Eris (formerly Xena), that are the same size as Pluto or larger and we don’t consider these planets.


If we compare the circumference of Pluto with Neptune, Pluto is 21x smaller. If we compare the estimated mass, then Pluto is almost 8000x smaller than Neptune.

And yet comparisons are a slippery slope. If we make the same comparison between Earth and Jupiter, then Earth’s circumference is 10x smaller than our Jovian giant, while Earth’s mass is an astounding 320x smaller, so does that make Earth a dwarf planet? It’s a good question when you think about it.

The International Astronomical Union (IAU) struggled with Pluto’s planetary designation for over a decade before finally relegating Pluto as a dwarf planet.

According to the IAU, a planet is:

  • is in orbit around the Sun
  • has sufficent mass to assume hydrostatic equilibrium (basically a round shape)
  • has “cleared the neighbourhood” around its orbit

The problem with this definition is:

Regardless, the IAU decided that Pluto’s inability to clear out its surroundings and its disproportionally large moon Charon, disqualified it from planethood (if there is such a term) and so Pluto was designated a dwarf planet.

Truth be told, even that category is generous, and leans more on the historical position of Pluto than it does on any of Pluto’s physical characteristics.

Pluto is a cosmic iceberg.

As best we understand Pluto’s composition at this time, Pluto doesn’t have the iron core we associate with regular planets and is probably a mixture of rock and ice.

Plutos Orbit

Given its composition and its highly irregular orbit, which is tilted by 17 degrees relative to the rest of the planets and how Pluto’s orbit cuts inside the orbit of Neptune from time to time, NASA even suggested Pluto may be a “failed” dormant comet. “Failed” in that it never got close enough to the Sun for its volatiles to give it a distinct comet tail.

Had Pluto fallen into an orbit that took it into the inner solar system it would have been the most spectacular comet of all time. And being accompanied by its icy moon Charon, Pluto would have had a highly irregular coma that may have appeared to vacillate over a period of six days (being the orbital period of Charon).

Pluto might not make much of a planet, but it would have made a sensational comet had its orbit taken it closer to the sun.

If you’d like to learn more about Pluto, check out Vintage Space.

14 thoughts on “Is Pluto a planet?

  1. If a planet is a celestial body orbiting a star, then all comets are also planets. If a planet is a celestial body of a certain size or mass orbiting a star, then what are rogue planets?

    I somehow think it wiser to define planets by their behavior, not their size.

    • yeah, it’s a tough one to define. Titan looks decidedly like a planet in orbit around another planet, so what’s a moon? Ah… they’re all designations of convenience. Most planets are obvious, but Pluto is the exception, sitting somewhere on the dividing line, and probably is a failed comet. Hey, thanks for reading, commenting and tweeting 🙂

  2. Dudes (& dudettes), they all have you fooled.

    Pluto is just the first-stage construction effort for some alien’s Death Star and they’re either sitting there, cruising around, watching us, OR

    It’s been abandoned and it just floats there (cause they don’t like the neighbors) OR

    it’s a vacation destination for their species, OR

    it’s an ANOMALY waiting for us to develop far enough along to land on it or…

    Oh crumbs, too much caffeine makes me way too “creative!”

    • Hah… Have you read Pushing Ice by Alistair Reynolds? He sets up Saturn’s moon Janus for a very interesting ride along these lines. It’s some great, speculative, hard sci fi.

  3. Pingback: Pluto is NOT (not?) a Planet - S4A

  4. Pingback: Yukarı Bak » Zavallı Plüton’un çilesi bitmez

  5. From Pluto’s strange (elliptical) orbit, it is clear to me Pluto doesn’t belong in our system, However that could be considered “Hate Speech” so lets be politically correct and say Pluto is a transgender planet.

  6. Such negativity over the status of Pluto. Maybe it isn’t a fully flegged planet and possibly doesn’t belong in our system, but that just makes me much more interested! I say leave the little guy alone. The little people of the universe have never done me any harm. One day when we finally get there we may well discover new things simply because the former 9th planet is so much different to the other 8. Besides, lets face it, Mercury and Venus are so completely different to each other yet each is potentially more interesting scientifically than other ways, albeit difficult to consider landing on either for a while. Mars, by contrast should be relatively easy to land on but once you go past that system, including Deimos and Phobos, we can only seriously consider the various moons, not to mention a couple of other “dwarf planets” like Ceres, which I believe is nowhere near as heavy as Pluto, for example. Apologies to Arthur C. Clarke and his story “The Sentinel” , later developed into “2001: A Space Odyssey”, but it will be difficult to land on any of the gas giants. At least we may potentially land on Pluto, possibly at a time when it orbits inside Neptune, for example. Maybe Plutos iron core can eventually enable it to retain a little extra status compared to many other bodies in the solar system.

    I wonder what Earth’s status will be in a thousand years time!

    • I think you’re right. The scientific sentiment is swinging back to Pluto being a dwarf planet but a planet nonetheless, which means there are at least another six Kuiper belt objects that should also become dwarf planets (some of them are bigger than Pluto)

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s