Yes. Also Ceres, Haumea, Makemake etc. We have lots of stars, lots of asteroids, so why not lots of planets. The IAU definition doesn’t work very well, it really distorts language using the word “dwarf” to mean something it doesn’t normally mean.
I think Alan Stern’s terminology is much better. He calls a planet that “clears its orbit” an “uber planet” and one that doesn’t, an “unter planet”.
Using this terminology, the IAU says that an uber planet is a planet, and an unter planet is a “dwarf planet” but “not a planet”. I.e. in one sentence they say it is a planet and in the next, that it isn’t.
But it gets much worse than that. The “dwarf planets” don’t have to be smaller than Earth even. According to the IAU definition a “dwarf planet” could be as large as Saturn or Jupiter. And we could find such a planet. The WISE survey only ruled out Saturn sized planets out to 10,000 AU and Jupiter sized ones out to 26,000 au. We could have gas giants that orbit our sun over 1.5 light years away, or over 100,000 au.
So - it is possible that any day we might discover a “Gas giant dwarf planet”.
Here is how.
In this diagram, “planet 9” if it exists (awful name for a planet but we are stuck with it for now) - they estimate mass same as Neptune. If so it just sneaks in as an orbit clearing uber planet. It’s above that diagonal line (the three lines show three possible definitions of an orbit clearing uber planet).
But Earth at the distance of Planet 9 (if it exists) would be a dwarf planet.
Planet 9 also may possibly consist of several planets in resonances rather than just one planet. If so, then some of those might be uber planets and some might be unter planets. We could end up with a group of several objects at the same distance in resonant orbits, all larger than Earth, and only some of them count as planets. Some others might be planets according to one definition and not according to another as the IAU definition doesn’t say which precisely of those three lines to use, or whether to use some other definition of orbit clearing.
But it gets much worse than that as I said. We can have planets orbiting well over a light year from Earth. The Nemesis and Tyche hypotheses were for a small red dwarf or brown dwarf. Those are pretty much ruled out though a very cold brown dwarf way out beyond one light year away is still possible.
But they haven’t ruled out gas giants like Jupiter or Saturn out there. So we could, easily, discover a gas giant too far out to clear its orbit.
If we find such, then according to the IAU definition then - well it would be a gas giant first, surely, if it is the mass of Jupiter. Or if you decide to call it something else, how can you not say it is a “something” giant? It’s nearly as large as a planet can be.
But it’s also a dwarf planet because it can’t clear its orbit at that distance from the Sun.
And it’s also not a planet because according to the IAU, dwarf planets are not planets.
So it is a
Gas giant
Dwarf planet
Non Planet
How can that be right? So our definition is not “future proof”. I think better to fix it right away.
Especially if we are to generalize it to other star systems. We are bound to find systems that have planets that are much closer to the borderline between a planet and a non planet than ours. For instance a simpler example, a Mercury sized object at the distance of Neptune in our solar system would be right on the boundary between a planet and a dwarf planet. It then wouldn’t be such a neat pattern any more.
But eventually - as we get to plot the orbits of gas giants far from their stars - surely we will find distant gas giants that then would be gas giant dwarf planet non planets, if we don’t find them in our solar system.
I think language matters. Obviously it makes no difference to Pluto itself. Or Ceres or Haumea etc. It’s not such a big deal really. But it helps us to think more clearly about these objects.
BTW I think “Planet 9” is a really poor name. It might not be an IAU planet. It might be several planets. It might be Planet 10 if there is another planet closer to us, for instance to explain the “Kuiper cliff” and when did we ever call planets by numbers, e.g. if you said “planet 4” how many would know you mean “Mars”?
But sadly there is no alternative name to it. I can’t call it “planet X” because there are lots of planet X candidates, and even today there are perhaps two other possible candidates not yet ruled out - Tyche (unlikely but not impossible) and the Kuiper cliff candidate and there may well be more ideas to come.
I wrote this up in more detail here.
About the Author
