Artist’s impression of a distant icy world like 2012 VP113. Credit: NASA/JPL-Caltech.

Astronomers Chad Trujillo and Scott Sheppard announced this week the discovery of a new icy world in the outer Solar System, named 2012 VP113. Trujillo is a colleague of Mike Brown, who discovered the dwarf planet Eris, which is larger than Pluto, among others. In a tradition of giving objects in the outer Solar System nicknames before they get real names, Trujillo has given 2012 VP113 the moniker “Biden” (because of the VP).

Now, 2012 VP113 is not very big. It’s believed to be about 500-600 kilometers across, only about a quarter the size of Pluto, but that’s not very well calculated yet. Historically, the sizes of objects in the outer Solar System tend to be overestimated at first, so it could be even smaller. But the interesting this is its orbit:

Orbits of Sedna and 2012 VP113 compared with the Kuiper Belt. Credit: Scott S. Sheppard/Carnegie Institution for Science.

That’s 2012 VP113’s orbit in red. In orange is the orbit of Sedna, another, larger object in the outer reaches of the Solar System. The purple rings are the orbits of the planets, and the blue ring in the Kuiper belt, where most of these ice balls like to hang out. These things are very far away.

Now, any astrophysicist can tell you there’s a problem with this picture: the orbits of 2012 VP113 and Sedna never go near anything big. Our newest ice ball never comes closer than 80 AU from the Sun, nearly three times as far as Neptune. The law of gravity says that even if an object is thrown out into deep space by swinging too close to a planet, it eventually has to come back to where it started. And, in fact, we see some Kuiper belt objects thrown onto long, comet-like orbits that take them far from the Sun by the gravity of Neptune, but they all come back to near the orbit of Neptune.

Except these two oddballs.

These two objects never go anywhere near anything, and we really have no idea how to put them out there in the first place. And it gets worse. We discovered both of these objects near their closets points to the Sun, and things on elliptical orbits don’t spend much time there, just like comets only spend a few months of their long orbits near the Sun. We can’t see very far past these closest points, and if there are two of these ice balls this close, there are probably dozens or even hundreds farther away!

So how did they get there? One theory is that the gravity of the passing star messed up their orbits billions of years ago. Another, more intriguing theory is that their orbits are being influenced by another “planet”, maybe larger than Earth, maybe 10 times larger than Earth, hidden away somewhere in the large, blank areas of that figure. Such an object would be hard to find, but very interesting if we did–more on that later.

On a different note, you may notice that I’ve been dancing around the issue of what to call 2012 VP113, or Sedna for that matter. “Objects”, “ice balls”, “things”? Here’s the problem: we acutally don’t know what they are! They aren’t “Kuiper belt objects” because they’re not in the Kuiper belt. Astronomers come up with all kinds of names like “detached objects”, “inner Oort cloud objects”, and so on.

But what about “dwarf planets”? Isn’t that what everybody else is calling them? Well, not exactly. The headlines may say that, but more careful articles will note that 2012 VP113 is a “possible dwarf planet”. This is because, unlike most things in astronomy, the International Astronomical Union (IAU) is the gatekeeper of the dwarf planets. Officially, a dwarf planet is any object that is large enough to become round under the force of its own gravity, but in practice. This is certainly true of Sedna, and there’s really no good reason to think it’s not true for this new object. But in practice, an object is only a dwarf planet when IAU say it is, and the IAU refuses to classify any new dwarf planets without better data that only the next generation of telescopes can provide, even if what we have is easily enough to say they have to be round by a wide margin.

This, in my opinion, is the biggest downside of the whole “definition of a planet” debate, which I’ll talk about more in my next post. It’s even worse that the whole “is Pluto a planet or not” thing because it’s just now how science is done. With exoplanets, you can have papers going back and forth saying, “this is a planet” and “no it’s not”, sometimes for years, but when you insist on knowing for sure, like in our Solar System, everything winds up being a “possible dwarf planet”, full stop, even when there’s no real debate, and I think that detracts from the fascinating diversity of objects in the outer Solar System. For more on that, you might check out this post on Mike Brown’s blog and move out from there.