When is a Minor Planet Not?

A curious little story about a minor planet becoming something else landed in my email box yesterday. I’ve been working on a project about exoplanets for the past few weeks. Part of that involves looking into the formation of such systems around distant stars. Obviously, planets form, but other objects get formed when larger worlds do.

As part of the project, I read up on the kinds of worlds we have in our own solar system. Of course, we have planets. We also have dwarf planets (same as planets, but with a few qualifiers). There are minor planets, asteroids, comets, moons, and rings (made up of particles of broken-up moons). Oh, and we have a star, the Sun.

And Now We Have the Naming of Parts: Solar System Edition

Just because solar system objects get “binned” into various categories doesn’t mean they always STAY in those categories. And, those categories sometimes say more about our understanding of solar system objects than about the worlds themselves.

We all know about the IAU’s silliness of “demoting” Pluto (based on a vote by a remnant of astronomers at the end of a meeting). They didn’t even bother to consult planetary scientists, who have the best knowledge of planetary system objects. It’s still a planet, albeit termed a “dwarf planet”. But, the name we call it didn’t change Pluto. It’s still a fascinating world out in the Kuiper Belt and it still orbits the Sun and has activity under its surface. Nothing has changed there.

But, what if an object appears to have changed? Say, like a comet. It dives in toward the Sun, and as it gets close, its ices start to sublimate. That creates a cloud of material around the nucleus, called a coma. A pair of tails sprouts out from the comet, and they (and the coma) last until the nucleus gets too far away from the Sun to sublimate. In that sense, the comet DID change, from being just a frozen chunk of ice to being one that acts when heated to form more characteristics. But it’s still a comet, whether it’s in its active state or a quiescent phase. What also changes, as we learn more about how comets work, is our understanding of them.

Minor Planet Discovery

Now, what if a minor planet did the same thing as a comet? Does that change the minor planet designation? In the case of at least one object, it has done exactly that. It’s called Centaur 2014 OG392. This little worldlet orbits out between the orbits of Jupiter and Neptune. Centaurs sometimes show features that comets usually exhibit: outgassing and coma-building. That’s pretty amazing since the region of space they orbit in pretty cold. That makes it harder for water to make the transition from a solid chunk of ice to a gas cloud.

A Minor Planet Becomes Something Else

Centaur 2014 OG392 caught the attention of a team of astronomers at Northern Arizona University, led by doctoral student and Presidential Fellow Colin Chandler. They studied it and found that this Centaur is pretty active. It’s outgassing and forming a coma. The team suspects that the culprit activity causing that outgassing is the sublimation of carbon dioxide or ammonia (or both). The team measured a coma that stretches out about 400,000 kilometers from the body of the minor planet.

Well, so does that coma formation make Centaur 2014 OG392 an active minor planet or a comet? Apparently, there are other active minor planets, so that’s an interesting question. The Minor Planet Center, which has the task of cataloging such objects, has responded to the discovery of the coma around Centaur 2014 OG392 by designating it as a comet. So, it’s new name is now C/2014 OG392 (PANSTARRS). And, that’s perfectly appropriate. It now reflects a better understanding of what this object really is.

Change is Good, Even for Minor Planets

I think this is all very interesting, and really reflects very nicely on our growing awareness of objects and activities in the solar system. It also shows how, as time goes by and new equipment and observing methods become available, our understanding of solar system objects is what changes. Comets and planets and asteroids and other objects will continue to do what they do, regardless of what we call them. But, the names we call them need to accurately reflect our understanding of them — not rely on outdated naming methods and misguided votes.

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