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So you want to talk about double planets? No sweat. November 30, 2009

Posted by Jorge Candeias in Definition of planet, double and multiple planets.
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3 comments

The post where I explain why 8 planets are bad science has been generating both good traffic and a rather interesting discussion in the comment boxes. Part if it is about double planets.

If you check the page, on this blog, where I present the current (and highly flawed) definition of planet and my alternative, you’ll find two things. One is that my alternative is quite simple and quite radical. Those long posts I keep mentioning but never get the time to write are mostly meant to explain all the reasoning behind that simplicity and radicality, along with why I think so poorly of the IAU’s definition. But I have been lacking the time to dive in those waters, and the best you may find for now are some hints spread here and there. One of the places where hints are to be found is the thread of comments in that post.

But maybe it’s time to actually write something a bit more solid than mere comments. And, since any place is good to start, why not taking the lead from the visitors to this blog and write about double planets?

The concept of double planet is very similar in its essence to that of a double star: two objects that share, more or less, the same characteristics, and that are gravitationally bound to eachother. However, whereas a double (or its extension: a multiple) planet has no definition anywhere, there is no question about what a double (or multiple) star is. A star is multiple if there is more than one star revolving about the same center of mass, the system’s baricenter. Note that nowhere is there any reference to where that barycenter lies. A small-mass star may be so close to a heavy star that the system’s barycenter lies inside the heavy one, and the system is still a double star. Undoubtedly.

The problem with planets arises because the only objects that are considered planets are those that revolve around stars (according to the IAU, it’s even worse: only the Sun can have planets, which is the most ridiculous aspect in that definition, but let’s forget about that particular nonsense for now). The fact that every planet that is part of a multiple-body system (i.e., the planet and its satellites) also revolves around that system’s center of mass murks the waters. True, in most situations the planet is much larger than its satellites, and the system’s center of mass lies deeply within it. But what if some day we’ll find two bodies of very similar sizes revolving around a center of mass that lies outside the planet? Which one is the planet then? Both? None?

And what to you mean “what if”? We already know one such system: Pluto-Charon. Even the Earth-Moon system may one day be in that scenario, for the Moon is constantly drifting away from our planet, which means that the system’s center of mass gets closer and closer to the Earth’s surface. But so far, it’s only Pluto-Charon. Pluto has traditionally been considered the planet and Charon the moon, but Pluto’s traditional standings have been getting a serious beating recently, and that one is no exception. In the first draft of the IAU definition of planet, swiftly defeated, Charon was to be “promoted” to the condition of planet and, together with Pluto, would form a double planet. The criterion was the position of the system’s barycenter.

That criterion is, however, just plain awful. Since the position of a system’s barycenter depends on the mass of the system’s components and on the distance between them, such a criterion could result in absolutely ridiculous situations. Imagine we find some fine day a system where the satellite’s mass is close to the planet’s and it’s on a highly eccentric orbit, meaning that the distance between the two objects varies a lot during an orbit. With the right masses and distances, when the two bodies get closer, the barycenter dips within the heaviest of the two bodies, and when they drift apart, the barycenter jumps from within the heaviest, hovers for a while above its surface only to dip again in the next orbit. Or, in other words, using that criterion, for part of each orbit the system would be composed of one planet and one satellite, and for the rest of each orbit it would be a double planet, obviously composed of two planets.

Sheer nonsense, don’t you agree? You do. I’m sure you do.

There are ways to solve this problem, of course. One is to say that there is no such thing as double planets: the heaviest of the set is a planet; the others are satellites and that’s it. Another one came up in the discussion of that post of mine: just establish an arbitrary limit of mass ratio between the two, above which the system would be considered a double planet, and below which it would just be a planet-satellite system. Since I’m very strongly opposed to establishing arbitrary limits (which is one of the reasons why I really hate the current IAU definition, but that’s for subsequent posts), I dislike the second option almost as much as I dislike the barycenter criterion. The first one is not arbitrary, so it’s fine with me.

Except that I have a better idea.

Let’s cover every part of the sizes’ scale. We’ve talked about stars and saw no problem there, we’ve talked about planets and saw a complete mess, let’s now see what happens in the lowest area, the asteroid, or small body, zone. Asteroids have also been found in associations of two or more gravitationally bound sets. The first asteroid found to be a binary was Ida, when Galileo (the probe, not the astronomer) photographed its moonlet Dactyl, in 1993, but in the last 16 years we’ve found almost 200 more such systems. Including systems with more than two components, the first of which was Sylvia, which has two (much) smaller companions: Remus and Romulus. What’s the terminology there?

Unsurprisingly for such a new set of concepts, it’s also a mess. People talk about asteroids and their moons, or moonlets, like they talk about planets and their satellites. However they also talk about binary asteroids and triple asteroids, without taking mass into account. The Ida-Dactyl system is a binary asteroid, despite the large difference in sizes between the two bodies. Hermes, number 69230 in the asteroid list, and composed of two components of almost the same size, is also a binary. That’s because, if taken independently, they both would surely be considered asteroids, so there’s no ambiguity. An asteroid moon is also an asteroid.

And that’s my great idea. If you look at my definition of planet, you’ll see that it only mentions roundness caused by self-gravity, not the position each body occupies in the great merry-go-round in the sky. This means that, yes, the Pluto sistem is a double planet, with two planets and two smaller bodies. An ice dwarf / ice dwarf kind of double planet. The Earth system is also a double planet, this time a terrestrial / terrestrial dwarf kind of double planet. Mars, on the contrary, is a single planet, despite being accompanied by two small bodies. Jupiter isn’t single and isn’t double: it’s a multiple planet, with 5 planets belonging to different categories (gas giant, terrestrial dwarf, maybe also ice dwarf) and a lot of smaller bodies. Saturn and Uranus are the “multiplest” of the planets, the first composed of 8 planets and a lot (really, a lot) of smaller bodies, the second comprising 6 planets plus debris. And Neptune is, again, a double planet. A gas giant / ice dwarf kind of double planet. Or perhaps an ice giant / ice dwarf. Plus small worlds, of course.

This way you get coherence along the whole scale of celestial objects. And solve easily and without ambiguity the whole double planet controversy. That’s on the plus side. On the minus side, it would make us change radically the way we look at these things. But maybe that’s not really a minus; you see, there are other reasons to do it.

But that would be for other posts.

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