Pluto’s Fourth Moon and Exoplanets around Binary Star Systems

by Dr. Tahir Yaqoob on June 14, 2012

You are probably wondering what the two parts of the title have to do with each other. Well, an interesting preprint appeared in May (2012) that describes some theoretical work on trying to measure the mass of two of Pluto’s moons, using the newly discovered fourth moon, and then the authors scale up the calculation to draw some conclusions about exoplanets orbiting binary stars.

Let’s rewind a bit. Pluto’s largest satellite, Charon, has a size of about half that of Pluto (the latter having a diameter of about 2300 km or 1430 miles), but a mass of only about 11 percent of Pluto’s mass. Pluto’s satellites Nix and Hydra on the other hand are much smaller than Charon and have diameters of the order of tens of kilometers/miles. It is difficult to measure the actual sizes because of the unknown reflectivity and other uncertainties. Likewise, the masses of Nix and Hydra are uncertain, even when one is forced to assume a density, and are estimated to be of the order of a millionth of the mass of Pluto or less. It is worth pointing out that even though the form of Newtonian gravity is very simple, if you have a system of more than two objects, there is a huge range in possible behavior, including chaos and instability. To obtain orbital solutions and masses is a formidable task. In 2011 a fourth satellite was discovered in Hubble images (in retrospect earlier Hubble images going back to 2006 also showed evidence for the fourth moon). Pluto’s fourth moon is called P4 (apparently this dull and unimaginative name is temporary – I sincerely hope so because it will really make my blood boil if it’s not!).

Anyway, back to the preprint (which is a draft) by Youdin et al., entitled, Circumbinary Chaos: Using Pluto’s Newest Moon to Constrain the Masses of Nix and Hydra. The authors take on the challenge of constructing a theoretical model for the entire system consisting of Pluto and its four companions, and using available data to constrain the models and hence derive some desired parameters. Two things are worth bearing in mind. The first is that the members of the system orbit the center of mass of the system, which, due to Charon’s mass, lies outside of Pluto. The second thing is that the fourth moon is even tinier than Nix and Hydra, possibly smaller than the size of say, greater London. To even be able to attempt constraining the mass of the fourth moon at such a huge distance from Earth is truly mind-boggling.

There are several conclusions the authors come to. First, a circular or near-circular orbit of P4 is strongly preferred (more quantitative details can be found in the paper). Second, an exact 5:1 resonance of the orbital period of P4:Charon is not stable and the results favor a ratio somewhat higher than 5:1. Third, even the most stable orbital solutions of P4 are not stable over longer times unless Nix and Hydra have masses less than some critical amounts (that are given in the paper). These mass limits on Nix and Hydra turn out to be a factor of 20 and 10 lower (respectively) than previous upper limits. The authors point out the importance of this and similar work as preparation for the arrival of the New Horizons NASA mission at the Pluto system in July 2015. They also point out the importance of the Pluto system as a critical factor in understanding planet formation. Ultimately, constraining the masses and sizes of the satellites better will result in relaxing assumptions about the density and allow the density (and therefore composition) to be constrained independently. The authors discuss various scenarios for the formation of the Pluto system, favoring an impact collision, but this and other scenarios such as gravitational collapse involving rotational fission, all have their own problems.

Finally, the authors consider the fact that the Pluto-Charon binary and its satellites are an analogue of exoplanets orbiting a binary star system. Scaling up, they deduce that Charon would be the equivalent of a red dwarf star with a mass a little larger than 10 percent of the Sun’s mass, if Pluto is scaled to the Sun’s mass. The satellites would then scale to masses of a couple to a few times that of Mars. The main conclusion appears to be that multiplanet systems around binary stars are going to be unstable if the planets are too close to each other so binary star systems are likely to only have single planets, or planets that are widely separated (making their detection more difficult). In other words, multiplanet (remote) interactions are more violent around double-star systems than those around single stars.

By the way, the artist’s impression of an exoplanet system around a double star in the Universe Today article on Kepler-34 and Kepler-35 is an excellent illustration of why the Exoplanets Digest blog does not have pretty pictures. In the illustration the stars are shown as perfect spheres. At the separation depicted, the effect of gravity of the stars on each other and on the planet (tidal interactions) will be enormous. You have also seen plenty of pictures where a huge planet is seen in the sky from another planet. Again, at that separation the gravitational interaction (nevermind the magnetic interactions) will be so fierce, things should be depicted to be in great turmoil. Debris should be everywhere, stress and strains on the planet will cause acts of gruesome violence. Yet the scenes are depicted as tranquil and peaceful. Think about it: the Moon has only 1.2 percent the mass of Earth, yet its effect on Earth in the form of tides and associated interactions is not negligible. Imagine if Saturn or Jupiter were in the sky as large as depicted in the artist’s impressions! (Jupiter is 26,000 times more massive than the Moon.) Quite simply, the scenes as commonly depicted could not exist. Now, I’m not against artistic license per se – it has it’s place and plays an important role. However, I cannot endorse its use in reporting science news. What’s the point of teaching science if it is going to be undone when reporting science news? Besides, it makes my job of explaining what’s really going down much harder. It’s not quite up there with the common depiction of an atom as a “universally accepted science symbol” (you know, the one with the nice little balls orbiting nicely around a ball bearing for a nucleus). Why is this so bad? We’ve known about quantum mechanics for over a hundred years, yet the 19th century view of the atom is persuasively reinforced everywhere you look, and has been ever since you can remember. It is absolutely horrific that this view is also reinforced in school text books. And don’t get me started on the white coats scientists have to wear in movies, even though their work might bring them nowhere near anything that could leave a stain on their otherwise trendy clothing (not).

Next time a kid (or indeed, a fully grown adult) asks you about atoms, be honest, draw an amorphous scribble and say that this is a better representation of what most atoms “look like” compared to the thing in your textbook, but you can’t actually say they “look like” such-and-such, in any regular sense of the phrase.

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