Scientists Discovered Mysterious Musical Rhythms in Space—and Found the Secret Culprit
The most common type of planet found around Sun-like stars in our universe is the sub-Neptune—a planet that sits in size between Earth and Neptune, and typically has a pretty thick atmosphere.
As they’ve found more and more sub-Neptunes, scientists have begun to notice that the puffier these planets are, the more likely they are to be in rhythmic, almost musical systems called resonant systems.
It turns out that this is isn’t observational bias—it’s a physical fact, and it likely has to do with how these planets form.
The Moon goes around the Earth. The Earth goes around the Sun. The Sun, along with its many stellar brethren, goes around the black hole at the center of our Milky Way galaxy.
Things orbiting other things is one of the most common occurrences in the universe. But somethings orbit in much more satisfying ways than others. There’s not a very strong relationship between, say, the orbit of Earth and the orbit of Neptune. But there are some stellar systems where the planets orbit round their stars in an almost musical way.
And according to recent research, there’s a key to forming these musical orbits—you have to be puffy.
Or, at least, it makes things easier. A recent study published in the journal Astronomy and Astrophysics has shown that when it comes to the most common planets orbiting Sun-like stars found throughout our universe—a slightly poorly-defined category called sub-Neptunes, which includes planets between the sized of Earth and Neptune that usually have fairly thick atmospheres—you’re more likely to really get along with your neighboring planets if you’re light and “puffy” than if you’re dense.
We refer to these systems in which the planets “really get along” as being ‘resonant,’ and the planets in these systems as being ‘in resonance’ with one another. This resonance occurs when the orbits of planets synch up in whole-number ratios. For example, a planetary system called K2-138 contains a string of 5 planets that are all in 3:2 harmonies with their neighbors. That means for every 3 orbits completed by the innermost planet, the next one out completes 2. For every 3 orbits that second planet completes, the next one out completes 2, and so on. If you speed those orbits up to human-audible speeds, they are all in harmonies of perfect fifths with each other.
But, why does the density matter here? What is it about puffy planets that makes them more likely to synch themselves up like this?
Well, for a long time, scientists thought it might be nothing—the propensity of puffy planets to make up resonant systems being a mere side effects of how their densities were measured. Experts can measure exoplanet densities as a part of the methods used to spot them in the first place, the two most common being the transit method (looking for a dip in star brightness caused by a planet passing in front of said star) and the radial velocity method (watching a star wiggle around slightly as it’s pulled by the gravity of an orbiting planet).
The planets for which density was calculated using the transit method were, on average, much lower than those calculated by the radial velocity method, meaning that the transit planets were puffier. And according to the researchers behind the new paper, the transit method planets were “predominantly nearly resonant”—most of them were in resonant systems.
That’s the kind of weird that makes scientists ask if there could be a causation present, rather than just a correlation. But when the team started their research, it was “still unclear whether the discrepancy is attributed to detection biases or to astrophysical differences between the nearly resonant and non-resonant planet populations.”
So, they dug in. The team isolated a selection of 36 sub-Neptunes that they knew could be stripped of method-based bias, and took a look to see if there were real differences between puffy planets and denser ones. And it turns out there. The puffier planets were more likely to be at least close to resonance. Later expanding their sample size to 133, they found the same thing. Subsequent computer simulated systems told the same story. And even when just comparing planets measured by the radial velocity method to others measured with the same method, the pattern held—the puffier the planet, the closer it would be to resonance.
This kind of result is a pretty good indicator that the preference of resonant systems for puffiness was a physical phenomenon rather than a quirk of our detection methods. But for right now, we’re still not exactly sure why it’s the case.
That’s not to say, however, that there aren’t theories. For example, according to the research team, the gravitational influence on one planet by another in a resonant system could cause excess tidal heating, causing the planets to expand and puff up from the increased temperature. Another theory says the puffier planets could have formed in different, less metal-rich locations than denser planets, and fallen into resonance with each other as they were pulled further in towards their stars.
Perhaps most intriguingly, however, the answer could be that these puffier planets are indicators of a stable celestial childhood. Basically, there’s an idea that when stellar systems form, they often settle into resonant systems early on. But as the new planets migrate in and eat up all the stuff in the disk of debris surrounding their star, that resonance tends to start to decay.
That decay then starts to cause collision between brand new planets before everything settles down. We’re pretty sure this kind of turmoil is a part of our own Solar System’s history. Those collisions, in turn, can knock the puff off of young planets and sometimes make them reform entirely, causing them to be denser than the young planets that were just hanging out in resonance.
The researchers don’t confidently stand behind one theory over another just yet, and acknowledge that there’s still a lot of work to be done to ferret out the definitive reason behind this odd happenstance. But they’re already proposing next steps towards that very investigation, so hopefully, we’ll soon be able to explain our puffy planetary pals even better.
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