How fine-tuned harmonies saves TRAPPIST-1 from destruction

Author: Dan Tamayo

In February of this year, a European-led team of astronomers announced that TRAPPIST-1, one of the 300 nearest stars to us, hosts seven Earth-like planets. Several of these could have the right temperatures to host liquid water, and are thus prime candidates in the search for life outside our solar system.

But a puzzle in the original paper was that when they tried to simulate the system, planets would start colliding after a short time. It seems unlikely that we would catch the system at such a special time just before its destruction—more likely there’s something else going on to keep the system together. The discovery team also pointed out that the time it took each planet to go around the star formed an incredible progression. In the solar system, the planets’ orbital periods are fairly randomly related. But in TRAPPIST-1, for every 2 orbits of the outermost planet, the next one in does 3 orbits, the next one 4…, 6, 9, 15, and 24. This is called a chain of resonances, and this is the longest one that has ever been discovered in a planetary system.

We thought that this could explain why things collided, because resonances turn out to be the seeds of chaos in planetary systems. In fact, they are the reason dinosaurs are extinct today. If you look at the asteroid belt today, you see huge gaps at many locations where asteroids would have a whole number period ratio with Jupiter. These resonances therefore act as escape hatches from the asteroid belt that can lead to collision courses with Earth.

But the tricky thing is that resonances can also have the opposite effect and keep things extremely stable. For example, Neptune does three orbits for every two orbits of Pluto. That’s a good thing for Pluto, because the two planets’ orbits cross one another. If it weren’t for this special relationship, the two would have eventually met, and Pluto would long ago have been demoted straight past dwarf planet to interplanetary smithereens.

So resonances are this double-edged sword, and in packed systems like TRAPPIST-1, it turns out that your ultimate fate depends on how well you’ve tuned your other orbital parameters, like the ellipticity of the orbits and how the orbits are oriented relative to one another.

There’s a good analogy with an orchestra. It’s not enough for members to merely keep time. If they don’t also tune their instruments to one another beforehand, there won’t be any harmony. In the same way, special ratios of orbital periods in packed planetary systems like TRAPPIST-1 are not enough to guarantee stability. If the other orbital parameters (ellipticities, alignments) are not also finely tuned before the symphony commences, that dissonance introduces chaos that can propagate to disrupt the system.

The resolution to why the TRAPPIST-1 simulations would lead to collisions is that it is very difficult to measure those additional parameters from the observations of the system. These uncertainties mean that the best recreations we can make of the system today are not tuned to one another, and quickly disrupt.

Since we couldn’t get those additional parameters from the system today, that motivated us to rewind the tape and think about how the system would have formed. Planets form in disks of gas and dust, and as planets grow and interact with the surrounding disk, they move around relative to one another. If this process is gentle enough, then planets can naturally tune all their orbital parameters to one another, just like the orchestra does before a symphony.

What we found is that when we create these harmonized systems, we find that the majority survive for as long as we can run our supercomputer simulations.

The conclusion is that this unprecedented chain of resonances is crucial in keeping the system together, but that it must be finely tuned to avoid the fate of the dinosaurs. This also suggests quiescent planet formation conditions in the TRAPPIST-1 birth disk that allow the planets to tune to one another without getting kicked around too much.

This is interesting in the broader picture, since around the many massive stars like the sun that have been discovered to host planets, resonances are rare. A leading hypothesis is that this is due to turbulent disk conditions that kick and detune planetary systems.

But TRAPPIST-1 is much smaller than the sun, barely able to ignite nuclear fusion and be called a star. These dim stars are harder to see and less well studied. It may be that the formation conditions around low mass stars are gentler and better able to form harmonious, long lived planetary systems.

This has implications for the prevalence of planets in the universe, since there are many small stars for every big one. The exciting part is that this will be tested by upcoming missions like the Transiting Exoplanet Survey Satellite (TESS) launching next year.

This research was published on The Astrophysical Journal Letters by CITA/ DAA/ CPS researchers Dan Tamayo, Hanno Rein, Cristobal Petrovich and Norman Murray. CITA researcher Matt Russo and his bandmate Andrew Santaguida worked with them to create a software System Sounds that directly translates the motion of planetary systems into music so that the rhythmic and harmonic structure can be seen and heard. The resonant chain of TRAPPIST-1 produces a surprisingly beautiful and eerily human result. How this came about was explained in an animation video created together with a Toronto studio. This work has been covered on New York Times and other newspapers and new sites.