With several Earth-sized exoplanets orbiting within its habitable zone, the TRAPPIST-1 system is one of the hottest prospects for harboring extraterrestrial life. But simulations suggested we would want to get moving before the crowded system comes to a catastrophic climax in the next million years or so. But new research from the University of Toronto has found that TRAPPIST's worlds orbit in what's called a "resonant chain," which keeps the system stable – and has been translated into a piece of music.

The planets in the TRAPPIST system orbit their star much, much closer than the planets in our Solar System orbit the Sun – in fact, all seven of them are packed into an orbit far tighter than that of Mercury. The initial research on the system showed that this proximity would destabilize the system pretty quickly, meaning either we spotted it just in time, or there's some other force at play.

"If you simulate the system, the planets start crashing into one another in less than a million years," says Dan Tamayo, co-author of the study. "This may seem like a long time, but it's really just an astronomical blink of an eye. It would be very lucky for us to discover TRAPPIST-1 right before it fell apart, so there must be a reason why it remains stable."

To investigate why that might be, the team looked a little closer to home. The orbits of Neptune and Pluto actually intersect, and yet the two bodies have survived for billions of years without colliding into each other. That's because they exist in a resonant configuration, meaning their orbits form a ratio of whole numbers: Neptune will zip around the Sun three times in the same amount of time as it takes Pluto to orbit twice. The 3:2 ratio is as regular as clockwork, so they'll never be at the same place at the same time.

As the researchers discovered, TRAPPIST works in the same way, but ups the ante with all seven of its planets syncing into what the researchers call a resonant chain. Each planet's orbit forms a whole number ratio with its neighbors, and the end result is a system that's incredibly stable in the long term.

So why did the original simulations keep predicting a TRAPPIST apocalypse? The Toronto team points out that no matter how synchronized a set of orbits may be, it can still end in disaster if our measurements are even slightly off, and chances are that the original calculations missed some of the nuance.

"It's not that the system is doomed, it's that stable configurations are very exact," says Tamayo. "We can't measure all the orbital parameters well enough at the moment, so the simulated systems kept resulting in collisions because the setups weren't precise."

To try to get a better picture of the system, the researchers investigated how these precise patterns may have formed in the first place. They found that as the planets were forming from a disk of gas and rock, they probably migrated in relation to each other, which would naturally form a stable resonant configuration.

"This means that early on, each planet's orbit was tuned to make it harmonious with its neighbors, in the same way that instruments are tuned by a band before it begins to play," says Matt Russo, co-author of the study.

Continuing that band analogy, the team decided the best way to illustrate the regularity of the TRAPPIST system was through music. The set of whole-number ratios between the planets' orbits is remarkably similar to what makes pairs of musical notes sound pleasing to the human ear, so the team composed a little ditty, "composed" by the planets.

"Most planetary systems are like bands of amateur musicians playing their parts at different speeds," says Russo. "TRAPPIST-1 is different; it's a super-group with all seven members synchronizing their parts in nearly perfect time."

Each planet was assigned a pitch by scaling up its orbital frequency by 212 million times, bringing it into the range of human hearing. These piano notes are played every time the planet passes in front of the star, and a drum beat kicks in whenever a planet overtakes its nearest neighbor. The end result can be seen and heard in the video below.

"It seems somehow poetic that this special configuration that can generate such remarkable music can also be responsible for the system surviving to the present day," says Tamayo.

The research appears in The Astrophysical Journal Letters.

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