The Harmony That Keeps Trappist-1’s 7 Earth-size Worlds From Colliding

The scientist in the office next door to Dr. Tamayo found musical inspiration from the Trappist-1 planets. Matt Russo, an astrophysicist who is also a musician, turned to Dr. Tamayo’s computer simulations for help turning the orbits into notes, and they have produced a sort of music of the spheres for the 21st century.

“I think Trappist is the most musical system we’ll ever discover,” Dr. Russo said. “I hope I’m wrong.”

While the planets are roughly the size of Earth, the Trappist-1 system is very different from our solar system. Trappist-1 is a dwarf star that is much smaller and colder than our sun, and all seven of the planets orbit within six million miles of the star. By contrast, Mercury, the innermost planet of our solar system, is 36 million miles from our sun. Earth is nearly 93 million miles away.

Since the Trappist-1 planets are so close to their star, they orbit quickly, and their “year” — the time to complete one orbit — ranges from 1.5 days to 19 days.

The original discoverers noted that those orbits were almost exactly in what scientists call “resonance.” That is, the second planet completes five orbits in almost exactly the time the first planet makes eight. The third planet completes three orbits for every five orbits of the second planet, and the fourth planet makes two orbits for every three orbits of the third. The other planets are also in resonance. (In our solar system, Pluto is in resonance with Neptune, with Pluto making two orbits for every three of Neptune.)

Yet when they plugged the data into computer simulations, the orbits quickly became unstable, falling apart in less than a million years. Even when they added the effects of tides on the planets, which tend to push planets toward more circular, stable orbits, the system still often fell apart within a few million years, a cosmic instant compared with the estimated age of the Trappist-1 star (three billion to eight billion years).

“We were missing some physics,” said Amaury H.M.J. Triaud, an astronomer at the University of Cambridge in England and a member of the team that described the Trappist-1 planets. Also missing: exact information about the shape and tilt of the orbits.

Dr. Tamayo and his colleagues took a different approach.

Instead of just looking at the orbits of the planets today, they looked at possible ways that the planets got to where they are now. The planets formed out of a disk of gas and dust. After that formation, the remaining disk would have nudged the planets inward, and those nudges tend to push the planets toward the stable resonances.

Dr. Tamayo offered the analogy of musicians in an orchestra. “It’s not enough for members to merely keep time,” he said.

The missing information about orbits is like musicians playing out of tune, he said. “By…

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