This simulation shows the complex but predictable pattern, called an orbital resonance, which occurs as the recently discovered TRAPPIST-1 planets exert a periodic gravitational tug on each other as they orbit their star. The three-body resonance of the outer three planets causes the planets to repeat the same relative positions. The expect effects of that resonance was used to predict the orbital period of TRAPPIST-1h.
The animation shows the planets orbiting for 90 Earth-days. After 15 Earth-days, the animation only shows the outer three planets: TRAPPIST-1f, TRAPPIST-1g, TRAPPIST-1h. The the simulations stops briefly each time two adjacent planets pass each other, and an arrow appears pointing to the location of the third planet.
Video Credit: NASA