Planetary system evolution. Artwork of two theories of how planetary systems evolve with unusual orbits. The first two frames (left) show the planets condensing from a disc of dust around a star. In the Tidal Parade" theory (top), three planets form, and the large outer one (red) slows the orbits of the inner two until they fall into the star. The large planet is slowed by dust, and begins to fall in, but stabilises close to the star as the dust dissipates. The lower path (the "Jumping Jupiters" theory) shows how gravitational interactions between planets can send one (purple) out of the system altogether, and leave the survivors in highly eccentric orbits."