Evolution of outer solar system orbits, illustration. At upper left, the outer giant planets are forming from the remnants of the solar nebula, with the orbits shown as red lines. The largest and closest of these planets to the Sun is Jupiter. The other three (Saturn, Neptune and Uranus) start to spiral outwards (upper centre) due to gravitational effects. Alignments between the planets (yellow arrow, upper right) starts to distort the orbits. This results in relatively eccentric orbits for the outer three giant plants (lower left). A close encounter between Saturn and Uranus affects their axial tilts (lower centre). Finally, at lower right, the planets are seen in their existing orbits.