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Why are Uranus's distant outer moons thought to have been captured?

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Images obtained by Voyager 2. — Uranus Two views of the southern hemisphere of Uranus, produced from images obtained by Voyager 2 on Jan. 17, 1986. In colours visible to the unaided human eye, Uranus is a bland, nearly featureless sphere (left). In a colour-enhanced view processed to bring out low-contrast details, Uranus shows the banded cloud structure common to the four giant planets (right). From the polar perspective of Voyager at the time, the bands appear concentric around the planet's rotational axis, which is pointing nearly toward the Sun. Small ring-shaped features in the right image are artifacts arising from dust in the spacecraft's camera.

Uranus’s distant outer moons are considered to be captured objects because they are classified as irregular satellites, which possess orbital characteristics distinct from the planet's regular moons.12

Unlike the regular moons, which formed in equatorial orbits at the same time as the planet, these outer moons exhibit highly elliptical orbits that are inclined at large angles to Uranus’s equator. Furthermore, most of these irregular satellites orbit in the retrograde direction. Because these orbital properties—high eccentricity, high inclination, and often retrograde motion—are inconsistent with formation within the circumplanetary disk that produced the regular satellites, astronomers conclude that these bodies were originally in orbit around the Sun before being gravitationally captured by Uranus after the planet had already formed.12