BACKWARDS MOON

摘要

Neptune is the farthest planet from the sun and home to the only backwards moon of substantial size in the solar system. While Neptune spins one way on its axis, its largest moon, Triton, circles the planet the other way. Large moons form out of the debris swirling around a newborn planet, so they should circle in the same direction as the planet's rotation. The fact that Triton, whose diameter is one-fifth that of the Earth, does not circle in this way suggests that its origins lie elsewhere.