Why do astronomers believe triton may have been a planet that was captured by neptune? why do astronomers believe triton may have been a planet that was captured by neptune? it is colder than any other moon or planet. it undergoes seasonal changes. it orbits neptune in the opposite direction of neptune's rotation. it has an atmosphere and a measurable greenhouse effect. it is too large to have been formed in the jovian nebula that formed neptune.