Where the Forest Meets the Stars by Glendy Vanderah

Life has taught Jo Teale and Gabriel Nash to trust in science and logic rather than miracles. But when a mysterious girl appears in the woods claiming to be from the stars, their carefully constructed worlds begin to shift. The child, calling herself Ursa Major, says she needs to witness five miracles before she can return home. As Jo and Gabe help her on her quest, they discover that the greatest miracle might be their own healing. Between scientific certainty and stellar magic, three wounded souls learn that love sometimes requires a leap of faith.