---

.planet {
    width: 150px;
    height: 150px;
    position: absolute;
    top: 10%;
    left: 40%;
    background-color: purple;
    border-radius: 100%;
    animation: move 2s infinite;
}

@keyframes move {
    0% { top: 10%; }
    50% { top: 80%; }
    100% { top: 10%; }
}