.planet {
    perspective: 500px;
    margin: 20px;
    display: inline-block;
}

.planet-content {
    position: relative;
    height: 250px;
    width: 250px;
    transition: transform 1s;
    transform-style: preserve-3d;
}

.planet:hover .planet-content {
    transform: rotateY(180deg);
    transition: transform 0.5s;
}

.planet-front,
.planet-back {
    position: absolute;
    backface-visibility: hidden;
    width: 100%;
    height: 100%;
}

.planet-front {
    background-image: url('https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Mercury_in_color_-_Prockter07-edit1.jpg/480px-Mercury_in_color_-_Prockter07-edit1.jpg');
    background-repeat: no-repeat;
    background-color: #000000;
    background-position: center;
    background-size: 80%;
}

.planet-back {
    background: #000000;
    color: #ffffff;
    padding: 10px;
    transform: rotateY( 180deg );
}