header {
    text-align: center;
    background-color: blue;
    padding: 20px;
}
footer {
    padding: 20px;
    background-color: blue;
}
main {
    background-color: black;
    color: white;
}
.planetes {
    align-items: center;
    display: flex;
    flex-wrap: wrap;
    
}
.planetes img {
    width: 400;
    opacity: 0.5;
}
.planetes {
    padding: 40px;
}
.systeme_solaire {
    text-align: center;
    align-items: center;
}
h2 {
    text-align: center;
}