* {
    box-sizing: border-box;
}

article,
aside {
    margin-bottom: 2.5%;
    padding: 5%;
    border: 1px solid lightgray;
    text-align: center;
}
aside {
    background: #ddd;
}

@media (min-width: 900px) {
    main {
        display: flex;
    }
    article {
        flex: 3;
    }
    aside {
        flex: 1;
    }
}

/* planets */

.planets {
    margin: 0;
    padding: 0;
}

.planet {
    list-style: none;
    border: 2px solid black;
    border-radius: 50%;
    height: 200px;
    width: 200px;
    margin-bottom: 20px;
    margin-left: auto;
    margin-right: auto;
    padding: 10px;
}

@media (min-width: 900px) {
    .planets {
        display: flex;
        flex-wrap: wrap;
        justify-content: space-around;
    }

    .planet {
        margin-left: 20px;
        margin-right: 20px;
    }
}

@media (min-width: 1200px) {
    .planet {
        width: 300px;
        height: 300px;
     }
}