My son, who is almost four, has recently become obsessed with the universe. He can recite the planets of the solar system and tell anyone who will listen that Pluto – his favourite – was once a planet but was reclassified as a “dwarf planet” in 2006. Now that he has a sense of scale and size, he has developed a habit of comparing everything: an ant is smaller than a leaf, the Earth is smaller than the sun, the sun is just a dot in the galaxy, and the galaxy is a tiny part of the universe. “And...