The poetic physicist Alan Lightman sieves four centuries of scientific breakthroughs, from Kepler’s revolutionary laws of planetary motion to the thousands of habitable exoplanets discovered by NASA’s Kepler mission, to estimate that even with habitable planets orbiting one tenth of all stars, the faction of living matter in the universe is about one-billionth of one-billionth: If all the matter in the universe were the Gobi desert, life would be but a single grain of sand.