Among Kepler and TESS's many surprising exoplanet discoveries are the ultra-short-period (USP) planets. These planets move very quickly and complete an orbit in less than 24 hours. Ancient astronomers thought that Mercury was so fast—it goes around the sun every 88 days—that they named it after the Roman god Mercury, who was the fast-traveling messenger of the gods. They couldn't have known how sluggish Mercury is compared to the USPs.
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