The solar system’s retinue of known, faraway worlds has gained another member: a small, icy body that takes 40,000 years to plod once around the sun, traveling farther away from our home star than all known solar system objects except for comets. The last time 2015 TG387 was anywhere within whispering distance of the sun, mammoths and cave bears trampled Eurasian grasses, and modern humans were crafting tools from stone.

Called 2015 TG387 (and nicknamed the Goblin), the world is likely spherical and about as wide as the state of Massachusetts. And—like a handful of other distant solar system inhabitants—its orbital behavior might signal the presence of an unseen Planet X lurking in the distant outer dominions of the