TEEGARDEN'S STAR is a tiny, dim object in the zodiacal constellation of Aries. It has a tenth of the sun's mass and emits most of its light in the infrared part of the spectrum. That makes it too faint to see with the naked eye, even though it is only 12 light-years away. So far, so unremarkable. But when astronomers at Calar Alto Observatory, in Spain, started scrutinising it, they spotted tiny wobbles in its motion. In 2019, after three years of careful measurement, they concluded that these are a consequence of the gravitational fields of two planets tugging the star around. The innermost, Teegarden b, has roughly the same mass as Earth, receives a similar amount of illumination from its host star and is probably rocky.
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