Today the number of known planets in the galaxy exceeds 4000. Extrapolating from that figure reveals that the Milky Way harbors more planets than it does stars.

Yet just a quarter century ago, the prospect of conducting such a galactic planet census seemed dim. Although planet-sized bodies had been detected around a neutron star,1 those objects probably didn’t form the way most planets do, and their dead host star is unlikely to foster an environment conducive to life. The only sunlike star known to host planets was the Sun.

It wasn’t that astronomers didn’t know how to go about looking for extrasolar worlds. By the 1980s a handful of researchers were trying to apply a 19th-century star-velocity measurement technique, known as the radial-velocity (RV) method or Doppler spectroscopy, to search for planet-sized companions.

Focusing on nearby bright stars, the astronomers inspected spectral lines for periodic redshifts and blueshifts stemming...

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