Nature 494, 452–454 (2013); doi:10.1038/nature11914

In this Letter, the Centro de Astrobiología affiliation (number 11, associated with authors David Barrado and Jorge Lillo-Box) was listed incorrectly; the correct address is: ‘Departamento Astrofísica, Centro de Astrobiología (INTA-CSIC), ESAC campus, PO Box 78, E-28691 Villanueva de la Cañada, Spain’. This has been corrected in the HTML and PDF versions of the manuscript.