G. Anglada-Escudé, P. Amado, J. Barnes, Z. Berdiñas, R. Butler, G. Coleman, I. de la Cueva, S. Dreizler, M. Endl, B. Giesers and 21 other author(s). (2016)cite arxiv:1609.03449Comment: Version accepted for publication by Nature (unedited, July 7th, 2016), figures inserted in text for easier read. Article body : 10 pages, 3 figures, 1 table. Methods section : 23 pages, 9 figures, 1 table. IMPORTANT NOTE : Acknowledgment to IAA/CSIC researchers Javier Pascual Granado and Rafael Garrido added for useful discussions and feedback on the mathematical properties of time-series.