In 2016, E. R. Newton and collaborators pointed out that for early M dwarfs, planets in their habitable zones may have orbital periods coinciding with the stellar rotation period (or in rare cases, such as Gliese 581d, multiples of it); this aspect seriously complicates the verification of any of those planets. Hence, this kind of number relation does not constitute a valid counterargument against the reality of such a planet, which tends to further strengthen the case for the existence of Gliese 581d.