More than 500 planets orbiting other stars have been recorded since 1995, detected mostly by a tiny wobble in stellar light.
Exoplanets are named after their star and listed alphabetically, in order of discovery.
Until now, the big interest in Gliese 581's roster of planets focused on Gliese 581g.
It leapt into the headlines last year as "Zarmina's World," after its observers announced it had roughly the same mass as Earth's and was also close to the Goldilocks zone.
But that discovery has since been discounted by many. Indeed, some experts suspect that Gliese 581g may not even exist but was simply a hiccup in starlight.
Its big brother, Gliese 581d, has a mass at least seven times that of Earth and is about twice our planet's size, according to the new study, which appears in a British publication, The Astrophysical Journal Letters.
The planet, spotted in 2007, had initially been dismissed as a candidate in the hunt for life.
It receives less than a third of the solar radiation Earth gets, and may be "tidally locked", meaning that one side of it always faces the sun, which would give it permanent dayside and nightside.