  The planetary nebula (PN) stage is the ultimate fate of stars with mass 1 to
8 solar masses (M$_\odot$). The origin of their complex morphologies is poorly
understood, although several mechanisms involving binary interaction have been
proposed. In close binary systems, the orbital separation is short enough for
the primary star to overfill its Roche lobe as it expands during the Asymptotic
Giant Branch (AGB) phase. The excess material ends up forming a common-envelope
(CE) surrounding both stars. Drag forces would then result in the envelope
being ejected into a bipolar PN whose equator is coincident with the orbital
plane of the system. Systems in which both stars have ejected their envelopes
and evolve towards the white dwarf (WD) stage are called double-degenerates.
Here we report that Henize 2-428 has a double-degenerate core with a combined
mass unambiguously above the Chandrasekhar limit of 1.4 M$_\odot$. According to
its short orbital period (4.2 hours) and total mass (1.76 M$_\odot$), the
system should merge in 700 million years, triggering a Type Ia supernova (SN
Ia) event. This finding supports the double-degenerate, super-Chandrasekhar
evolutionary channel for the formation of SNe Ia.
