  The Earth, Venus, Mars, and some extrasolar terrestrial planets have a mass
and radius that is consistent with a mass fraction of about 30% metallic core
and 70% silicate mantle. At the inner frontier of the solar system, Mercury has
a completely different composition, with a mass fraction of about 70% metallic
core and 30% silicate mantle. Several formation or evolution scenarios are
proposed to explain this metal-rich composition, such as a giant impact, mantle
evaporation, or the depletion of silicate at the inner-edge of the
proto-planetary disk. These scenarios are still strongly debated. Here we
report the discovery of a multiple transiting planetary system (K2-229), in
which the inner planet has a radius of 1.165+/-0.066 Rearth and a mass of
2.59+/-0.43 Mearth. This Earth-sized planet thus has a core-mass fraction that
is compatible with that of Mercury, while it was expected to be similar to that
of the Earth based on host-star chemistry. This larger Mercury analogue either
formed with a very peculiar composition or it has evolved since, e.g. by losing
part of its mantle. Further characterisation of Mercury-like exoplanets like
K2-229 b will help putting the detailed in-situ observations of Mercury (with
Messenger and BepiColombo) into the global context of the formation and
evolution of solar and extrasolar terrestrial planets.
