  The bulk chemical composition and interior structure of rocky exoplanets are
of fundamental importance to understanding their long-term evolution and
potential habitability. Observations of the chemical compositions of the solar
system rocky bodies and of other planetary systems have increasingly shown a
concordant picture that the chemical composition of rocky planets reflects that
of their host stars for refractory elements, whereas this expression breaks
down for volatiles. This behavior is explained by devolatilization during
planetary formation and early evolution. Here, we apply a devolatilization
model calibrated with solar system bodies to the chemical composition of our
nearest Sun-like stars -- $\alpha$ Centauri A and B -- to estimate the bulk
composition of any habitable-zone rocky planet in this binary system
("$\alpha$-Cen-Earth"). Through further modeling of likely planetary interiors
and early atmospheres, we find that compared to Earth, such a planet is
expected to have (i) a reduced (primitive) mantle that is similarly dominated
by silicates albeit enriched in carbon-bearing species (graphite/diamond); (ii)
a slightly larger iron core, with a core mass fraction of $38.4_{-5.1}^{+4.7}$
wt% (cf. Earth's 32.5 $\pm$ 0.3 wt%); (iii) an equivalent water-storage
capacity; and (iv) a CO$_2$-CH$_4$-H$_2$O-dominated early atmosphere that
resembles that of Archean Earth. Further taking into account its $\sim$ 25%
lower intrinsic radiogenic heating from long-lived radionuclides, an ancient
$\alpha$-Cen-Earth ($\sim$ 1.5-2.5 Gyr older than Earth) is expected to have
less efficient mantle convection and planetary resurfacing, with a potentially
prolonged history of stagnant-lid regimes.
