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The bulk chemical composition and interior structure of rocky exoplanets areof fundamental importance to understanding their long-term evolution andpotential habitability. Observations of the chemical compositions of the solarsystem rocky bodies and of other planetary systems have increasingly shown aconcordant picture that the chemical composition of rocky planets reflects thatof their host stars for refractory elements, whereas this expression breaksdown for volatiles. This behavior is explained by devolatilization duringplanetary formation and early evolution. Here, we apply a devolatilizationmodel calibrated with solar system bodies to the chemical composition of ournearest Sun-like stars -- $\alpha$ Centauri A and B -- to estimate the bulkcomposition of any habitable-zone rocky planet in this binary system("$\alpha$-Cen-Earth"). Through further modeling of likely planetary interiorsand early atmospheres, we find that compared to Earth, such a planet isexpected to have (i) a reduced (primitive) mantle that is similarly dominatedby 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-storagecapacity; and (iv) a CO$_2$-CH$_4$-H$_2$O-dominated early atmosphere thatresembles 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 haveless efficient mantle convection and planetary resurfacing, with a potentiallyprolonged history of stagnant-lid regimes.

A model Earth-sized planet in the habitable zone of $\alpha$ Centauri A/B