The first detailed chemical abundance analysis of the M dwarf ( M4.0 ) exoplanet-hosting star Ross 128 is presented here , based upon near-infrared ( 1.5–1.7 \micron ) high-resolution ( R \sim 22,500 ) spectra from the SDSS-APOGEE survey .
We determined precise atmospheric parameters T _ { eff } =3231 \pm 100K , log g =4.96 \pm 0.11 dex and chemical abundances of eight elements ( C , O , Mg , Al , K , Ca , Ti , and Fe ) , finding Ross 128 to have near solar metallicity ( [ Fe/H ] = +0.03 \pm 0.09 dex ) .
The derived results were obtained via spectral synthesis ( 1-D LTE ) adopting both MARCS and PHOENIX model atmospheres ; stellar parameters and chemical abundances derived from the different adopted models do not show significant offsets .
Mass-radius modeling of Ross 128b indicate that it lies below the pure rock composition curve , suggesting that it contains a mixture of rock and iron , with the relative amounts of each set by the ratio of Fe/Mg .
If Ross 128b formed with a sub-solar Si abundance , and assuming the planet ’ s composition matches that of the host-star , it likely has a larger core size relative to the Earth despite this producing a planet with a Si/Mg abundance ratio \sim 34 % greater than the Sun .
The derived planetary parameters – insolation flux ( S _ { Earth } =1.79 \pm 0.26 ) and equilibrium temperature ( T _ { eq } =294 \pm 10K ) – support previous findings that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone .