Context : We obtained an LBT/PEPSI spectrum with very high resolution and high signal-to-noise ratio ( S/N ) of the K0V host Kepler - 444 , which is known to host 5 sub-Earth size rocky planets .
The spectrum has a resolution of R \approx 250,000 , a continuous wavelength coverage from 4230 Ã Â to 9120 Ã , and S/N between 150–550:1 ( blue to red ) .

Aims : We performed a detailed chemical analysis to determine the photospheric abundances of 18 chemical elements , in order to use the abundances to place constraints on the bulk composition of the five rocky planets .

Methods : Our spectral analysis employs the equivalent width method for most of our spectral lines , but we used spectral synthesis to fit a small number of lines that require special care .
In both cases , we derived our abundances using the MOOG spectral analysis package and Kurucz model atmospheres .

Results : We find no correlation between elemental abundance and condensation temperature among the refractory elements ( T _ { \mathrm { C } } Â > 950 K ) .
In addition , using our spectroscopic stellar parameters and isochrone fitting , we find an age of 10 \pm 1.5 Gyr , which is consistent with the asteroseismic age of 11 \pm 1 Gyr .
Finally , from the photospheric abundances of Mg , Si , and Fe , we estimate that the typical Fe-core mass fraction for the rocky planets in the Kepler - 444Â system is approximately 24 \% .

Conclusions : If our estimate of the Fe-core mass fraction is confirmed by more detailed modeling of the disk chemistry and simulations of planet formation and evolution in the Kepler - 444Â system , then this would suggest that rocky planets in more metal-poor and \alpha -enhanced systems may tend to be less dense than their counterparts of comparable size in more metal-rich systems .