The super-Earth exoplanet 55 Cnc e , the smallest member of a five-planet system , has recently been observed to transit its host star .
The radius estimates from transit observations , coupled with spectroscopic determinations of mass , provide constraints on its interior composition .
The composition of exoplanetary interiors and atmospheres are particularly sensitive to elemental C/O ratio , which to first order can be estimated from the host stars .
Results from a recent spectroscopic study analyzing the 6300 Å [ O I ] line and two C I lines suggest that 55 Cnc has a carbon-rich composition ( C/O=1.12 \pm 0.09 ) .
However oxygen abundances derived using the 6300 Å [ O I ] line are highly sensitive to a Ni I blend , particularly in metal-rich stars such as 55 Cnc ( [ Fe/H ] = 0.34 \pm 0.18 ) .
Here , we further investigate 55 Cnc ’ s composition by deriving the carbon and oxygen abundances from these and additional C and O absorption features .
We find that the measured C/O ratio depends on the oxygen lines used .
The C/O ratio that we derive based on the 6300 Å [ O I ] line alone is consistent with the previous value .
Yet , our investigation of additional abundance indicators results in a mean C/O ratio of 0.78 \pm 0.08 .
The lower C/O ratio of 55 Cnc determined here may place this system at the sensitive boundary between protoplanetary disk compositions giving rise to planets with high ( > 0.8 ) versus low ( < 0.8 ) C/O ratios .
This study illustrates the caution that must applied when determining planet host star C/O ratios , particularly in cool , metal-rich stars .