Observations of small extrasolar planets with a wide range of densities imply a variety of planetary compositions and structures .
Currently , the only technique to measure the bulk composition of extrasolar planetary systems is the analysis of planetary debris accreting onto white dwarfs , analogous to abundance studies of meteorites .
We present measurements of the carbon and oxygen abundances in the debris of planetesimals at ten white dwarfs observed with the Hubble Space Telescope , along with C/O ratios of debris in six systems with previously reported abundances .
We find no evidence for carbon-rich planetesimals , with C/O < 0.8 by number in all 16 systems .
Our results place an upper limit on the occurrence of carbon-rich systems at < 17 percent with a 2 \sigma confidence level .
The range of C/O of the planetesimals is consistent with that found in the Solar System , and appears to follow a bimodal distribution : a group similar to the CI chondrites , with \log ( { < } \mathrm { C / O } { > } ) = -0.92 , and oxygen-rich objects with C/O less than or equal to that of the bulk Earth .
The latter group may have a higher mass fraction of water than the Earth , increasing their relative oxygen abundance .