physical chemistry - Rotational Constant for CO2 from an IR Vibrational-Rotational spectra

I have been trying to calculate the rotational constants ($B$) for $\ce{CO}$ and $\ce{CO2}$ from IR Vibrational-Rotational spectra.

I know that for $\ce{CO}$ the peak spacing is approximately $2B$ (ignoring effects due to centrifugal distortion) however for $\ce{CO2}$ it seems the peak spacing represents $4B$ instead when I compare my results to literature?

Does anybody have any explanation as to why this is, or could provide reading on this subject to explain why the peak spacing for $\ce{CO2}$ is $4B$?