  The exploration of carbon-to-oxygen ratios has yielded intriguing insights
into the composition of close-in giant exoplanets, giving rise to a distinct
classification: carbon-rich planets, characterized by a carbon-to-oxygen ratio
$\ge$ 1 in their atmospheres, as opposed to giant planets exhibiting
carbon-to-oxygen ratios close to the protosolar value. In contrast, despite
numerous space missions dispatched to the outer solar system and the proximity
of Jupiter, Saturn, Uranus, and Neptune, our understanding of the
carbon-to-oxygen ratio in these giants remains notably deficient. Determining
this ratio is crucial as it serves as a marker linking a planet's volatile
composition directly to its formation region within the disk. This article
provides an overview of the current understanding of the carbon-to-oxygen ratio
in the four gas giants of our solar system and explores why there is yet no
definitive dismissal of the possibility that Jupiter, Saturn, Uranus, or
Neptune could be considered carbon-rich planets. Additionally, we delve into
the three primary formation scenarios proposed in existing literature to
account for a bulk carbon-to-oxygen ratio $\ge$ 1 in a giant planet. A
significant challenge lies in accurately inferring the bulk carbon-to-oxygen
ratio of our solar system's gas giants. Retrieval methods involve integrating
in situ measurements from entry probes equipped with mass spectrometers and
remote sensing observations conducted at microwave wavelengths by orbiters.
However, these methods fall short of fully discerning the deep carbon-to-oxygen
abundance in the gas giants due to their limited probing depth, typically
within the 10-100 bar range.
