  Ocean planets are volatile rich planets, not present in our Solar System,
which are thought to be dominated by deep, global oceans. This results in the
formation of high-pressure water ice, separating the planetary crust from the
liquid ocean and, thus, also from the atmosphere. Therefore, instead of a
carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide
concentration is governed by the capability of the ocean to dissolve carbon
dioxide (CO2).
  In our study, we focus on the CO2 cycle between the atmosphere and the ocean
which determines the atmospheric CO2 content. The atmospheric amount of CO2 is
a fundamental quantity for assessing the potential habitability of the planet's
surface because of its strong greenhouse effect, which determines the planetary
surface temperature to a large degree. In contrast to the stabilising
carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth
atmosphere, we find that the CO2 cycle feedback on ocean planets is negative
and has strong destabilising effects on the planetary climate. By using a
chemistry model for oceanic CO2 dissolution and an atmospheric model for
exoplanets, we show that the CO2 feedback cycle can severely limit the
extension of the habitable zone for ocean planets.
