  The carbon-silicate cycle regulates the atmospheric $CO_2$ content of
terrestrial planets on geological timescales through a balance between the
rates of $CO_2$ volcanic outgassing and planetary intake from rock weathering.
It is thought to act as an efficient climatic thermostat on Earth and, by
extension, on other habitable planets. If, however, the weathering rate
increases with the atmospheric $CO_2$ content, as expected on planets lacking
land vascular plants, the carbon-silicate cycle feedback can become severely
limited. Here we show that Earth-like planets receiving less sunlight than
current Earth may no longer possess a stable warm climate but instead
repeatedly cycle between unstable glaciated and deglaciated climatic states.
This has implications for the search for life on exoplanets in the habitable
zone of nearby stars.
