Extrasolar planets are being discovered at a rapid rate, with 4,531 planets in 3,363 systems (with another 7,798 candidates awaiting confirmation). Of these, 166 have been identified as rocky planets (aka. “Earth-like”) while another 1,389 have been rocky planets that are several times the size of Earth (“Super-Earths). As more and more discoveries are made, the focus is shifting from the discovery process towards characterization.
In order to place tighter constraints on whether any of these exoplanets are habitable, astronomers and astrobiologists are looking for ways to detect biomarkers and other signs of biological processes. According to a new study, astronomers and astrobiologists should be on the lookout for indications of a carbon-silicate cycle. On Earth, this cycle ensures that our climate remains stable over the course of eons and could be the key to finding life on other planets.
The study, titled “Carbon cycling and habitability of massive Earth-like exoplanets,” was conducted by Amanda Kruijver, Dennis Honing, and Wim van Westrenen – three Earth scientists with the Vrije Universiteit Amsterdam. Honing is also a fellow with the Origins Center, a Netherlands-based national science institute committed to researching the origins and evolution of life in our Universe. Their study is currently being reviewed for publication in The Planetary Science Journal.
Diagram of the fast carbon cycle, showing the movement of carbon between land, atmosphere, and oceans. Credit: U.S. DOE/BERIS
On Earth, this two-step cycle ensures that carbon dioxide (CO2) levels in our atmosphere remain relatively consistent over time. This first step consists of carbon dioxide being removed from our atmosphere by reacting with water vapor to form carbonic acid, which weathers and dissolves silicate rock. The products of this weathering are washed into the oceans (forming carbonate rock), which sink to the seafloor and become part of the Earth’s mantle.
This is where the second step comes into play. Once in the mantle, carbonate rocks are melted down to create silicate magma and CO2 gas, the latter of which is released back into the atmosphere through volcanic eruptions. As Dr. Honing explained to Universe Today via email, the process is also affected by changes in surface conditions:
“Importantly, the speed of this process depends on the surface temperature: If the surface gets hotter, weathering reactions speed up, and more CO2 can be removed from the atmosphere. Since CO2 is a greenhouse gas, this cools mechanism down the surface, so we have a stabilizing feedback. We have to point out that this stabilizing feedback needs a long time to be efficient, in the order of hundreds of thousand years or even millions of years.”
A key consideration here is how the Sun has been getting hotter with time, Dr. Honing added. Compared to Earth’s early history, our planet now receives roughly 30% more energy from the Sun, which is why atmospheric CO2 levels were higher in the distant past. Therefore, it is safe to say that weathering becomes more pronounced as a planet gets older and that atmospheric CO2 levels will drop at an increasing rate at this point in their evolution.
The terrestrial planets of our Solar System at approximately relative sizes (left to right): Mercury, Venus, Earth, and Mars. Credit: LPI
Since this is a simple chemical process, there is no reason to think that a carbon-silicate cycle couldn’t function on other planets – provided they have liquid water on their surfaces. For exoplanet researchers and astrobiologists, the presence of liquid water has been a key biosignature in the ongoing search for extraterrestrial life. The issue of plate tectonics has also been raised since this plays a significant role in maintaining Earth’s habitability over time. Said Dr. Honing: