Rocky planets might need to be the right age to support life
Extrasolar planets are being discovered at a rapid rate—4,531 planets in 3,363 systems (with another 7,798 candidates awaiting confirmation). Of these, 166 have been identified as rocky planets (also known as "Earth-like"), while another 1,389 have been categorized as rocky planets that are several times the size of Earth ("Super-Earths"). As more and more discoveries are made, the focus of astronomers is shifting from the discovery process towards characterization.
In order to quantify 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, indications of a carbon-silicate cycle could be the key. On Earth, this cycle ensures that our climate remains stable for eons, and it 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 Höning, and Wim van Westrenen—three Earth scientists with the Vrije Universiteit Amsterdam. Höning 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 was recently published in The Planetary Science Journal.
On Earth, this two-step cycle ensures that carbon dioxide (CO2) levels in our atmosphere remain relatively consistent over time. In the first step, carbon dioxide is 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, creating carbonate rock that sinks to the seafloor and becomes 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. Höning 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 mechanism cools 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 is how the Sun has been getting hotter with time, Dr. Höning 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.
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 critical to 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. Höning:
"In our own solar system, only planet Earth has plate tectonics and therefore subduction. The reason for this is not entirely clear and subject to modern studies—probably it has to do with the rock composition, planet size, surface temperature, or with the existence of liquid water on the surface itself.
"If we would have weathering on an exoplanet but no subduction, the produced carbonates would accumulate on the surface and may become unstable again after millions of years. We explored this scenario in earlier work and found that the climate would still be regulated to some extent, although somewhat less efficiently than with plate tectonics as assumed in the present paper."
Dr. Höning and his colleagues are hardly alone when it comes to investigating whether plate tectonics and geological activity are essential for life. In recent years, similar research has been conducted that has considered if stagnant lid planets (where the surface and mantle consists of one inactive plate) covered in oceans could still have a carbon cycle—with encouraging results.
For the sake of their study, Dr. Höning and his colleagues sought to determine if a carbon-silicate cycle would be possible on other rocky planets that range from being "Earth-like" to "Super-Earths." To this end, they created a model that reproduced Earth's carbonate-silicate cycle and took all of the relevant processes into account, including interior evolution, volcanic outgassing, weathering, and subduction. They then considered how the model could be sensitive to changes in size and mass.
"For example, the pressure within massive planets increases more strongly with depth since the gravity is higher," said Dr. Höning. "The pressure has an effect on the melting depth and also on the strength of mantle convection, which determines the interior cooling rate. So we updated all model parts that are sensitive to the size or mass of the planet and could therefore explore the influence of these parameters on exoplanet habitability."
What they found was that an increase in mass (to a point) would result in higher average surface temperatures, thereby altering what would be considered the planet's circumsolar habitable zone (also known as the "Goldilocks Zone"). Said Dr. Höning:
"We found that exoplanets of Earth's age but ~3 times more massive should have higher volcanic outgassing rates, since their interior is much hotter and mantle convection therefore more vigorous. The carbonate-silicate cycle can still regulate the climate on these planets, nevertheless we expect a hotter surface. Therefore, the optimal distance between the planet and the star in order to maintain liquid water on the planet surface is a bit further away than Earth's distance to the Sun."
However, the results were the opposite when they increased the mass of a rocky planet up to 10 times that of Earth (which corresponds to ~2 Earth radii). "Here, the pressure within these planets is so large that volcanic activity and outgassing of CO2 becomes smaller," he said. "However, since the heat from their interior is not lost as efficiently, outgassing of CO2 becomes particularly efficient in the later evolution. Unfortunately, stellar luminosity also increases with time, so the planet might then become too hot for any liquid water to exist."
There are many takeaways from these results. For one, the study demonstrates that size and mass are important parameters for planetary habitability. At the same time, size and mass are among the very few parameters that scientists have access to right now. As with the available means of detection—the Transit Method, for example, is very good at constraining these two properties—scientists are somewhat limited by indirect means and must rely on extrapolations and modeling.
However, these two parameters are still very useful for constraining what types of rocky planets could be habitable and which ones aren't likely to support life. What's more, they show how a planet's age and mass play a significant role in maintaining a carbon cycle, and therefore the planet's habitability. By considering these factors together, scientists will be able to say whether a planet is "potentially habitable more confidently." As Dr. Höning summarized:
"One main finding of our paper is that we should really look at the combination of planet size and age to get an idea about habitability. Earth-sized planets should be habitable for a very long period of time, but their atmospheres are of course more difficult to characterize than for larger planets. Planets of 3 times Earth's mass (receiving the same stellar flux) should have a hotter surface than Earth (difference ~10K). Even more massive planets receiving the same stellar flux are a bit cooler, but would significantly heat up later in their evolution."
What's more, this study will be beneficial when next-generation telescopes become available and can conduct direct observations of exoplanets. This is something astronomers expect from the upcoming James Webb Space Telescope (JWST), the Nancy Grace Roman Space Telescope, and ground-based observatories like the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT).
By directly observing light reflected by an exoplanet's atmosphere, astronomers will obtain spectra that reveal the atmosphere's chemical composition. This research could be used for future studies to place the detection of atmospheric CO2 into its proper context. In short, astrobiologists will determine if it's an indication of geological activity and can therefore be interpreted as a possible indication of habitability.
Another encouraging aspect of the study is that even where rocky planets of varying masses and sizes are concerned, the carbonate-silicate cycle remains an efficient regulator of the climate. If scientists detect evidence of this cycle on exoplanets, they can rest assured that it indicates potential habitability, no matter how massive the planet is. "So, we can remain optimistic about finding extraterrestrial life in the future." said Dr. Höning.