While a planet can orbit around a Sun-like star and sit comfortably within its habitable zone, it could also potentially not meet any of the fundamental requirements for sustaining life.
Specifically, a planet can entirely lose its atmosphere.
The University of California, Riverside has produced research on this problem regarding exoplanets in the habitable zone of other stars: how small of a planet will remain habitable if it were to exist in the habitable zone of a Sun-like star? Using a model developed to depict how planets develop, including cooling, outgassing, and atmospheric loss, researchers were able to identify a distinct size threshold. Rocky planets that are smaller than 0.8 times Earth’s radius will have very low chances of being able to maintain an atmosphere over billions of years while in the habitable zone around a Sun-like star.
This information is critical because of the increasing quantity of known exoplanets. With limited time to conduct telescope observations, astronomers must now develop improved processes to determine which smaller rocky exoplanets would have a high probability of developing intelligent life on them.
It is proposed that size should be amongst the first criteria astronomers look for in assessing rocky planets that have the potential to hold life.
The size threshold between small and large planetary bodies is determined by geophysical (gravity) and thermal (cooling) effects.
It may seem intuitive to date these small rocky bodies because of their lower gravity. Gas will escape from small planets more readily than gas from larger planets due to the lower gravity. After all, small rocky planets will also have the lowest escape velocity, thus making them less able to hold on to fast-moving particles within their upper atmospheres, especially when they are subjected to extreme amounts of high-energy radiation from their parent stars when they are young.
However, gravity is only part of the story. Worlds that are smaller than Earth will shed their internal heat at a much more rapid rate than larger planets because they have a higher ratio of surface area to volume. Over time, this process eventually creates a thick lithosphere surrounding the planet, which is the outer layer of a solid body. This stops volcanic activity. Once volcanic activity has stopped, the only method of returning gases from the pummeled crust to the atmosphere is lost.
Sub-Earth worlds are especially vulnerable to this dual mechanism: an easier method of escaping the atmosphere, and an earlier transition away from volcanic outgassing.
Researchers created a model called the “Smaller Than Earth Habitability Model” (STEHM) to study this interaction. The STEHM follows planets between 1.0 and 0.5 Earth radii and estimates how their interiors change, how much CO2 they can release, and how quickly their atmosphere could be removed by extreme UV radiation from a solar-type star.
Using the STEHM model, the results are significant. For the time spans considered, a planet with a radius of 1.0, 0.9, and 0.8 Earth radii will retain an atmosphere for a long time. However, planets that are 0.7 Earth radii or smaller will not.
For example, a 0.7 Earth radius planet would lose its atmosphere in approximately 600 million years, and a 0.6 Earth radius planet would lose its atmosphere in about 400 million years. In contrast, a 0.5 Earth radius planet would completely lose its atmosphere in approximately 30 million years.
That is an extremely short time when compared to other planets.
The STEHM model represents an upper limit, or a best-case scenario, rather than a lower limit or worst-case scenario.
The STEHM model is actually biased in favor of retaining an atmosphere. The team developed a model assuming a pure carbon dioxide atmosphere on these planets, which is a reasonable assumption because CO2 is a dense and cool gas. It is more difficult to lose than the lighter gases that exist on other planets. Further, the planets were modeled as stagnant-lid planets, which means they have one continuous outer shell and do not exhibit plate tectonics like Earth does.
The stagnant-lid assumption is not arbitrary because Earth is the only known planet with active plate tectonics, whereas all other rocky bodies in our solar system are considered to either lack plate tectonics completely or exhibit only limited activity. Although these assumptions are favorable, the cutoff still appeared between 0.7 and 0.8 Earth radii.
The researchers also used Mars and Venus as calibration points for their model, since these are two examples of stagnant-lid planets that experienced very different historical developments.
The simulated results for a Venus-like planet showed that it accumulated and maintained thick CO2 atmospheres. Meanwhile, the simulated results for a Mars-like planet showed that it formed an initial CO2 atmosphere greater than four bars of pressure and lost that atmosphere in less than 200 million years. These results validate the broad expectations for both planets and provide additional validation for applying the model to exoplanets.
There are some limitations to the study because it is based on a 1D model. The current model does not consider weathering to draw down carbon dioxide. The model also does not consider sputtering or ion pickup processes, non-thermal escape processes, magnetic fields, tidal heating, or the unique conditions found in close proximity to M-dwarf stars. Additionally, the model relies on a conservative assumption regarding the Sun’s initial high-energy output. This means that if the early stellar radiation was greater than the model assumes, the upper limit for habitability could be raised.
This definition of what constitutes life is not particularly generous. Rather, it could be seen as the minimum amount of atmosphere necessary for retention.
The threshold defined does not preclude all possibilities for planets below the threshold.
Certain unusual circumstances provide small planets with better prospects. The most important factor was the initial inventory of carbon. The modeling suggests a planet with an initial carbon inventory greater than what is believed to be the case for Earth would have significantly better chances of retaining its atmosphere throughout the modeling run. A planet with 1.0 × 10²³ moles of carbon was shown to be capable of retaining its atmosphere in nearly all instances except for those planets with a radius of 0.5 Earth radii.
Planet core radius played an equally important role in this parameter. Planets with smaller cores and larger mantle sizes were able to retain more of their volatiles and heat-generating elements, which allowed them to undergo degassing for a longer period of time. Under certain modeling conditions, even planets with a radius of 0.7 or 0.6 Earth radii could form thin atmospheres after the harsh early radiation faded away.
Having a cooler initial state for the mantle was also beneficial.
For astronomers looking to identify potentially habitable planets orbiting Sun-like stars, there are some straightforward implications of this research on planet size.
Astronomers can increase their chances of identifying long-term habitability by using a simple threshold of 0.8 Earth radii as an indicator of potentially habitable planets that might still retain an atmosphere. However, just because a planet is larger than 0.8 Earth radii does not guarantee its potential for long-term habitability. Many of these simulated planets ended up with very thick CO2 atmospheres, reaching up to 150 times the atmosphere on Earth, which are likely inhospitable to complex forms of life.
The research also suggests that plate tectonics play a critical role in maintaining carbon dioxide at levels conducive to the development of complex life. Despite these problems, the findings may serve as an important tool by providing astronomers with a way to streamline their search for potentially habitable worlds by evaluating the size of a planet earlier in the process.
Research findings are available online in the journal arXiv.
The original story “Rocky planets smaller than Earth may be too small to stay habitable” is published in The Brighter Side of News.
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