Researchers from Europe have developed a new model in order to study whether liquid water could be maintained on planetary surfaces in various conditions. Their model considers the effect of land/ocean distribution and even the fraction of vegetation that could cover a planet’s surface. In their findings, one planet — TRAPPIST-1d — stood out as being potentially the most habitable planet in that system.
“We can investigate the main features of each planet by using a simple model, particularly useful when little information is known about planetary characteristics, since for more complex models we need to know several planetary features,” said Tommaso Alberti, a physicist at the University of Calabria in Italy, in an email to Seeker.
Alberti and his colleagues say in their paper that their model is a simple climate-vegetation energy-balance model. They used it to study the seven TRAPPIST-1 planets to determine their climate dependence on three things: the global albedo (i.e. the energy from the star that is reflected back to space from the planet), the fraction of vegetation that could cover their surfaces, and the different greenhouse conditions.
“The model allows us to investigate whether liquid water could be maintained on the planetary surfaces (i.e., by defining a ‘surface water zone’) in different planetary conditions, with or without the presence of greenhouse effect,” the team wrote in their paper.
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Determining the potential habitability of a specific exoplanet is more complicated than just figuring out if the planet is in the host star’s habitable zone. Numerous other characteristics factor into it, as the planets in our own solar system attest. While Venus, Earth, and Mars are all considered to be in the Sun’s habitable zone, Earth is currently the only habitable planet — by human standards, anyway. The runaway greenhouse atmosphere on Venus is too thick to support life as we know it, while Mars’ thin atmosphere doesn’t allow liquid water to stay on the planet’s surface.
On Earth, we know that everywhere there is water, there is life. But even though the TRAPPIST-1 planets are Earth-size, with some planets in the habitable zone, the conditions in that planetary system are very different from ours. The seven planets orbit quite close to their ultra-cool red dwarf star, with orbits lasting between 1.5 and 20 days, compared to Earth's 365 days. Because they are so close to their star, the planets are likely tidally locked, with one side always facing the star. This means conditions could vary widely from one side of the planet to the other. Additionally, the star gives off different wavelengths of light than our Sun. TRAPPIST-1 is much redder, and emits longer wavelengths, including those in the near-infrared.
So, how do you try to answer a complicated question like potential habitability with so many variables? Alberti and his colleagues responded by making their model as simple as possible.
The research team said that one of the drawbacks of using detailed climate models is the necessarily large pool of assumptions of atmospheric and surface conditions. So instead, they used a simple zero-dimensional energy-balance model “which allows the extraction of global information on the climate evolution by using the actual knowledge about the planetary system.”
Zero-dimensional energy balance models have been used to study Earth’s climate. Such a model takes a single planet’s characteristics separately instead of using a broader model for the entire solar system. More specifically, the model uses the planet as a single point, using an average global temperature.
“The term zero-dimensional is related to the fact that we do not take into account latitudinal and longitudinal variations into the energy balance,” Alberti told Seeker, “between incoming stellar radiation, which depends on the star-planet distance, and the outcoming planetary radiation, depending on the vegetation (and so on the albedo) and on the atmospheric composition (and so on the greenhouse effect).”
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This model has the advantage of transparency through minimal assumptions, allowing comparative sets of models to be studied. The researchers looked at several situations, from completely barren, rocky planets to Earth-like conditions, both neglecting and considering the greenhouse effect, to explore different possible climates and make a comparative study of TRAPPIST-1 planetary system climates.
“We are able to investigate different scenarios for TRAPPIST planetary system moving from rocky planets to Earth-like planets with similar and/or different greenhouse conditions and to underline the role of vegetation in defining a particular climate state,” Alberti said. “Finally, by defining the surface water zone, defined as the circumstellar region where a planet can host liquid water on its surface, we showed that this zone is strongly dependent on the different parameters of the model and, in particular, on the initial fraction of vegetation coverage, the presence of oceans and the greenhouse effect.”
Alberti said it is well known that vegetation is able to change planetary albedo (i.e., the fraction of incidental radiation reflected back to space), and consequently it affects temperature evolution. Previous studies have shown that if vegetation is widespread enough, it would affect the reflective properties of the whole planet. The albedo on a vegetated planet is much less than on a non-vegetated planet.
Alberti and his team previously investigated an energy-balance model with two types of vegetation using the famous Daisyworld model, which is a 1983 computer simulation that creates a hypothetical world covered with either white daisies or black daisies to study elements in the Earth-Sun system.
“In that study, we found that vegetation is one of the main feedbacks which affect temperature evolution, together with the greenhouse effect,” Alberti said. Additionally, an ocean vs. land also has a big effect, as oceans have a very low albedo.
“By defining the surface water zone, defined as the circumstellar region where a planet can host liquid water on its surface,” Alberti explained, “we showed that this zone is strongly dependent on the different parameters of the model and, in particular, on the initial fraction of vegetation coverage, the presence of oceans and the greenhouse effect.”
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The team said they determined that outer planets (f, g, and h) are too cold and cannot host liquid water on their surfaces. But they found that the three “inner” planets — TRAPPIST1-b, c, and d — appear to have the ability to hold water on their surfaces.
However, the fourth planet, TRAPPIST-1d, was found to be the most stable from an Earth-like perspective, “since it resides in the surface water zone for a wide range of reasonable values of the model parameters.” This result differs from a paper that came out earlier this year that used a more detailed three-dimensional model, which took into account a variety of greenhouse gases and showed that the best candidate for a habitable ocean-covered planet was TRAPPIST1-e. Alberti’s team said their model showed water oceans could only exist on the “e” planet with greenhouse effect conditions different from the Earth.
But in their paper, Alberti and his colleagues did allow that the greenhouse effect needs to be properly considered, since it is one of the main feedbacks in regulating thermal energy balance. They are looking to continue their research using an improved version of their model.
“In particular, we are investigating a two-dimensional version in which both latitudinal and longitudinal variations are included,” Alberti said. “These aspects are crucial for TRAPPIST planetary system because planets are tidally locked implying a higher temperature gradient between the two sides of each planet.”
Using models like this to be able to garner this much information about distant exoplanets is exciting because, in theory, the same type of instruments on board satellites that study Earth could be used to confirm the presence of continents and vegetation on the surface of faraway worlds. Just as satellites observe the reflected light coming from Earth to determine crop growth and water distribution, more sophisticated and sensitive instruments could look at the light curves from exoplanets to determine more details about their surfaces and atmospheres.
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