For many years, astronomers have been looking for extraterrestrial life. In doing so, astronomers have narrowed their search to a small “habitable zone” around stars. This habitable zone defines the region in which the temperature of a planet is likely to be warm enough, but not too warm, for liquid water to exist at the surface of the planet.
By analyzing the data in a recent paper published in Astrophysical Journal, Professor Amri Wandel from the Hebrew University of Jerusalem argues that this long-held assumption may actually be too conservative. It may easily dismiss planets that once seemed inhospitable to liquid water as potentially habitable planets because they might still have liquid water on their surfaces.
Wandel’s studies were focused mainly on planets orbiting two types of stellar names, M Dwarf Stars and K Dwarf Stars. These stars are both smaller and cooler than our Sun and make up the majority of stellar types in our galaxies.
The majority of M and K Dwarf Stars are excellent targets for exoplanet detecting. The planets orbiting these stars transit more frequently. They can be easily detected and studied with instruments like the James Webb Telescope, etc. Therefore, they appear to have tremendous potential for finding and studying exoplanets.
However, most of the exoplanets orbiting M and K Dwarf Stars look very different than Earth. Almost all of these planets are “tidally locked.” A planet that is tidally locked is a planet that has one side always facing its host star. The other side of the planet remains in perpetual dark.
Experts once argued that a planet that is tidally locked would not likely be a good candidate for life because of the extreme temperature difference between the two sides of the planet. For instance, one side may have “scorching heat.” The opposite side would have “freezing temperatures.” This would make it impossible for the atmosphere to exist on the planet due to atmospheric collapse.
However, Wandel argues that this assumption is incorrect by performing a comparison of the heat flow on such planets using his analytical climate model. “We found that the amount of heat circulating from the dayside of a planet to the nightside is more efficient than previously thought. If there is sufficient transfer of heat, more than a hemisphere’s worth of the dark hemispheres on most worlds can be affected when they are closer to their stars using traditional models of astronomy,” he explained to The Brighter Side of News.
Instead of just focusing on how warm or cold a world can get on average, Wandel’s study also looks at the ability of any part of the surface within that planet’s orbit to support permanent liquid water under the same conditions.
In a tidally locked world, the nightside of the world might be the only place to support liquid water.
In addition, Wandel’s research has provided further understanding of previously observed phenomena based on the work of the James Comet Mike Space Telescope. This telescope has been able to detect the presence of gaseous forms of water (H₂O), as well as many gases associated with liquid water, in the atmospheres of warm, nearby, super-Earth-type planets orbiting around M-dwarf stars.
These are mostly found within what would generally be considered an inner boundary of the habitable zone. Due to these observations, it is now difficult to state that there should be no evidence to support the idea that H₂O would survive around such star systems.
In addition to the inner edge of the habitable zone being able to accommodate places for the presence of H₂O in its gaseous form, Wandel’s research also extends the outer edge of the habitable zone outwards. On other worlds within our solar system, for example Earth, there could be evidence for permanent liquid water being located under the covering of thick, ice-crusted layers. It could also exist below the covering of a permanent ice-frosted layer.
If conditions were to allow for sufficient subsurface pressure to build up during ice onset or onset of maximum freezing temperatures, such as when subglacial or intraglacial lakes were present, these places where permanent liquid water might have existed would also mean that there are places on these worlds where there is a chance of continued existence of permanent liquid water. This could occur despite atmospheric or environmental conditions remaining frozen.
While water is viewed as the most important of all life-supporting compounds, it is only one of several critical needs that living systems have for proper functioning. These needs ultimately include both chemical energy and long-term stability.
Traditional limits of habitability are typically based on two different sets of legibility limits. The inner limit represents the point of maximum water vapor loss through atmospheric escape. The outer limit represents the point at which any planet has frozen over completely.
This study proposed a simpler approach for estimating the habitability of exoplanets around different types of stars. The luminosity of main-sequence stars is directly related to the size and temperature of those stars. Main-sequence stars are in the process of fusing hydrogen to helium in their cores. This relationship means that their luminosities can be positively correlated to their mass and size.
Using the relationships established in this study, scaling laws can be devised for estimating the location on exoplanets that would support habitable conditions. This can be done without the necessity of performing complex climate models.
The intention of such an approach is to aid researchers in directing future observations. It is not intended to provide a substitute for the more detailed models.
A major conclusion of this work is that, generally, tidal locks on exoplanets are more likely to have habitable conditions at their star than those that have a faster rotation. These are exoplanets that are constantly in orbit around the same planet at the same distance from their star. In contrast, planets with a day-length cycle comparable to the rotation of a solar system every one to two years are less likely to be habitable when compared to Earth.
Whereas previously researchers tended to consider whether or not the entire zone would be too hot, Wandel proposed that researchers ask themselves if some part will remain cool enough.
In summary, the research has provided two potential scenarios. The first scenario is the presence of liquid water on the surface of the planet. The second scenario involves the possibility of the presence of frozen water beneath a solidified nightside of an exoplanet. This could occur even though the conditions at the surface may be entirely extreme.
Two parameters are established as frames for this study. One of these is the Heating Factor, which incorporates stellar energy, surface reflectivity (albedo), and greenhouse warming. The other is the Heat Transport Fraction, which describes how much incoming energy is distributed uniformly across the planet.
In the case of various heavenly bodies, drastically different ratios exist with respect to heat transport. For instance, with regard to Mars, close to 85 percent of the energy that is absorbed by Mars is distributed globally. On the other hand, Mercury absorbs nearly 100 percent of its received energy. There is little to no global redistribution of that energy.
Utilizing moderate heat transport values, the author deduces that liquid water on the night side of a planet could remain viable for many planets. This would apply to planets that receive multiple times the solar energy received by Earth.
Additionally, according to the research, a tidally locked planet could occupy an orbit nearly three times closer to the parent star than the inner boundary defined by conservative estimates. It could still maintain liquid water on its night side.
Before establishing a definition for habitability linked to location or distance, the author Wandel established definitions for habitability that were based instead on heating. This heating refers to energy input produced from incoming stellar energy. It represents the amount of energy reaching the planet from its parent star. It also includes the surface reflectivity (albedo) and the greenhouse warming effect.
With the heating-based model of habitability, habitability is defined by the extent to which a planet falls within the range of temperature between freezing and boiling. This range is related to its surroundings. All of this holds true in locations that are permanently covered by ice. It also applies in regions where there is water below the planet’s surface.
This study proposed various atmospheric patterns. Examples include hot day-side and cool night-side waters.
There are also other important findings from this research that have implications for searching for extrasolar planets. Based on previous studies, M-dwarf systems are easier to study than other types of stars. Thus, the findings of this research indicate that many more planets orbiting M-dwarfs will likely contain water.
If scientists do not have access to the atmospheric data of the exoplanets, the study suggests using infrared measurements of the exoplanets. This can serve as a method of testing for the presence of exoplanets with substantial atmospheres.
In this situation, scientists can use the heat transport to determine whether the basic atmospheric condition has a significant effect on the temperatures on the day and night sides. If the pattern, or heat transport subsystem, can be identified, then it supports the idea that a significant amount of atmosphere exists.
Finally, the recent discoveries of water vapour on close-in planets may suggest that there may be night-side or subsurface oceans of water on the planets. These are exoplanets orbiting stars. If this holds true, this would support an even broader definition of habitability than the currently accepted definition.
According to Wandel, based on the research findings, there may be far more potentially habitable exoplanets orbiting M-dwarfs and K-dwarfs than was previously theorized by conservative model estimates. In fact, the difference in potentially habitable exoplanets between M-dwarfs and Sun-like stars could be as great as a factor of 50.
Research findings are available online in the Astrophysical Journal.
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