Habitable Zone Research Paper

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Habitable Zone Research Paper

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Explaining the Circumstellar Habitable Zone

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In the past decade, the number of known exoplanets has reached just shy of , and many more are expected to be found once next-generation telescopes are put into service. And with so many exoplanets to study, research goals have slowly shifted away from the process of discovery and toward characterization. Unfortunately, scientists are still plagued by the fact that what we consider to be a " habitable zone " is subject to a lot of assumptions. Addressing this, an international team of researchers recently published a paper in which they indicated how future exoplanet surveys could look beyond Earth-analog examples as indications of habitability, and adopt a more comprehensive approach.

The paper, titled " Habitable Zone predictions and how to test them ," recently appeared online and was submitted as a white paper to the Astro Decadal Survey on Astronomy and Astrophysics. The team behind it was led by Ramses M. The purpose of the decadal survey is to consider previous progress in various fields of research and to set priorities for the coming decade.

One of the overriding priorities of exoplanet research is looking for planets where extra-terrestrial life could exist. In this respect, scientists designate planets as being "potentially habitable" and therefore worthy of follow-up observations based on whether or not they orbit within their stars' habitable zones HZ. For this reason, it is prudent to take a look at what goes in to defining a HZ. As Ramirez and his colleagues indicated in their paper, one of the major issues with exoplanet habitability is the number of assumptions made. To break it down, most definitions of HZs assume the presence of water on the surface since this is the only solvent currently known to host life.

These same definitions assume that life requires a rocky planet with tectonic activity orbiting a suitably bright and warm star. However, recent research has cast doubt on many of these assumptions. This includes studies that indicate that atmospheric oxygen does not automatically mean the presence of life — especially if that oxygen is the result of chemical dissociation and not photosynthesis. Other research has shown that the presence of oxygen gas during the early periods of a planet's evolution could prevent the rise of basic life forms. Also, there have been recent studies showing that plate tectonics may not be necessary for life to emerge, and that so-called "water worlds" may not be able to support life but still could.

On top of all that, you have theoretical work that suggests that life could evolve in seas of methane or ammonia on other celestial bodies. The key example here is Saturn's moon Titan, which boasts an environment that is rich in prebiotic conditions and organic chemistry, which some scientists think could support exotic lifeforms. In the end, scientists search for known biomarkers like water and carbon dioxide because they are associated with life on Earth, the only known example of a life-bearing planet.

But as Ramirez explained to Universe Today via email, this mindset where Earth analogues are considered suitable for life is still fraught with problems:. For instance, it assumes that multi-bar CO2 atmospheres can be supported on potentially habitable planets near the habitable zone outer edge. However, such high CO2 levels are toxic to Earth plants and animals, and thus without a better understanding of the limits of life, we do not know how reasonable this assumption is. In a previous study, Dr. Ramirez showed that the presence of methane and hydrogen gas could also cause global warning, and thus extend the classical HZ somewhat.

This came just a year after he and Lisa Kaltenegger an associate professor with the Carl Sagan Institute at Cornell University produced a study showing that volcanic activity which releases hydrogen gas into the atmosphere could also extend a star's HZ. Luckily, researchers will have the opportunity to test these definitions, thanks to the deployment of next-generation telescopes. Not only will scientists be able to test some of the longstanding assumptions on which HZs are based, they will able to compare different interpretations.

According to Dr. Ramirez, a good example is provided by the levels of CO2 gas that are dependent on a planet's distance from its star:. This would also test whether the carbonate-silicate cycle, which is what many believe has kept our planet habitable for much of its history, is a universal process or not. In this process, silicate rocks are converted to carbon rocks through weathering and erosion, while carbon rocks are converted to silicate rocks through volcanic and geological activity.

This cycle ensures the long-term stability of Earth's atmosphere by keeping CO2 levels consistent over time. It also illustrates that water and plate tectonics are essential to life as we know it. However, this type of cycle can only exist on planets that have land, which effectively rules out "water worlds. With this amount of water on their surfaces, "water worlds" are likely to have dense layers of ice at their core-mantle boundary, thus preventing hydrothermal activity.

But as noted already, there is some research that indicates that these planets could still be habitable. While the abundance of water would prevent the absorption of carbon dioxide by rocks and suppress volcanic activity, simulations have shown that these planets could still cycle carbon between the atmosphere and the ocean, thus keeping the climate stable. If these types of ocean worlds exist, says Dr. Ramirez, scientists could detect them through their lower planetary density and high pressure atmosphere. And then there is the matter of various greenhouse gases, which are not always an indication of warmer planetary atmospheres, depending on the type of star. We will be able to observe this in planetary spectra. Speaking of red dwarfs, the debate rages on as to whether or not planets that orbit these stars would be capable of maintaining an atmosphere.

In the past few years, multiple discoveries have suggested that rocky, tidally locked planets are common around red dwarf stars, and that they orbit within their stars' respective HZs. However, subsequent research has reinforced the theory that the instability of red dwarf stars would likely result in solar flares that would strip the atmospheres of any planets orbiting them. Lastly, Ramirez and his colleagues raise the possibility that habitable planets could be found orbiting main sequence type-A stars, which have until recently been considered unlikely candidates.

Main sequence type-A stars Sirius A, Altair, and Vega were thought to be too bright and hot to be habitable. Ramirez says, "I am also interested in finding out if life exists on habitable zone planets orbiting A-stars. There has not been a lot of published assessments of A-star planetary habitability, but some next-generation architectures plan to observe them. We will soon learn more about the suitability of A-stars for life. Ultimately, studies like this one, which question the definition of the "habitable zone," will come in handy when next-generation missions commence science operations.

With higher-resolution, more sensitive instruments, they will be able to test and validate many of the predictions that have been made by scientists. These tests will also confirm whether or not life could exist out there only as we know it, or also beyond the parameters that we consider to be "Earth-like. We need to be able to find and characterize as many habitable zone planets as possible if we wish to maximize our chances of finding life. However, I also hope that our paper inspires people to dream beyond just the next 10 years so.

I really believe that there will eventually be missions that will be far more capable than anything that we are currently designing. Our current efforts are just the beginning of a much more committed endeavor for our species. The Decadal Survey meeting is being hosted jointly by Board of Physics and Astronomy and the Space Studies Board of the National Academy of Sciences, and will be followed by a report to be released roughly two years from now. Thus far, this search has focused on Earth-like terrestrial worlds in the zone around a star that has the right temperature to allow liquid water to exist. The fact that this means a region that is neither too hot nor too cold, but just right, has led to it being dubbed the Goldilocks zone by astronomers.

Considering Hycean worlds expands the possible range of habitable planets because they can hang on to liquid water outside what is considered the traditional Goldilocks zone. Madhusudhan, who is the lead author of a study discussing Hycean worlds in The Astrophysical Journal , said that there are more of these planets beyond the limits of the solar system than there are rocky worlds like Earth, and that their atmospheres are ideal for analysis. As Hycean planets are large and abundant in the exoplanet population, it is easy to detect them and to study their atmosphere, said Madhusudhan. Madhusudhan and his team have already identified 11 Hycean world candidates orbiting nearby stars, with the conditions of a Hycean world first spotted by the astronomers in the water-rich atmosphere of the exoplanet Kb in The exoplanet which is identified as a Super-Earth—a planet like ours but much larger—orbits a red dwarf light-years from Earth, and the characterization of its atmosphere by Madhusudhan and his team pointed them to this exciting new class of exoplanets.

Olivier Demangeon, a researcher at Portugal's Institute of Astrophysics and Space Science who was not involved with the research, told Newsweek: "The new class of potentially habitable planet that the study presents could quite quickly affect the search for life. There are still several open questions regarding the real habitability of these planets and they are not as promising as terrestrial rocky planets in this regard. It significantly increases our capacity to detect biosignatures if they are present.

Tuchow and his co-author Jason Wright, a professor of astronomy and astrophysics at Penn State, took a different approach to the definition of habitable worlds. In a paper published in Research Notes of the American Astronomical Society, they pointed to the fact that star systems are dynamic and change over time. This means that Goldilocks zones can change too and planets may not remain habitable throughout their entire existence. This is a problem because the brightnesses and temperatures of stars evolve over time, causing their habitable zones to change with them," Tuchow told Newsweek.

Tuchow and Wright say this means there are two types of planets in habitable zones, planets that form there and stay there, and belatedly habitable planets that form outside Goldilocks zones and find themselves within them due to changes in their parent star. Astronomers should not only focus on whether a planet is in the current day habitable zone, Tuchow said, but they should also take the host star's evolution into account and consider how long a planet has remained in the habitable zone and whether it has remained habitable for its entire lifetime.

Demangeon, who was not involved in the study, said: "The fact the researchers have found a quite convincing name for these worlds might help to disseminate the idea. However, as they also wrote the real question is: does being in the belated habitable zone affect the capacity of a planet to have the conditions to host life?

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