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Monthly Space Topic

Join University faculty, graduates, and students as we explore space history, technology, astronomy, planetary geology, and exciting new space missions. A new topic will be selected each month, and a member of the APUS Space Studies Research Group will present a short paper. In addition to their insight, additional links will be provided for further investigation. So, buckle up and join us as we explore the vast wonders of Space.

Goldilocks is not just a character from a storybook; in astronomy, the Goldilocks zone refers to the "sweet spot" around a star where conditions are just right for a planet to be habitable. For many years, scientists believed a habitable planet could only exist around a star similar to our Sun. However, recent research has expanded the possibilities to include other types of stars. This month's topic, presented by Emma Follis, explores the parameters of habitable zones around these stars that could support life on other planets.

A little more about Emma...

Emma is an undergraduate student at APUS pursuing a bachelor's degree in space studies with a concentration in astronomy. Her space interests revolve around exoplanets, atmospheric science, and astrobotany. When Emma is not deep-diving into a space topic of interest (and excitedly giving new random space facts to her husband when he least expects it), she is either spending time with her family, reading a book, trying to recreate her favorite restaurant dishes, or identifying plants and collecting rocks while hiking. 

Stellar Environments and Life: A Study of Habitability Metrics in M-, K-, and G-Dwarf Systems

by Emma Follis

 

Section 1: Introduction

Our knowledge of the cosmos has been completely transformed by the discovery of exoplanets, which have shown us how diverse planetary systems are and brought up important issues regarding the possibility of extraterrestrial life. Central to this quest is the concept of the habitable zone (HZ), a region around a star where conditions may allow for liquid water to exist on a planet's surface which is a fundamental criterion for life as we know it. Often called the "Goldilocks Zone," this idea is not set in stone and instead varies greatly based on the star's luminosity, temperature, and size. Therefore, determining planets that could harbor life requires an understanding of how these habitable zones vary among stellar types.

 

Stars are categorized by their spectral types, which range from the massive, hot O- and B-types to the cooler, more stable M-dwarfs. In this paper, I focus on the three stellar types most relevant to habitability studies: M-dwarfs, K-dwarfs, and G-dwarfs. There are unique benefits and difficulties for planetary habitability associated with each type. Because they are the most common stars in the galaxy, M-dwarfs provide a large number of potentially habitable planets. But because of their small habitable zones near the star, planets are vulnerable to powerful stellar flares and radiation that could destroy atmospheres.

 

K-dwarfs, intermediate in size and temperature, are stable and long-lived, offering favorable conditions for habitability over extended periods. G-dwarfs, like our Sun, have wide habitable zones and sufficient stability to allow for the development of complex life, but they are less common compared to M- and K-dwarfs.

The goal of this research is to analyze how the size, location, and occupancy of habitable zones vary across these stellar types using data from the NASA Exoplanet Archive. Specifically, this paper investigates the following questions: How do the inner and outer boundaries of habitable zones differ among M, K, and G stars? What fraction of planets orbiting these stars fall within their respective habitable zones? Are certain stellar types more conducive to hosting potentially habitable planets? Here, I present an in-depth statistical analysis of stellar and planetary data to address these questions, highlighting trends and offering insights into the conditions that may favor the emergence of life.

 

In this study, the inner and outer boundaries of habitable zones are calculated using established equations that account for stellar luminosity and effective temperature (Kopparapu R. K., et al., 2013)These boundaries define the range where liquid water could exist, and their variations provide critical insights into the diversity of planetary environments. Since planets with elliptical orbits might only spend a portion of their orbit in the habitable zone, the influence of orbital eccentricity on habitability is also examined. In order to provide a solid assessment of the variations and patterns, statistical techniques are utilized to compare the size and occupancy of habitable zones among M, K, and G stars.

 

This study is an extension of earlier research. Dressing and Charbonneau (2015), for instance, looked at the possibility of habitable planets orbiting M-dwarfs, highlighting how common they are but also pointing out the difficulties caused by stellar activity. Using a framework for understanding how different star types differ in their habitability, Kaltenegger (2017) examined the effects of stellar properties on planetary atmospheres and surface conditions. By synthesizing these insights with new analyses of data from the NASA Exoplanet Archive, this study aims to advance our understanding of the factors that govern planetary habitability.

There are many ramifications for this research. The identification of trends in habitable zones across stellar types can help prioritize targets for future observational missions, including those equipped with next-generation telescopes that can characterize exoplanetary atmospheres.

 

Furthermore, knowing the circumstances that give rise to life can help answer more general queries concerning the uniqueness of Earth and the frequency of life in the universe. In order to find trends and make insightful deductions, I use statistical tools and visualization techniques to present a thorough analysis of habitable zones across M, K, and G stars in this paper. In doing so, this work contributes to the ongoing search for extrasolar life and expands our understanding of the cosmic environments that may support it.

Section 2: Hypothesis

The study's objective is to determine the percentage of exoplanets that reside in the habitable zones and to examine the differences in these zones among M, K, and G stellar types. This includes putting theories about the distribution of planets with habitable zones and the size of these zones to the test.

Qualitative Hypotheses

  1. The size and location of habitable zones differ significantly among M, K, and G stars due to differences in stellar luminosity and effective temperature.

  2. Planets orbiting M-dwarfs are more likely to be found within their habitable zones because M-dwarfs are the most abundant stellar type in the galaxy. However, their narrow zones and high stellar activity may offset this trend.

  3. K-dwarfs offer the most favorable conditions for long-term habitability due to their moderate-sized habitable zones and high stability compared to M- and G-dwarfs.

 

Null Hypothesis (H)

The null hypothesis assumes that there are no significant differences in the mean habitable zone sizes (u) or the fractions of planets within habitable zones () across the stellar types:

  and   

Where:

  • : Mean size of the habitable zone.

  • : Fraction of planets within the habitable zone.

  • Subscripts M, K, and G denote M-dwarfs, K-dwarfs, and G-dwarfs.

Figure 1

 

Figure 2

Image of Yuri Gagarin

Radiation Effects

On Earth, humans are typically exposed to 2.4 milli-sievert (mSv) per year from several different places such as the Earth itself, fallout and remnants of nuclear fallout/testing, coal and nuclear plants, x-rays at the doctor’s office or hospital, etc. (IAEA, n.d.). In space, astronauts get exposed to a range of radiation amounts depending on the altitude they are at. It can be anywhere from 50 to 2,000 mSv (Perez, 2019). For comparison, 1 mSv is equal to around 3 chest x-rays. Whether it is galactic cosmic rays (GCR) or solar particle events (SPE), space radiation can cause several issues for humans. Some of these include radiation sickness, cancer, degenerative diseases, and lower immunity capabilities.

According to the United States Environmental Protection Agency (2023), radiation sickness is the common name for acute radiation syndrome. For someone to get radiation sickness, the exposure would have to be extensive. Minimally, someone would have to get 75 rad[1] in a somewhat short time interval of minutes to hours to get radiation sickness. On Earth, this is the equivalent of getting 18,000 chest x-rays across your entire body within that time span. Other events that can cause this level of radiation explosion while on Earth are nuclear explosions, highly radioactive material ruptures close by, and accidental handling of radiative material. Being exposed to radiation can cause some long-term effects as well like cancer and cardiovascular disease.

Cancer caused by radiation happens because ionizing radiation damages the DNA within cells. Normally human bodies can repair those damaged cells, but with constant exposure (like an astronaut would have in space), that damage is not being repaired. When the cells die, they ultimately become cancerous and end up spreading as cancer does.

 

Physiological Effects

Physiological effects include a list of risks such as telomeres mutation/shortening, body mass loss, bone density loss, folate increase, inflammation (leading to artery wall thickening), gene mutation, gene expression, protein level instability, hypotension, fluid shifts, and visual impairments. Bone density loss is contributed to being in microgravity specifically. It has been observed that there is a rate of loss around 1 – 1.5% per month while in microgravity (Institute of Medicine, 2014). This is significant because in postmenopausal females (when bone density loss is a common occurrence), the average rate of loss is around 2-3% per year. Mitigation efforts include drinking enough water, diet (focused on nutritional intake), and exercise. Hypotension is thanks to microgravity as well, where blood ends up staying in the upper part of the body. This, by itself, can cause facial edema. As the person returns to Earth and gravity, the blood pools much lower and ends up causing orthostatic hypertension.

Visual impairments have been documented on multiple occasions both on the International Space Station (ISS) and the Mir Space Station (Institute of Medicine, 2014). Of the collective reports that focus on eye health of astronauts, the overall findings saw that hyperopia (or far-sightedness), scotomas (or blind spots), and papilledema (elevated intracranial pressure) were common occurrences. These were associated with fluid shifts (due to microgravity), diet, radiation exposure, and elevated carbon dioxide levels. To mitigate these visual impairment issues, there has been an extensive research effort by NASA to monitor and combat the after-effects once the astronauts are back on Earth.

Gene mutation, telomere mutation/shortening, and gene expression issues are attributed to radiation exposure primarily. Radiation has a way of getting into cells and either changing them or damaging them. When radiation is not damaging the cells, it is changing them, even if slightly. A great example of this was NASA’s Twins Study back in 2016. This study was done with a set of genetically identical twins (Mark and Scott Kelly) over the course of 25 months[2]. What made this an ideal research opportunity was the fact that they were both astronauts. This allowed NASA to study how the human body reacted to spaceflight and being in space for extended periods of time (Parks, 2018). Parks (2018) explained that the data collected and analyzed during the Twins Study (as seen in Figure 3) showed a few important things: telomere lengths changed during a mission[3], decreased body mass while in orbit, increased folate while in orbit, microgravity impacted cognitive functions[4], vaccine immunity was not impacted by the space environment, microgravity increased inflammation within the body[5], microgravity affected the microorganisms within the human gut[6], gene mutations are triggered by spaceflight[7], gene expression relied on certain environments to react the way they are expected to[8], thickening of artery walls[9], and the regulation of fluids via proteins increased[10].

Figure 3

Data collected pre-, in-, and post-flight during the 25 months of the NASA Twin Study

Psychological Effects

The psychological effects include cognitive effects once out of microgravity, degradation of fine motor function, gross motor function, and hand-eye coordination, dual-tasking, and isolation. As seen in NASA’s Twin Study, cognitive function is affected by leaving and reentering Earth’s gravity. Another concern is central nervous system (CNS) issues. CNS issues are attributed to radiation exposure, but they tend to cause psychological changes such as degradation of both fine and gross motor functions (Zarana S. Patel, 2020). Hand-eye coordination and dual-tasking capabilities fall under the category of cognitive function – so by extension, they are considered psychological effects. Impacted hand-eye coordination and dual-tasking capabilities can be detrimental to astronauts and the missions they are on.

Isolation is a very broad term for what has ended up being a slightly taboo subject for astronauts up until recent years. There was a reluctance to report certain psychological symptoms for fear of being removed from the mission and future projects (Institute of Medicine, 2014). Despite that, there has always been a consensus that irritability and anxiety are normal occurrences when it comes to isolation issues for astronauts. NASA has made mitigation efforts to recognize and combat psychological side effects attributed to isolation. The mitigation comes in the form of analog missions to monitor isolation effects on the psyche. One specific study in 2014 had six crew members isolate themselves for 520 days to simulate a mission to Mars. This study concluded with one crew member showing depression and mood swings towards the end of the mission, two crew members showing signs of sleep regulating issues which attributed to majority of the conflicts during mission between crewmembers, and the other crew members showing no behavioral or adverse psychological symptoms (Zarana S. Patel, 2020).

Conclusion

Studying and conquering the radiation, physiological, and psychological challenges humans face are vital to our long-duration space travel abilities for the future. Without proper mitigation efforts, long-duration space travel just will not be a viable possibility for humans. Theoretical concepts of artificial gravity have been a topic of conversation when talking about possible long-term space travel without many of the current risks and issues that are seen during missions. Artificial gravity, while the technology is not there yet, is a viable concept when considering that it would negate the risks attributed to microgravity. The only mitigation efforts humanity can truly make when it comes to radiation exposure is advancement of technology and material being used for space crafts and space suits. Neither are easy tasks, but they are at least being considered and studied in hopes of safer space travel for astronauts.

 

[1] Rad is the United States’ standard measure for radiation units. Internationally, it is Gray (Gy). 1 Gy is equivalent to 100 rads.

[2] The actual time spent in space by one of the twins was 340 days, but they were both monitored for a total of 25 months to get pre-, in-, and post-flight data.

[3] Telomeres lengthen during flight and time in space but rapidly shorten within about 48 hours of coming back within Earth’s gravity. This has been hypothesized to be attributed to intense exercise and restricted diet during the mission.

[4] It was hypothesized that the twin that went into space (Scott) had declining cognitive function when returning to Earth due to Earth’s gravity.

[5] As seen by blood tests that measured lipids and cytokines.

[6] The biggest difference was a decrease in Bacteroidetes numbers while in space – but they did come back to normal levels upon touchdown.

[7] Test results showed that hundreds of gene mutations were experienced by Scott during the mission. 93% returned to normal after the fact, but a surprising amount of them did not.

[8] The two aspects that were unexpected during this part of the study were the gene expressions near the telomere length regulation and collagen production regulation areas.

[9] Unfortunately, it’s still unclear if this was due to the space environment or specific to genetics within the individual.

[10] The specific biomarker that was focused on was aquaporin 2. Aquaporin 2 is meant to help regulate water being transferred around within the body which gave indications of hydration status. This was associated to abnormally high levels of plasma sodium (which is a dehydration indictor), but overall, this is hypothesized to be attributed to fluid shifting and ultimately microgravity.


References:

Garrett-Bakelman, Francine E.; et al. (2019). The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight. Science. doi:10.1126/science.aau8650

 

IAEA. (n.d.). Radiation in Everyday Life. Retrieved May 17, 2023, from International Atomic Energy Agency (IAEA): https://www.iaea.org/Publications/Factsheets/English/radlife

 

Institute of Medicine. (2014, June 23). Health Standards for Long Duration and Exploration Spaceflight: Ethics Principles, Responsibilities, and Decision Framework. (J. Kahn, C. Liverman, & M. McCoy, Eds.) 3. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK222157/

 

Keeter, B. (Ed.). (2020, September 4). Long-Term Challenges to Human Space Exploration. Retrieved from National Aeronautics and Space Administration (NASA): https://www.nasa.gov/centers/hq/library/find/bibliographies/Long-Term_Challenges_to_Human_Space_Exploration

 

Lewis, C. (2017, January 24). Studying Long-Duration Human Spaceflight. Retrieved from Air and Space: https://airandspace.si.edu/stories/editorial/studying-long-duration-human-spaceflight

 

Meleshko, G., Shepelev, Y., Averner, M., & Volk, T. (2001). Risks to Astronaut Health During Space Travel. (C. E. JR Ball, Ed.) Safe Passage: Astronaut Care for Exploration Missions. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK223785/

 

Parks, J. (2018, February 16). How does space change the human body? Retrieved from Astronomy: https://astronomy.com/news/2018/02/how-does-space-change-the-human-body

 

Pearlman, R. Z. (2015, March 26). One Year in Space: A History of Ultra-Long MIssions Off Planet Earth. Retrieved from Space.com: https://www.space.com/28947-yearlong-space-missions-history.html

 

Perez, J. (Ed.). (2019, October 8). Why Space Radiation Matters. Retrieved from NASA.gov: https://www.nasa.gov/analogs/nsrl/why-space-radiation-matters

 

Radiation Health Effects. (2023, February 15). Retrieved from EPA.gov: https://www.epa.gov/radiation/radiation-health-effects

 

Romero, E., & Francisco, D. (2020). The NASA human system risk mitigation process for space exploration. Acta Astronautica,, 175, 606-615. doi:https://doi.org/10.1016/j.actaastro.2020.04.046.

 

Space Facts. (2023). Astronauts and Cosmonauts (sorted by "Time in Space"). Retrieved from http://www.spacefacts.de/english/e_tis.htm

 

Tays, G. D., Hupfeld, K. E., McGregor, H. R., Salazar, A. P., Dios, Y. E., Beltran, N. E., . . . Seidler, R. D. (2021). The Effects of Long Duration Spaceflight on Sensorimotor Control and Cognition. Frontiers in Neural Circuits, 15. doi:10.3389/fncir.2021.723504

 

Zarana S. Patel, T. J. (2020). Red risks for a journey to the red planet: The highest priority human health risks for a mission to Mars. npj Microgravity, 6(1). doi:https://doi.org/10.1038/s41526-020-00124-6

 

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