During the past couple of weeks, we have been working as a group, researching parameters that might be appropriate to determine a planet as habitable, and their boundaries according to past research.
Our data lead Harry did some research into boundaries of what gravity humans could withstand. Harry found an article claiming that humans could withstand a gravity of up to 3-4 times the strength of Earth’s. It is conclusive that these humans would have to have had considerable training to increase their muscle mass and cardiovascular fitness to endure this new strength of gravity. A test was carried out with participant Hafthor Bjornsson(aka The Mountain), former world’s strongest man) where he was put under the simulated conditions of 5 times the Earth’s gravity. He had passed the test but not everyone has the freak genetics of this elite athlete. For most humans, 5g causes near impossible locomotive motion due to the stress on the bones. We concluded that habitable exo-planets have an upper bound value of 4g.
Searching for the lower bound proved to be more difficult as most articles only discuss 0g. However, this could be an interesting discussion point as our journey to exoplanets may have to be on spaceships, where we would be under 0g. The effects of prolonged periods on a spaceship with 0g would weaken our bones and muscles, which would be bad news if we were migrating to an exo-planet with gravity stronger than Earths. Further studies shows that bones lose about 1% density per month (elderly Earth-bound humans lose about 1.5% per year) when in 0g environments. Without any exercise, muscle mass could fall by 20% after 6-11 days.
With further research, an article was found stating they found a critical mass of 0.3 Earth Masses as the lower bound for planetary masses life can exist. This is as a result of deteriorating tectonic activity needed for life to survive.
Amaia, our group coordinator, did the research for atmospheres of habitable exo-planets. One important property of atmospheres that was explored was pressure. Pressure is significant as it affects the boiling point of water, which is essential for life. Amaia came across a significant term called the Armstrong Limit. This is the pressure at which atmosphere pressure is sufficient for water to boil at the temperature of the human body. This leads to no life. This led to the conclusion that the lower limit of habitable pressure would be around 75kPa.
The upper limit of pressure was interestingly found through deep sea diving. It is confirmed that with a pressure below 4 bar, the body can function relatively normally but for no change in functioning, the limit is 2 bar.
Amaia also investigated the chemical composition of exo-planets with signs of life and explored deeply into their biosignatures. These compounds consisted of methane, oxygen, nitrous oxide and ammonia (caused by bacteria). We concluded that signs of some of these biosignatures were more signs for already existing life and that discovering a planet with these signatures would be groundbreaking, and therefore an unrealistic expectation. However, properties that are still essential to look for ozone-to protect us from solar flares and harmful UV rays, and water, in order to sustain life.
Spectral Types of Stars
Owen, the communications lead, researched which spectral types of stars would be the best candidates for habitable exo-planets to be orbiting. The types of stars were narrowed down to F, G, and K type stars. This because star types higher up on the spectrum have short life spans on the main sequence. This forbids the evolution of life and prohibits the sufficient time to develop complex life on land like trees and ither types of vegetation.
On the opposite extreme, stars with less than half of the Sun’s mass are more likely to tidally lock planets that are orbiting close enough to have liquid water on their surface too quickly, before life can develop. Tidally locking (or synchronous rotation of the star and planet) may eventually cause the destruction of a life-sustaining atmosphere through condensation on the cold, perpetually dark side of the planet. Moreover, most M-type red dwarf stars would tend to sterilize life on a close-orbiting Earth-type planet regularly with large stellar flares.
Therefore, NASA’s proposed Kepler Mission will search for habitable planets at nearby main sequence stars that are less massive than spectral type A but more massive than type M –dwarf stars of types F, G, and K. However, since low-mass M-and K-type stars so numerous, some astronomers and planetary scientists are continuing to model low-mass stars and possible planetary environments that may be potentially suitable for Earth-type plant and animal life, as well as for microbes.