Habitability Of Planets: March 28, 2020 Background

The Habitability Of Planetsmarch 28 20201 Backgroundthe Presence Of L

The presence of liquid water is considered to be a prerequisite for life as we know it, which makes looking for water a practical way to begin our search for life beyond Earth. For water to exist on the surface of a planet, the planet must have the right temperature on its surface. The main driving force behind the surface temperature of any planet is the light it receives from its parent star. Around every star there is a region where the planet will receive just the right amount of light to give it temperatures that are conducive to liquid water - this region is called the star’s Habitable Zone. The orbit of the Earth currently falls within the Habitable Zone of our Sun.

To begin, load up the Habitable Zone simulator written by the University of Nebraska by entering the following URL in the address bar of your web browsers: The flash simulator will show you a visual diagram of the solar system in the top panel, a set of simulation settings in the middle panel, and a timeline of the habitability of the Earth in the bottom panel. To run the simulation, click the run in the bottom panel. This button immediately becomes a pause button which will allow you to pause the simulation at any time. To restart the simulation, press the restart button at the very top of the simulation. The blue region marked on the diagram is the Habitable Zone around our Sun.

Notice how there is both an inner edge and an outer edge - the planets interior to the habitable zone are too hot to support liquid water, while the planets exterior to it are too cold. The simulation is currently set to zero-age - this is the Solar System as it was when it first formed, 5 billion years ago. Which planets were in the Habitable Zone at this time? 1 2) Press the start button and watch the Habitable Zone change with time. Pause the simulation when it reaches an age of 5 billion years (you can keep track of the time by looking at the timeline marker in the bottom panel). This is the Solar System as it is today - which planets are in the Habitable Zone now? 3) Allow the simulation to run until the Earth is no longer in the Habitable Zone. At what age does this happen? How long from now until this happens? You can use the timeline bar in the bottom panel to determine your answers. . 4) After the Earth is no longer within the Habitable Zone, what do you think the conditions on Earth will be like? 5) Resume the simulation and let it run until the end. Which planets other than the Earth fell within the Habitable Zone at any point during the Sun’s life? 6) If you had to choose planets of our Solar System for future colonization based on their future habitability, which would you choose, and why?

The Habitable Different Kinds of Stars Now that you’ve simulated the Habitable Zone around our Sun, we’ll run the same simulation for other stars. Astronomers classify stars with letters, O, B, A, F, G, K, and M. The O stars are the hottest and brightest, while the M stars are the dimmest and coolest. Every kind of star has a Habitable Zone, but the brighter the star the farther out the Habitable Zone. Imagine putting an extra log on a campfire - the campers all have to back off a few feet to maintain the same comfortable temperature. But in order for complex life to have a chance to develop, a planet must remain habitable for an extended period of time. How long? We only have Earth to use as an example, so we really don’t know. For the purpose of this exercise, we’ll assume that Earth is “typical” and that planets around other stars mostly follow the timeline of events on Earth shown below: 2 Billion years ago development toward complex life 4.5 Earth forms 4.3-4.4 Earth cools, oceans form 3.8 first bacterial fossils 2.4 rise of oxygen in atmosphere 2.0 first complex cells 0.55 first complex animals (fossils in Clapp). The next table shows several different kinds of stars. Notice how they each have a different mass - the mass of a star determines what kind of star it is. Reset the Habitable Zone simulator with the reset button at top, and then adjust the star mass with the initial star mass slider bar in the middle panel. Notice how the Habitable Zone immediately changes size. Notice also that you can adjust the orbit of “Earth” by adjusting the initial planet distance slider bar in the middle panel. The units of distance from the star are AU - astronomical units, the distance of the Earth from the Sun. The Earth is one AU from the Sun. For each of the star types in the table below, find the planet orbit that remains habitable the longest. To do this you’ll need to run the simulation many times for each star type, each time adjusting the initial planet distance until you find a distance that keeps the planet habitable the longest. Record in the table 1) the size of this orbit, in AU, 2) how long this orbit remains habitable, 3) the most advanced type of life that can develop during this time frame, assuming the Earth’s timeline for life is typical. Type Star Mass Longest Habitable Orbit Habitable Lifetime Most Advanced Life [Solar Masses] [Astronomical Units] [Billions of Years] O 15 B 5.0 A 2.0 F 1.3 G (Sun) 1.0 K 0.7 M 0.4

Unfortunately, for low-mass M type stars the habitable zone is quite close to the star - so close that planets in this zone are likely to be tidally locked. This means that the same side of the planet will always face the star, just as the same side of the Earth’s moon always faces the Earth. The simulator indicates that a planet is tidally locked when it is split between one brown and the other side being light gray. In this section we’ll experiment with planets around M type stars. Adjust the stars “initial stellar mass” to 0.3 (30% of the Sun’s mass), 3 and adjust the “initial planet distance” until the planet is in the star’s Habitable Zone. The planet should switch to the tidally locked icon, even if the planet is in the Habitable Zone.

1) What impact do you think tidal locking would have on the prospect of life on this planet? 2) Try adjusting the star’s mass. What is the lowest mass star that would allow a non- tidally locked planet in the Habitable Zone at the beginning of the star’s life? 3) What is the lowest mass star that would allow a non-tidally locked planet in the Habitable Zone at any point during the star’s life?

Given what you’ve learned so far, what type of star is the best place to look for life? The Sun is a G-type star. What do you think the development of life on planets orbiting hotter types of stars would be like? What about cooler types of stars? Do you think that life in such conditions is even possible? Justify your answer. If you were the director of a NASA program to search for life beyond Earth, toward which type of star would you direct your attention? Why? Justify your answer using the evidence above, and also any other lines of reasoning you like. Percent of all stars are M-type; does your answer to the above question change? Why?

Paper For Above instruction

The quest to understand planetary habitability hinges on several key factors, with the presence of liquid water being paramount. Water’s role as a fundamental solvent for life makes it a prime criterion in the search for extraterrestrial life. Landed planets must reside within their star’s habitable zone—the region where temperatures allow water to remain liquid on the surface. This zone's outer and inner edges are dictated by stellar luminosity and distance, respectively. The Earth's current position within the Sun’s habitable zone exemplifies the importance of this concept. Through simulation, we observe that the habitable zone has evolved over time due to stellar and planetary changes, impacting the potential for life’s development and sustainability.

The initial stage of the solar system, approximately 5 billion years ago, saw the planets positioned differently relative to the habitable zone. Simulations show that early on, some planets may have been within this zone, allowing conditions suitable for the emergence of life. As the Sun aged, the habitable zone migrated outward; currently, Earth remains in a favorable position for life. However, in about 1 to 2 billion years, the increasing luminosity of the Sun will cause Earth to exit the habitable zone, rendering surface conditions inhospitable. Future conditions may lead to a frozen Earth or a planet with extreme temperatures, limiting the prospects for surface-based life—though subsurface life could persist in some environments.

Beyond our solar system, stars vary significantly in mass, luminosity, and lifespan, affecting their habitable zones. Astronomers classify stars into spectral types O, B, A, F, G, K, and M, with O-type stars being the hottest and most luminous, and M-type stars the coolest and dimmest. Larger, brighter stars tend to have habitable zones farther out but shorter stable lifespans, limiting the window for life to develop. Smaller stars, especially M-dwarfs, have their habitable zones very close in, often leading to tidal locking—where one side of the planet perpetually faces the star. Such tidal locking could have profound implications for climate, atmospheric circulation, and the potential for life.

Simulation studies reveal that planets around lower-mass stars are likely to be tidally locked, raising concerns about habitability due to temperature extremes on the permanent day and night sides. Tidal locking could result in harsh surface conditions, but some models suggest that atmospheric and oceanic circulation might distribute heat effectively, potentially allowing habitable conditions despite tidal effects. The minimum stellar mass that can support a non-tidally locked planet in the habitable zone at the start of the star's life appears to be around 0.3 solar masses, but this can vary with planetary orbital adjustments.

Assessing the prospects for life, certain stellar types emerge as more promising. G-type stars, like our Sun, strike a balance with moderate luminosity, sufficiently long stable lifespans, and habitable zones at comfortable distances. Hotter stars (O, B, A, and F types) have shorter lifespans, often too brief for complex life to develop, although they might host planets with early life forms. Cooler stars (K and M types) offer longer lifespans, but the proximity of their habitable zones increases the likelihood of tidal locking and other extreme conditions.

From a strategic perspective, a NASA search for extraterrestrial life might prioritize G and K-type stars, given their stability and the longevity of their habitable zones, which provides a better window for complex life to evolve. Although M-dwarf stars are the most abundant—comprising roughly 75-80% of all stars—the challenges associated with their habitable zones—especially tidal locking and stellar activity—make them less ideal targets. Nonetheless, emerging research suggests that with adequate atmospheric conditions, life could still be possible around M-dwarfs. Consequently, a balanced approach would consider both the abundance and the habitability potential, emphasizing stars with stable, long-lived habitable zones.

References

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