habitable-zones-report

Name: Habitable Zones Exercises Please read through the background pages entitled Life, Circumstellar Habitable Zones, and The Galactic Habitable Zone before working on the exercises using simulations below. Circumstellar Zones Open the Circumstellar Zone Simulator. There are four main panels: ● The top panel simulation displays a visualization of a star and its planets looking down onto the plane of the solar system. The habitable zone is displayed for the particular star being simulated. One can click and drag either toward the star or away from it to change the scale being displayed. ● The General Settings panel provides two options for creating standards of reference in the top panel. ● The Star and Planets Setting and Properties panel allows one to display our own star system, several known star systems, or create your own star-planet combinations in the none-selected mode. ● The Timeline and Simulation Controls allows one to demonstrate the time evolution of the star system being displayed. The simulation begins with our Sun being displayed as it was when it formed and a terrestrial planet at the position of Earth. One can change the planet’s distance from the Sun either by dragging it or using the planet distance slider. Note that the appearance of the planet changes depending upon its location. It appears quite earth-like when inside the circumstellar habitable zone (hereafter CHZ). However, when it is dragged inside of the CHZ it becomes “desert-like” while outside it appears “frozen”. 1. Drag the planet to the inner boundary of the CHZ and note this distance from the Sun. Then drag it to the outer boundary and note this value. Lastly, take the difference of these two figures to calculate the “width” of the sun’s primordial CHZ. Credit ~ NAAP CHZ Inner Boundary CHZ Outer Boundary Width of CHZ 0.848 1.15 0.302

Name: 2. Let’s explore the width of the CHZ for other stars. Complete the table below for stars with a variety of masses. Solar Mass ( ) ? ⨀ Star Luminosity ( ) ? ⨀ CHZ Inner Boundary (AU) CHZ Outer Boundary (AU) Width of CHZ (AU) 0.3 0.0132 0.112 0.154 0.042 0.7 0.134 0.350 0.502 0.152 1.0 0.739 0.819 1.17 0.351 2.0 16.5 3.89 5.56 1.67 4.0 241 14.8 21.2 6.4 8.0 2690 49.3 71.1 21.8 15.0 19000 131 188 57 3. Using the table above, what general conclusion can be made regarding the location of the CHZ for different types of stars? when the CHZ increases the habitable zone is pushed further away. 4. Using the table above, what general conclusion can be made regarding the width of the CHZ for different types of stars? higher mass means wider and it means it would be less if decreased. Exploring Other Systems Begin by selecting the system 51 Pegasi. This was the first planet discovered around a star using the radial velocity technique. This technique detects systematic shifts in the wavelengths of absorption lines in the star’s spectra over time due to the motion of the star around the star-planet center of mass. The planet orbiting 51 Pegasi has a mass of at least half Jupiter’s mass. 5. Zoom out so that you can compare this planet to those in our solar system (you can click-hold-drag to change the scale). Is this extrasolar planet like any in our solar system? In what ways is it similar or different? It is similar in that it has a circular orbit around the sun although it has the closest orbit to our sun in comparison to the other planets. 6. Select the system HD 93083. Note that planet b is in this star’s CHZ. Now in fact this planet has a mass of at least 0.37 Jupiter masses. Is this planet a likely candidate to have life like that on Earth? Why or why not? This planet would likely not host life, the main reason being that part of its orbit is not in a “habitable zone” so life as we define it would most likely exist though it might host forms of life unrecognizable to our human understanding of life. Credit ~ NAAP

Name: 8. Return to the none selected mode and configure the simulator for Earth (a 1 star at a ? ⨀ distance of 1 AU). Note that immediately after our Sun formed Earth was in the middle of the CHZ. Drag the timeline cursor forward and note how the CHZ moves outward as the Sun gets brighter. Stop the time cursor at 4.6 billion years to represent the present age of our solar system. Based on this simulation, how much longer will Earth be in the CHZ? earth will be in the habitable zone until 5.44 gy so earth will be in the habitable zone for approximately 800 million more years. 9. What is the total lifetime of the Sun (up to the point when it becomes a white dwarf and no longer supports fusion)? 12 billion years 10. What happens to Earth at this time in the simulator? The planet is destroyed. You may have noticed the planet moving outwards towards the end of the star’s life. This is due to the star losing mass in its final stages. We know that life appeared on Earth early on, but complex life did not appear until several billion years later. If life on other planets takes a similar amount of time to evolve, we would like to know how long a planet is in its CHZ to evaluate the likelihood of complex life being present. To make this determination, first set the timeline cursor to time zero, then drag the planet in the diagram so that it is just on the outer edge of CHZ. Then run the simulator until the planet is no longer in the CHZ. Record the time when this occurs – this is the total amount of time the planet spends in the CHZ. Complete the table for the range of stellar masses. 11. It took approximately 4 billion years for complex life to appear on Earth. In which of the systems above would that be possible? What can you conclude about a star’s mass and the likelihood of it harboring complex life? Credit ~ NAAP Solar Mass ( ) ? ⨀ Initial Planet Distance (AU ) Time in CHZ 0.3 0.157 380 Gy 0.7 1.0 2.0 4.0 8.0 15.0

Name: Tidal Locking We have learned that large stars are not good candidates for life because they evolve so quickly. Now let’s take a look at low-mass stars. Reset the simulator and set the initial star mass to 0.3 . Drag the planet in to the CHZ. ? ⨀ Notice that the planet is shown with a dashed line through its middle. What has happened is that the planet is so close to its star that is has become tidally locked due to gravitational interactions. This is analogous to Earth’s moon which always presents the same side towards Earth. For a planet orbiting a star, this means one side would get very hot and the other side would get very cold. ( However, a thick atmosphere could theoretically spread the heat around the planet as happens on Venus. In answering the following questions, please put aside this possibility. ) 12. What would happen to Earth’s water if it were suddenly to become tidally locked to the Sun? What would this mean for life on Earth? If Earth were to stop spinning and always had one side facing the Sun, it would become extremely hot on one side and very cold on the other, making it almost impossible for most forms of life, including humans, to survive 13. Complete the table below by resetting the simulator, setting the initial star mass to the value in the table, and positioning the planet in the middle of the CHZ at time zero. Record whether or not the planet is tidally locked at this time. If tidal locking reduces the likelihood of life evolving on a planet, which system in the table is least conducive towards life? 0.3 and 0.5 CHZ Summation We have seen that low-mass stars have very small CHZs very close to the star and that planets become tidally locked at these small distances. We have seen that high-mass stars have very short lives – too short for life as we know it to appear. The combination of these two trains of thought is often referred to as the Goldilocks hypothesis – that medium-mass stars give the optimal opportunity for complex life to appear. Credit ~ NAAP Solar Mass ( ) ? ⨀ Tidally Locked? 0.3 yes 0.5 yes 0.8 no 1.0 No